rus
Issue #6/2021
A. B. Lyukhter, V. I. Krivorotov, K. V. Skvortsov
Two-Level Method for Assessing The Operational Reliability of a Laser Impact Active-Protection Modular Cabin (APMC)
Two-Level Method for Assessing The Operational Reliability of a Laser Impact Active-Protection Modular Cabin (APMC)
DOI: 10.22184/1993-7296.FRos.2021.15.6.454.472
The results of a comprehensive assessment of the operational reliability of a laser impact active-protection modular cabin (APMC) are presented. A two-level computational and experimental technique has been developed and practically implemented, which includes, at the first level, the determination of the «actuation» time of the laser radiation emergency shutdown system (LRESS) under the conditions of APMC with a continuous fiber laser generator operating with a maximum power of 6 kW. At the second level, the stress and structural state of the metal of the panels was assessed, as well as the bearing capacity of the APMC elements after repeated laser impact. The reliability and advantages of APMC in terms of equipping with LRESS protection means, excluding the possibility of laser radiation leaving the cabin, are shown.
The results of a comprehensive assessment of the operational reliability of a laser impact active-protection modular cabin (APMC) are presented. A two-level computational and experimental technique has been developed and practically implemented, which includes, at the first level, the determination of the «actuation» time of the laser radiation emergency shutdown system (LRESS) under the conditions of APMC with a continuous fiber laser generator operating with a maximum power of 6 kW. At the second level, the stress and structural state of the metal of the panels was assessed, as well as the bearing capacity of the APMC elements after repeated laser impact. The reliability and advantages of APMC in terms of equipping with LRESS protection means, excluding the possibility of laser radiation leaving the cabin, are shown.
Теги: laser technological equipment; active-protection modular cabins reflected and scattered radiation of laser impact; magnetometric
Two-Level Method for Assessing The Operational Reliability of a Laser Impact Active-Protection Modular Cabin (APMC)
A. B. Lyukhter, V. I. Krivorotov, K. V. Skvortsov
Vladimir State University named after A. G. and N. G. Stoletovs (VlSU), Vladimir, Russia
NTO «IRE-Polyus: LLC, Fryazino, Moscow region, Russia
Vladimir Law Institute of the Federal Penitentiary Service of Russia (VLI of FPS of Russia), Vladimir, Russia
The results of a comprehensive assessment of the operational reliability of a laser impact active-protection modular cabin (APMC) are presented. A two-level computational and experimental technique has been developed and practically implemented, which includes, at the first level, the determination of the «actuation» time of the laser radiation emergency shutdown system (LRESS) under the conditions of APMC with a continuous fiber laser generator operating with a maximum power of 6 kW. At the second level, the stress and structural state of the metal of the panels was assessed, as well as the bearing capacity of the APMC elements after repeated laser impact. The reliability and advantages of APMC in terms of equipping with LRESS protection means, excluding the possibility of laser radiation leaving the cabin, are shown.
Keywords: laser technological equipment; active-protection modular cabins (APMC); laser radiation emergency shutdown system (LRESS); fiber-optic laser generator with a power of 6 kW; protection against direct, reflected and scattered radiation of laser impact; magnetometric stress state control; bearing capacity; reliability of APMC elements.
Received on: 04.06.2021
Accepted on: 20.08.2021
Fulfillment of the mandatory requirements for laser safety specified in domestic and world regulatory documents is the most important factor determining the widespread introduction into industrial production of laser complexes, installations and systems, as well as laser processing technologies. In 2020, a number of regulatory documents of the Russian Federation, such as SN 5804-91, SanPiN 2.2.4.3359-16, etc. are losted a legal force.
However, the importance and relevance of solving the problem of ensuring the conditions for the safe operation of laser equipment at domestic enterprises, despite the absence of these documents in the legal framework of the Russian Federation, is beyond doubt. In new projects, when developing operational documents, a laser generator without collective protection equipment (cabins, posts, etc.) is still classified as hazard class 4 [1–9]. However, during the operation of laser robotic systems (LRS), the likelihood of hazardous and emergency situations is not excluded: due the formation of powerful reflected laser radiation (LR) from the treated surface; LR going beyond the limits of the workpiece or processing area in the direction of the likely presence of personnel (burn-in of the workpiece, inaccurate positioning, failure of the control program).
In the event of an emergency, there is a high likelihood of serious harm to the health of working personnel by laser radiation. In the opinion of the authors of this work, referring to their own experience, as well as publications of other researchers [5–7, 10–13], a laser generator used as part of laser systems (installations, complexes, etc.), equipped with means of collective active protection, can be with sufficient justification to refer to the 1st hazard class.
The purpose of this work is to develop and apply a two-level methodology for assessing the operational reliability of an active-protection modular cabin (APMC) from unauthorized laser impact while fulfilling its functional properties.
The paper presents the test results of the developed modular protective cabin equipped with a laser radiation emergency shutdown system (LRESS). APMC was created as a means of collective protection [8] from unauthorized influence of LR when placing laser technological complexes in its space for various technological processes of laser processing (welding, surfacing, hardening, etc.). In terms of equipping with protection means, APMC has undoubted advantages over passive protective cabins (without active protection).
The essence of a specially developed design and experimental two-level methodology for assessing the operational reliability of APMC is as follows. At the first level of the technique, tests are carried out for the efficiency of the LRESS generator of continuous laser radiation with a maximum power of 6 kW.
At the second level, the study of the effect of LR on the structural and physical-mechanical characteristics of metal panels, the stress state, as well as the operational properties of APMC elements was carried out. That is, the assessment of the correctness of the structural solutions laid down in the design and implemented in the manufacture of the cabin was carried out.
The main provisions of the LRESS principle of operation are to generate an alarm signal for switching off the laser radiation source to prevent spread of LR outside the APMC. This solution is performed by placing the sensor’s sensitive element in the space between the sides of the double-sideed panel (Fig. 1). When direct laser radiation hits the front side of the panel, it heats up with subsequent burning. Direct, reflected or diffusely scattered (re-reflected) laser radiation, the spectrum of which falls within the sensitivity range of the applied photodiode, passed as a result of burning into the inner cavity of the APMC panel, is recognized by the LRESS module. The control circuit filters and amplifies the photocurrent, and then generates a relay signal for the safety circuits of the laser complex.
Thus, LRESS provides on laser equipment of the 4‑th hazard class the formation of an emergency signal for switching off the source of laser radiation, which excludes its propagation outside the cabin. Accordingly, a laser technological complex (LTC) with such a reliable protection from the beam going out of the cabin can be classified under the definition of laser products of the 1st hazard class.
Efficiency tests were carried out according to the program regulating the test procedure in accordance with the selected experimental modes. At the first level of the methodology, the reliability of the LRESS system at APMC for laser cladding equipment was experimentally determined. The error-proof of the emergency automatic shutdown of laser radiation was determined with an operating continuous laser generator with a maximum power of 6 kW with a fiber optic system and a focal length of 250 mm.
The method of the first level prescribed operations in the following sequence, taking into account the selected positive test result – the absence of a through hole in the rear side of the APMC panel:
According to the test methodology, the range of distances from the panel to the optical head of LTC (L) was equal to one (F), three (3F) and five (5F) focal lengths of the optical system. The test at each distance was repeated at least 5 times. Modes and results of impact to laser radiation on the APMC panel are presented in table. 1. Images of panels after testing are shown in Fig. 2.
As a result of a thorough visual inspection of the rear side of the APMC panel, no through holes and traces of heat impact were found, which is evidence that the laser radiation did not go beyond its limits. An exception was the burn-through of the front and rear sides with the LRESS was disabled (Fig. 2a).
During the tests carried out, it was found that LRESS, with a very high degree of probability, performs an emergency shutdown of the laser generator from the mains supply, which ensures high efficiency of active protection on serial laser processing complexes with the APMC.
It is known that reflected and diffusely scattered laser radiation may pose a great danger to the organs of vision and skin areas unprotected by personal protective equipment [9–13]. These types of radiation are able to penetrate outside the protective cabin through the gaps, slots between the posts, panels and other elements, which significantly reduces the level of protective properties of the cabin.
Therefore, at the second level of the methodology, an investigation of the features of the effect of laser radiation (direct and reflected) was carried out and an assessment of the degree of influence of thermal effects from direct laser radiation on the stressed state of structural elements and metal structures of APMC as a whole over a given estimated time of its operation.
First of all, to determine the stress state indicators in order to exclude the effect of mechanical and thermal factors that cause the occurrence of residual deformations in the form of shape distortion « and other macro- and micro-dimensional distortions and imperfections that reduce the effectiveness of the protective (operational) properties of individual assembly elements and the cabin as a whole.
An increase in stresses in the metal structure of the cabin panels contributes to the appearance of shape distortions and loss of dimensions, the formation of gaps, cracks and other imperfections, which is unacceptable. It is known that there is a fairly stable relationship between the chemical composition of ferromagnetic materials and the value of their coercive force Hc [14]. To establish this dependence for the material of the studied APMC panels, were taken 5 samples and the chemical composition of the metal was determined by the optical emission method on a Magellan Q‑8 analyzer. The results of determining the chemical composition of the panel samples are presented in table. 2.
Based on the results of determining the chemical composition of the metal of the panels using the formulas proposed by N. D. Bogacheva [14] performed preliminary calculations of the theoretical mean value of the coercive force (Hc). The calculations used the average quantitative content of elements (C, Si, Mn, Cr and Ni) in steel. The results of calculations performed according to the formula (1) are presented in table 3.
Hc = KC ∙ C% + KSi ∙ Si% + KMn ∙ Mn% + KCr ∙ Cr% + KNi ∙ Ni%. (1)
In formula (1): are the empirical coefficients (Table 3) before the content of the corresponding elements in steel (in%).
Нс = (16 ∙ 0.07) + (2 ∙ 0.012) + (0.9 ∙ 0.289) +
+ (0.6 ∙ 0.014) + (3 ∙ 0.015)
1.12 + 0.024 + 0.26 = 1.46 ≈ 1.5 А / cm.
That is, the approximate average (expected) value of Нс of the investigated rolled metal products within the grade composition of steel 08Yu in the initial state is Нс = 1.5 A / cm (Table 3).
The resulting calculated value of Нс was used as a reliable primary information parameter, including for the subsequent assessment of the stress state sides of the panel.
To determine the mechanical properties and stress state of elements and APMC as a whole, modern methods of non-destructive magnetometric testing, metallographic and structural analysis, statistical data processing, together with mechanical tests, hardness measurements, etc. were used.
The APMC panel (front and rear sides) was conventionally divided into sections according to the degree of impact to laser radiation and marked accordingly (Fig. 2b, c). To exclude the propagation of diffusely scattered laser radiation inside the panel through the holes formed by the burn-through, they were covered (muffled) with an opaque tape before each subsequent switching on of the laser. Thus, the reliability of the determination of the irradiance in the experiments was increased.
At the end of the experiments on the first level of the methodology – assessing the efficiency of the LRESS actuation, the Нс values were measured on the front side (from the side where the sensor was located) and rear side outer sides of the panel at pre-marked areas (Fig. 2b, c). After Нс measurements, samples were taken from the panel for further studies of the metallographic structure, as well as the stress state of the metal of the APMC panels.
As a result of investigations, it was found that LRESS, with a high degree of reliability, performs an emergency disconnection of the laser generator from the mains supply for a specified minimum period of time and ensures the effectiveness of the APMC application on serial laser processing complexes.
MACROSTRUCTURAL METALLOGRAPHIC ANALYSIS
Macrostructural metallographic analysis was performed on thin sections made from specially selected templates. Templates were selected from the characteristic sections of the panel after their irradiation with a laser instrument with variable parameters: different power, focal length of the laser head and duration (duration) of impact; and also after measuring the Hc values of the panel metal. A general view of individual macrosections and a panorama of the zone of thermal effect of laser radiation with a characteristic type of macro- and microstructures is shown in Fig. 3. In fig. 3a shows traces (dark areas) characterizing the consequences of thermal effect of direct laser radiation on the panel side under study.
This causes the grain boundaries to burn through (Fig. 3b) and ultimately burns through the entire thickness of the cab panel (Figure 3c). The characteristic separate areas with visible structural distortions (overburning) of the grain boundaries are clearly visible, as well as the closely spaced areas of the grains themselves, caused by the action of laser radiation, are quite distinguishable. It should be noted that as one approaches the reflow zones (near the burn-through) or directly near the through-burn-through of the panel under study, the results of the manifestation of these effects on structural changes noticeably increase. This is expressed, first of all, by the fact that the grain boundaries themselves are distorted by «overburning» and, as a consequence, the loss of the mechanical strength of the metal (Fig. 3b, c).
MICROSTRUCTURAL ANALYSIS
To increase the reliability of the results and ensure the identification of samples taken from the panel side, microstructural analysis was performed on specially selected mcrosections. Microsections were obtained from those templates that were used to make macrosections. The preparation of microsections (pressing, grinding, polishing, etc.) was carried out using a set of equipment of the company «Struers», Denmark. The etching of microsections was carried out in a 20% alcohol solution of nitric acid. The microstructure of 08Yu steel before and after impact to laser radiation (LR) is shown in Fig. 4. The grain size was determined using a microscope GX51 (Olympus, Japan). The measurement of the grain size was carried out using a special PC program SIAMS∆, as well as by the secant method at a two-hundredfold magnification of the image [23, 24].
The microstructure of 08Yu steel before (a) and after (b) impact to laser radiation is shown in Fig. 4. It follows from Fig. 4a that in the initial (before impact to LR) state, the structure of the panel metal is characteristic of steels type 08Yu in a normalized state.
The average grain size is 0.013 mm (13 μm). In the area of impact to direct laser radiation, in the areas located in the immediate vicinity of the through holes formed as a result of burn-through of the panel under study, the microstructure has a characteristic appearance of a heat-treated metal material. There is a noticeable increase in grain size in these areas, on average, from 0.013 mm (13 μm) with the presence of individual fragments from to 0.1 mm (100 μm) (Fig. 4b).
In parallel with metallographic studies of the structure in the zones of thermal effect of laser radiation, the microhardness of the metal of the cabin sides was determined on microhardness [25, 26]. The microhardness was determined using a DuraScan 20 microhardness tester (Emco Test, Austria) at a load of 100 grams. The microhardness in the studied areas of the panel in the initial (before the thermal effect of LR) state was, on average, HV0.1 = 100. The microhardness in the areas exposed to laser radiation, depending on the proximity to the through-burn zone, is much higher 1.9 times) and amounted to HV0.1 = 130...190. This also indicates an increase in stresses in the panel metal after laser impact [27–30]. The graph of the distribution of microhardness along the length of the sample (Fig. 5a) depending on the location from the through hole (burn-through) on the panel under investigations and the structure of the sample are shown in Fig. 5b. It follows from the graph (Fig. 5a) that the average values of the microhardness of the metal increase with the approach in the through hole formed as a result of the panel burn-through due to impact to direct laser radiation.
The uneven distribution of microhardness values over the section also increases, there are separate areas with visible structural distortions caused by thermal effects (overburning) of grain boundaries. By the uneven distribution of microhardness values in the measured areas corresponding to the original structure, the transition zone and the through-burn zone, one can judge the inhomogeneity of the stress state in these areas.
Taking into account the recommendations of the authors of works [31, 32], the fields of thermal impact on the metal of the panel were constructed in these experiments. The diagram (Fig. 6) shows an image of isotherm fields characterizing the typical thermal effect of laser radiation during stationary heating. From the results of computational and experimental studies, it follows that the stresses from the thermal effect of laser radiation do not exceed the value of the ultimate strength of the metal of the APMC panels (steel 08Yu). This prevents the occurrence of critical deformations from the thermal effect of laser radiation, which can lead to distortion of the panel shape, causing the panels to loosely adhere to the elements of the cabin frame. Moreover, consequently, it practically excludes the possibility of propagation through slots, gaps and other discontinuities between panels and struts (elements) of the frame of reflected and diffusely scattered laser radiation outside the cabin.
The resulting nomogram (Fig. 6) was used to study the features of the thermal effect of laser radiation on the structural elements of the protective cabin panel. These nomograms were taken into account when sampling and cutting out of the characteristic sections of the panel of samples. For the manufacture of macro- and microsections and subsequent metallographic examination, determination of hardness, as well as mechanical testing.
To determine the physical and mechanical properties of rolled metal and the stress level in individual sections and the cabin as a whole, a special magitometric control technique was used, based on measuring the coercive force of the metal.
It is known, that there is a fairly stable relationship between the chemical composition of a ferromagnetic material, its mechanical properties, structural, stressed state and coercive force [19–22]. In turn, the mechanical properties are largely determined by the chemical composition of the material, its state after thermomechanical impact (for example, rolling, forging, etc. and types of heat treatment: hardening, normalization, improvement, annealing, etc.). This makes it possible to carry out preliminary calculations of Нс and use them in the future when assessing the stress state of elements, panels and the APMC as a whole.
Measurements of the values of the coercive force were carried out using a coercimeter (magnetic strukturoscope) KRM-K2C-M developed by the research and production company «Special Scientific Research» (SSR). The measurements were carried out in accordance with the requirements of RD ICC «KRAN» – 007-97 / 02 [22] and the Operation Manual for the КРМ–К2Ц-М device. The values of Нс measured on separate cards – blanks before cutting out samples for further tests were, on average, Нс = 2 A / cm. This means that rolled metal products of 08Yu steel were in the delivery condition in a normalized condition.
The Hc values of the front and rear sides of the protective panel were measured, which was a skin in the form of a thin sheet metal-roll welded to the frame by an intermittent seam by arc welding.
Primary Hc values were obtained at each marked area on both sides of the panel, taking into account the location (polarization) of the sensor poles of the magnetic sensors, along and across the rolling direction. Next, the Hc values of the panel sheet were determined, on the characteristic sections of the panel:
The measurement results were processed using a standard statistical program. Frequency histograms of the distribution of Hc measurements were constructed with the determination of the mathematical expectation (ME) and the standard deviation SD (Fig. 7a). Using specially developed software built into the coercimeter, we built color magnetograms characterizing the distribution of Hc values over the object surface after processing the results of the KRM–K2C–M model coercimeter readings obtained during measurements. The image (appearance) of typical histograms and diagrams (color magnetograms) of the measurement results of Hc of the front (inner) side, as well as the metal of the entire protective panel as a whole, is shown in Fig. 7.
Analysis of the measurement results, expressed by the corresponding histograms and magnetograms, allows us to conclude that the average values of Hc of rolled metal on the reverse side in the horizontal (4.67 A / cm) and vertical (4.95 A / cm) directions and metal on the front side in the horizontal (4,72 A / cm) and vertical (5.01 A / cm) directions, indicate that rolled metal in the initial state had minimal anisotropy of mechanical properties in surfacing along and across rolling. The average value of the mathematical expectation (ME) of the sheathing sheet of the protective panel (front and rear sides) is:
МEav. = (МEint.→ + МОext.↑ + МEint.→ + МОext.↑) / 4 =
= (4.67 + 4.95 + 4.72 + 5.01) / 4 = 4.838 ≈ 4.8 А / cm (2)
In formula (2), arrows indicate the location (polarization) of the sensors of the magnetic sensors of the KRM–K2C–M model device when measuring Нс: → – horizontal (in the direction of rolling) and ↑ – vertical (at an angle of 90 ° to the direction of rolling).
DETERMINATION OF THE LEVEL OF THE ACTIVE RESIDUAL STRESSES
The method for determining the level of effective residual stresses is based on the following measurement procedures:
Static tensile tests of flat specimens cut from a protective panel (08Yu steel) were carried out on an universal testing machine LFM‑250 (Walter + Bai AG, Switzerland). Simultaneously with the stretching of the sample, the values of the coercive force (Hc) were measured using a magnetic structroscope (coercitimeter) of the KRM–K2C–M model.
Based on the results of measuring the Hc values obtained directly in the process of mechanical tests, a graphical dependence was built in the coordinates: «flow stress – coercive force», which was used as a calibration curve (Fig. 8).
Using the constructed calibration curve, according to the experimental values of the coercive force, the stress level in the metal of the panel was determined and the stress state in the panel of the protective cabin as a whole was estimated. It should be noted that previously measured on separate blank cards cut from a sheet before making samples for further tests, the values of Hc were, on average, Нс = 2 A / cm. The experimentally obtained value of Нс of the protective panel after welding the sheet to the frame before laser action on the front and rear (outer) sides of the protective panel is on average, Нс = 4.8 A / cm (Table 4).
As you can see from the table. 4, an increase in 2.4 times (4.8 / 2) values of Нс characterizes an increase in stresses in the metal structure of the APMC panel caused by welding a thin-sheet sheathing to the frame.
It follows from the graph (Fig. 8) that according to the values of the coercive force Нс, measured in the metal of the panel under study after laser impact (Нс = 6.3–6.6 A / cm), the stresses in the metal structure of the protective cabin panel do not exceed σ = 26 kg / mm2 (284 MPa), which is lower than the tensile strength values for steel 08Yu, which is:
Thus, it was found by the calculation and experimental method for measuring the coercive force that stresses σ = 26 kg / mm2 (284 MPa) are not so critical in order to contribute to the accumulation of damage in the metal of the panels under the influence of the factors under study during the operation of the APMC. The results of the assessment of the stress state of the panel indicate the functional reliability of the APMC elements and the cabin as a whole, which is an independent prefabricated metal structure.
As a result of investigation carried out according to the developed methodology, it was experimentally established and confirmed in practice that the created APMC provide high strength indicators and a level of comprehensive protection against the effects of laser radiation (reflected and diffusely scattered) on personnel outside the protective cabin. This is an important factor aimed at accelerating the solution of the problems of expanding the field of implementation of laser processing technologies in the industrial production of domestic metallurgy and mechanical engineering enterprises.
The results of evaluating the operational reliability of cabins with active protection using a two-level method indicate that APMC of this design provide a high level of protection against the effects of laser radiation on serial laser processing complexes. First of all, from the impact of direct laser radiation (due to the equipment of the LRESS) and, in addition, from the impact of side negative phenomena: reflected and diffusely scattered laser radiation on personnel outside the protective cabin when performing specific technological laser processing processes.
CONCLUSION
For a comprehensive assessment of the operational reliability of APMC, a two-level design and experimental technique has been developed and implemented in practice. This study presents the results of experiments performed using a two-level technique.
At the first level of the methodology, the experimental determination of the time of the «response» of the LRESS on the APMC during the implementation of the technological process of laser cladding. Evaluation of the efficiency of emergency automatic shutdown of laser radiation was carried out when operating a continuous laser generator with a fiber optic system with a maximum power of 6 kW. At the second level, the effect of laser radiation on the structural and physical-mechanical characteristics of the metal, the stress state, as well as the operational properties of the APMC elements was investigated.
It is established by means of calculation and experiments:
The activation efficiency of the emergency shutdown system of laser radiation with a working laser source with a power of 6 kW at a serial laser processing complex has been experimentally proved. LRESS provides «triggering» of sensors configured for emergency disconnection of the laser generator from the mains supply for a specified minimum period of time.
The increase in the level of stresses in the elements of the metal structure under the influence of the investigated factors caused by LR are not so critical in order to contribute to the accumulation of damage in the metal of the panels during the operation of the APMC.
The thermal effect of LR on the structure, physical and mechanical properties of the metal and the stress state of the investigated element of the protective cabin with an installed sensor sensitive to LR does not reduce the performance of the sensor itself and the metal structure of the panel, which indicates the reliability of the cab protection system as a whole.
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AUTHORS
A. B. Lyuchter. Cand. of Science. Vladimir State University named after A.G. and N. G. Stoletovs, 3699137@mail.ru, Vladimir, Russia.
ORCID: 0000-0003-1523-0637
V. I. Krivorotov. Cand. of Science. NTO IRE-Polyus LLC, vKrivorotov@ntoire-polus.ru, Fryazino, Moscow region, Russia
K. V. Skvortsov. Cand. of Science. Vladimir Law Institute of the Federal Penitentiary Service of Russia (VLI of FPS of Russia), k-skv@yandex.ru, Vladimir, Russia.
ORCID 0000-0001-8611-3353
A. B. Lyukhter, V. I. Krivorotov, K. V. Skvortsov
Vladimir State University named after A. G. and N. G. Stoletovs (VlSU), Vladimir, Russia
NTO «IRE-Polyus: LLC, Fryazino, Moscow region, Russia
Vladimir Law Institute of the Federal Penitentiary Service of Russia (VLI of FPS of Russia), Vladimir, Russia
The results of a comprehensive assessment of the operational reliability of a laser impact active-protection modular cabin (APMC) are presented. A two-level computational and experimental technique has been developed and practically implemented, which includes, at the first level, the determination of the «actuation» time of the laser radiation emergency shutdown system (LRESS) under the conditions of APMC with a continuous fiber laser generator operating with a maximum power of 6 kW. At the second level, the stress and structural state of the metal of the panels was assessed, as well as the bearing capacity of the APMC elements after repeated laser impact. The reliability and advantages of APMC in terms of equipping with LRESS protection means, excluding the possibility of laser radiation leaving the cabin, are shown.
Keywords: laser technological equipment; active-protection modular cabins (APMC); laser radiation emergency shutdown system (LRESS); fiber-optic laser generator with a power of 6 kW; protection against direct, reflected and scattered radiation of laser impact; magnetometric stress state control; bearing capacity; reliability of APMC elements.
Received on: 04.06.2021
Accepted on: 20.08.2021
Fulfillment of the mandatory requirements for laser safety specified in domestic and world regulatory documents is the most important factor determining the widespread introduction into industrial production of laser complexes, installations and systems, as well as laser processing technologies. In 2020, a number of regulatory documents of the Russian Federation, such as SN 5804-91, SanPiN 2.2.4.3359-16, etc. are losted a legal force.
However, the importance and relevance of solving the problem of ensuring the conditions for the safe operation of laser equipment at domestic enterprises, despite the absence of these documents in the legal framework of the Russian Federation, is beyond doubt. In new projects, when developing operational documents, a laser generator without collective protection equipment (cabins, posts, etc.) is still classified as hazard class 4 [1–9]. However, during the operation of laser robotic systems (LRS), the likelihood of hazardous and emergency situations is not excluded: due the formation of powerful reflected laser radiation (LR) from the treated surface; LR going beyond the limits of the workpiece or processing area in the direction of the likely presence of personnel (burn-in of the workpiece, inaccurate positioning, failure of the control program).
In the event of an emergency, there is a high likelihood of serious harm to the health of working personnel by laser radiation. In the opinion of the authors of this work, referring to their own experience, as well as publications of other researchers [5–7, 10–13], a laser generator used as part of laser systems (installations, complexes, etc.), equipped with means of collective active protection, can be with sufficient justification to refer to the 1st hazard class.
The purpose of this work is to develop and apply a two-level methodology for assessing the operational reliability of an active-protection modular cabin (APMC) from unauthorized laser impact while fulfilling its functional properties.
The paper presents the test results of the developed modular protective cabin equipped with a laser radiation emergency shutdown system (LRESS). APMC was created as a means of collective protection [8] from unauthorized influence of LR when placing laser technological complexes in its space for various technological processes of laser processing (welding, surfacing, hardening, etc.). In terms of equipping with protection means, APMC has undoubted advantages over passive protective cabins (without active protection).
The essence of a specially developed design and experimental two-level methodology for assessing the operational reliability of APMC is as follows. At the first level of the technique, tests are carried out for the efficiency of the LRESS generator of continuous laser radiation with a maximum power of 6 kW.
At the second level, the study of the effect of LR on the structural and physical-mechanical characteristics of metal panels, the stress state, as well as the operational properties of APMC elements was carried out. That is, the assessment of the correctness of the structural solutions laid down in the design and implemented in the manufacture of the cabin was carried out.
The main provisions of the LRESS principle of operation are to generate an alarm signal for switching off the laser radiation source to prevent spread of LR outside the APMC. This solution is performed by placing the sensor’s sensitive element in the space between the sides of the double-sideed panel (Fig. 1). When direct laser radiation hits the front side of the panel, it heats up with subsequent burning. Direct, reflected or diffusely scattered (re-reflected) laser radiation, the spectrum of which falls within the sensitivity range of the applied photodiode, passed as a result of burning into the inner cavity of the APMC panel, is recognized by the LRESS module. The control circuit filters and amplifies the photocurrent, and then generates a relay signal for the safety circuits of the laser complex.
Thus, LRESS provides on laser equipment of the 4‑th hazard class the formation of an emergency signal for switching off the source of laser radiation, which excludes its propagation outside the cabin. Accordingly, a laser technological complex (LTC) with such a reliable protection from the beam going out of the cabin can be classified under the definition of laser products of the 1st hazard class.
Efficiency tests were carried out according to the program regulating the test procedure in accordance with the selected experimental modes. At the first level of the methodology, the reliability of the LRESS system at APMC for laser cladding equipment was experimentally determined. The error-proof of the emergency automatic shutdown of laser radiation was determined with an operating continuous laser generator with a maximum power of 6 kW with a fiber optic system and a focal length of 250 mm.
The method of the first level prescribed operations in the following sequence, taking into account the selected positive test result – the absence of a through hole in the rear side of the APMC panel:
- to impact with LR on the surface of the front side of the panel in the zone, determined randomly before the operation of the LRESS, which turns off the LR generator;
- carry out a visual inspection of the rear side of the panel for the presence of a through hole;
- if it is detected on the surface of the rear side of the panel (paint burnout, metal melting, etc.), it is necessary to study this zone with the help of additional optical devices in order to establish the results of the effect of the thermal effect of LR on the material of the rear side.
According to the test methodology, the range of distances from the panel to the optical head of LTC (L) was equal to one (F), three (3F) and five (5F) focal lengths of the optical system. The test at each distance was repeated at least 5 times. Modes and results of impact to laser radiation on the APMC panel are presented in table. 1. Images of panels after testing are shown in Fig. 2.
As a result of a thorough visual inspection of the rear side of the APMC panel, no through holes and traces of heat impact were found, which is evidence that the laser radiation did not go beyond its limits. An exception was the burn-through of the front and rear sides with the LRESS was disabled (Fig. 2a).
During the tests carried out, it was found that LRESS, with a very high degree of probability, performs an emergency shutdown of the laser generator from the mains supply, which ensures high efficiency of active protection on serial laser processing complexes with the APMC.
It is known that reflected and diffusely scattered laser radiation may pose a great danger to the organs of vision and skin areas unprotected by personal protective equipment [9–13]. These types of radiation are able to penetrate outside the protective cabin through the gaps, slots between the posts, panels and other elements, which significantly reduces the level of protective properties of the cabin.
Therefore, at the second level of the methodology, an investigation of the features of the effect of laser radiation (direct and reflected) was carried out and an assessment of the degree of influence of thermal effects from direct laser radiation on the stressed state of structural elements and metal structures of APMC as a whole over a given estimated time of its operation.
First of all, to determine the stress state indicators in order to exclude the effect of mechanical and thermal factors that cause the occurrence of residual deformations in the form of shape distortion « and other macro- and micro-dimensional distortions and imperfections that reduce the effectiveness of the protective (operational) properties of individual assembly elements and the cabin as a whole.
An increase in stresses in the metal structure of the cabin panels contributes to the appearance of shape distortions and loss of dimensions, the formation of gaps, cracks and other imperfections, which is unacceptable. It is known that there is a fairly stable relationship between the chemical composition of ferromagnetic materials and the value of their coercive force Hc [14]. To establish this dependence for the material of the studied APMC panels, were taken 5 samples and the chemical composition of the metal was determined by the optical emission method on a Magellan Q‑8 analyzer. The results of determining the chemical composition of the panel samples are presented in table. 2.
Based on the results of determining the chemical composition of the metal of the panels using the formulas proposed by N. D. Bogacheva [14] performed preliminary calculations of the theoretical mean value of the coercive force (Hc). The calculations used the average quantitative content of elements (C, Si, Mn, Cr and Ni) in steel. The results of calculations performed according to the formula (1) are presented in table 3.
Hc = KC ∙ C% + KSi ∙ Si% + KMn ∙ Mn% + KCr ∙ Cr% + KNi ∙ Ni%. (1)
In formula (1): are the empirical coefficients (Table 3) before the content of the corresponding elements in steel (in%).
Нс = (16 ∙ 0.07) + (2 ∙ 0.012) + (0.9 ∙ 0.289) +
+ (0.6 ∙ 0.014) + (3 ∙ 0.015)
1.12 + 0.024 + 0.26 = 1.46 ≈ 1.5 А / cm.
That is, the approximate average (expected) value of Нс of the investigated rolled metal products within the grade composition of steel 08Yu in the initial state is Нс = 1.5 A / cm (Table 3).
The resulting calculated value of Нс was used as a reliable primary information parameter, including for the subsequent assessment of the stress state sides of the panel.
To determine the mechanical properties and stress state of elements and APMC as a whole, modern methods of non-destructive magnetometric testing, metallographic and structural analysis, statistical data processing, together with mechanical tests, hardness measurements, etc. were used.
The APMC panel (front and rear sides) was conventionally divided into sections according to the degree of impact to laser radiation and marked accordingly (Fig. 2b, c). To exclude the propagation of diffusely scattered laser radiation inside the panel through the holes formed by the burn-through, they were covered (muffled) with an opaque tape before each subsequent switching on of the laser. Thus, the reliability of the determination of the irradiance in the experiments was increased.
At the end of the experiments on the first level of the methodology – assessing the efficiency of the LRESS actuation, the Нс values were measured on the front side (from the side where the sensor was located) and rear side outer sides of the panel at pre-marked areas (Fig. 2b, c). After Нс measurements, samples were taken from the panel for further studies of the metallographic structure, as well as the stress state of the metal of the APMC panels.
As a result of investigations, it was found that LRESS, with a high degree of reliability, performs an emergency disconnection of the laser generator from the mains supply for a specified minimum period of time and ensures the effectiveness of the APMC application on serial laser processing complexes.
MACROSTRUCTURAL METALLOGRAPHIC ANALYSIS
Macrostructural metallographic analysis was performed on thin sections made from specially selected templates. Templates were selected from the characteristic sections of the panel after their irradiation with a laser instrument with variable parameters: different power, focal length of the laser head and duration (duration) of impact; and also after measuring the Hc values of the panel metal. A general view of individual macrosections and a panorama of the zone of thermal effect of laser radiation with a characteristic type of macro- and microstructures is shown in Fig. 3. In fig. 3a shows traces (dark areas) characterizing the consequences of thermal effect of direct laser radiation on the panel side under study.
This causes the grain boundaries to burn through (Fig. 3b) and ultimately burns through the entire thickness of the cab panel (Figure 3c). The characteristic separate areas with visible structural distortions (overburning) of the grain boundaries are clearly visible, as well as the closely spaced areas of the grains themselves, caused by the action of laser radiation, are quite distinguishable. It should be noted that as one approaches the reflow zones (near the burn-through) or directly near the through-burn-through of the panel under study, the results of the manifestation of these effects on structural changes noticeably increase. This is expressed, first of all, by the fact that the grain boundaries themselves are distorted by «overburning» and, as a consequence, the loss of the mechanical strength of the metal (Fig. 3b, c).
MICROSTRUCTURAL ANALYSIS
To increase the reliability of the results and ensure the identification of samples taken from the panel side, microstructural analysis was performed on specially selected mcrosections. Microsections were obtained from those templates that were used to make macrosections. The preparation of microsections (pressing, grinding, polishing, etc.) was carried out using a set of equipment of the company «Struers», Denmark. The etching of microsections was carried out in a 20% alcohol solution of nitric acid. The microstructure of 08Yu steel before and after impact to laser radiation (LR) is shown in Fig. 4. The grain size was determined using a microscope GX51 (Olympus, Japan). The measurement of the grain size was carried out using a special PC program SIAMS∆, as well as by the secant method at a two-hundredfold magnification of the image [23, 24].
The microstructure of 08Yu steel before (a) and after (b) impact to laser radiation is shown in Fig. 4. It follows from Fig. 4a that in the initial (before impact to LR) state, the structure of the panel metal is characteristic of steels type 08Yu in a normalized state.
The average grain size is 0.013 mm (13 μm). In the area of impact to direct laser radiation, in the areas located in the immediate vicinity of the through holes formed as a result of burn-through of the panel under study, the microstructure has a characteristic appearance of a heat-treated metal material. There is a noticeable increase in grain size in these areas, on average, from 0.013 mm (13 μm) with the presence of individual fragments from to 0.1 mm (100 μm) (Fig. 4b).
In parallel with metallographic studies of the structure in the zones of thermal effect of laser radiation, the microhardness of the metal of the cabin sides was determined on microhardness [25, 26]. The microhardness was determined using a DuraScan 20 microhardness tester (Emco Test, Austria) at a load of 100 grams. The microhardness in the studied areas of the panel in the initial (before the thermal effect of LR) state was, on average, HV0.1 = 100. The microhardness in the areas exposed to laser radiation, depending on the proximity to the through-burn zone, is much higher 1.9 times) and amounted to HV0.1 = 130...190. This also indicates an increase in stresses in the panel metal after laser impact [27–30]. The graph of the distribution of microhardness along the length of the sample (Fig. 5a) depending on the location from the through hole (burn-through) on the panel under investigations and the structure of the sample are shown in Fig. 5b. It follows from the graph (Fig. 5a) that the average values of the microhardness of the metal increase with the approach in the through hole formed as a result of the panel burn-through due to impact to direct laser radiation.
The uneven distribution of microhardness values over the section also increases, there are separate areas with visible structural distortions caused by thermal effects (overburning) of grain boundaries. By the uneven distribution of microhardness values in the measured areas corresponding to the original structure, the transition zone and the through-burn zone, one can judge the inhomogeneity of the stress state in these areas.
Taking into account the recommendations of the authors of works [31, 32], the fields of thermal impact on the metal of the panel were constructed in these experiments. The diagram (Fig. 6) shows an image of isotherm fields characterizing the typical thermal effect of laser radiation during stationary heating. From the results of computational and experimental studies, it follows that the stresses from the thermal effect of laser radiation do not exceed the value of the ultimate strength of the metal of the APMC panels (steel 08Yu). This prevents the occurrence of critical deformations from the thermal effect of laser radiation, which can lead to distortion of the panel shape, causing the panels to loosely adhere to the elements of the cabin frame. Moreover, consequently, it practically excludes the possibility of propagation through slots, gaps and other discontinuities between panels and struts (elements) of the frame of reflected and diffusely scattered laser radiation outside the cabin.
The resulting nomogram (Fig. 6) was used to study the features of the thermal effect of laser radiation on the structural elements of the protective cabin panel. These nomograms were taken into account when sampling and cutting out of the characteristic sections of the panel of samples. For the manufacture of macro- and microsections and subsequent metallographic examination, determination of hardness, as well as mechanical testing.
To determine the physical and mechanical properties of rolled metal and the stress level in individual sections and the cabin as a whole, a special magitometric control technique was used, based on measuring the coercive force of the metal.
It is known, that there is a fairly stable relationship between the chemical composition of a ferromagnetic material, its mechanical properties, structural, stressed state and coercive force [19–22]. In turn, the mechanical properties are largely determined by the chemical composition of the material, its state after thermomechanical impact (for example, rolling, forging, etc. and types of heat treatment: hardening, normalization, improvement, annealing, etc.). This makes it possible to carry out preliminary calculations of Нс and use them in the future when assessing the stress state of elements, panels and the APMC as a whole.
Measurements of the values of the coercive force were carried out using a coercimeter (magnetic strukturoscope) KRM-K2C-M developed by the research and production company «Special Scientific Research» (SSR). The measurements were carried out in accordance with the requirements of RD ICC «KRAN» – 007-97 / 02 [22] and the Operation Manual for the КРМ–К2Ц-М device. The values of Нс measured on separate cards – blanks before cutting out samples for further tests were, on average, Нс = 2 A / cm. This means that rolled metal products of 08Yu steel were in the delivery condition in a normalized condition.
The Hc values of the front and rear sides of the protective panel were measured, which was a skin in the form of a thin sheet metal-roll welded to the frame by an intermittent seam by arc welding.
Primary Hc values were obtained at each marked area on both sides of the panel, taking into account the location (polarization) of the sensor poles of the magnetic sensors, along and across the rolling direction. Next, the Hc values of the panel sheet were determined, on the characteristic sections of the panel:
- without visible traces of the effects of direct radiation, that is, taken as the original;
- near the pronounced effect of laser radiation on the general condition of the panel.
The measurement results were processed using a standard statistical program. Frequency histograms of the distribution of Hc measurements were constructed with the determination of the mathematical expectation (ME) and the standard deviation SD (Fig. 7a). Using specially developed software built into the coercimeter, we built color magnetograms characterizing the distribution of Hc values over the object surface after processing the results of the KRM–K2C–M model coercimeter readings obtained during measurements. The image (appearance) of typical histograms and diagrams (color magnetograms) of the measurement results of Hc of the front (inner) side, as well as the metal of the entire protective panel as a whole, is shown in Fig. 7.
Analysis of the measurement results, expressed by the corresponding histograms and magnetograms, allows us to conclude that the average values of Hc of rolled metal on the reverse side in the horizontal (4.67 A / cm) and vertical (4.95 A / cm) directions and metal on the front side in the horizontal (4,72 A / cm) and vertical (5.01 A / cm) directions, indicate that rolled metal in the initial state had minimal anisotropy of mechanical properties in surfacing along and across rolling. The average value of the mathematical expectation (ME) of the sheathing sheet of the protective panel (front and rear sides) is:
МEav. = (МEint.→ + МОext.↑ + МEint.→ + МОext.↑) / 4 =
= (4.67 + 4.95 + 4.72 + 5.01) / 4 = 4.838 ≈ 4.8 А / cm (2)
In formula (2), arrows indicate the location (polarization) of the sensors of the magnetic sensors of the KRM–K2C–M model device when measuring Нс: → – horizontal (in the direction of rolling) and ↑ – vertical (at an angle of 90 ° to the direction of rolling).
DETERMINATION OF THE LEVEL OF THE ACTIVE RESIDUAL STRESSES
The method for determining the level of effective residual stresses is based on the following measurement procedures:
- on carrying out mechanical tests of samples for static tension;
- on the measurement in the process of loading the coercive force (Hc) with the construction of a calibration curve in the coordinates «coercive force – stress» in tension;
- on the measurement of Нс of the investigated panel of the cabin and on the determination of the stresses acting in the metal structure of the panel according to the calibration curve.
Static tensile tests of flat specimens cut from a protective panel (08Yu steel) were carried out on an universal testing machine LFM‑250 (Walter + Bai AG, Switzerland). Simultaneously with the stretching of the sample, the values of the coercive force (Hc) were measured using a magnetic structroscope (coercitimeter) of the KRM–K2C–M model.
Based on the results of measuring the Hc values obtained directly in the process of mechanical tests, a graphical dependence was built in the coordinates: «flow stress – coercive force», which was used as a calibration curve (Fig. 8).
Using the constructed calibration curve, according to the experimental values of the coercive force, the stress level in the metal of the panel was determined and the stress state in the panel of the protective cabin as a whole was estimated. It should be noted that previously measured on separate blank cards cut from a sheet before making samples for further tests, the values of Hc were, on average, Нс = 2 A / cm. The experimentally obtained value of Нс of the protective panel after welding the sheet to the frame before laser action on the front and rear (outer) sides of the protective panel is on average, Нс = 4.8 A / cm (Table 4).
As you can see from the table. 4, an increase in 2.4 times (4.8 / 2) values of Нс characterizes an increase in stresses in the metal structure of the APMC panel caused by welding a thin-sheet sheathing to the frame.
It follows from the graph (Fig. 8) that according to the values of the coercive force Нс, measured in the metal of the panel under study after laser impact (Нс = 6.3–6.6 A / cm), the stresses in the metal structure of the protective cabin panel do not exceed σ = 26 kg / mm2 (284 MPa), which is lower than the tensile strength values for steel 08Yu, which is:
- experimentally, with tension σ = 30.9 kg / mm2 (303 MPa);
- according to the reference books of properties of steel 08Yu σ = 37 kg / mm2 (360 MPa).
Thus, it was found by the calculation and experimental method for measuring the coercive force that stresses σ = 26 kg / mm2 (284 MPa) are not so critical in order to contribute to the accumulation of damage in the metal of the panels under the influence of the factors under study during the operation of the APMC. The results of the assessment of the stress state of the panel indicate the functional reliability of the APMC elements and the cabin as a whole, which is an independent prefabricated metal structure.
As a result of investigation carried out according to the developed methodology, it was experimentally established and confirmed in practice that the created APMC provide high strength indicators and a level of comprehensive protection against the effects of laser radiation (reflected and diffusely scattered) on personnel outside the protective cabin. This is an important factor aimed at accelerating the solution of the problems of expanding the field of implementation of laser processing technologies in the industrial production of domestic metallurgy and mechanical engineering enterprises.
The results of evaluating the operational reliability of cabins with active protection using a two-level method indicate that APMC of this design provide a high level of protection against the effects of laser radiation on serial laser processing complexes. First of all, from the impact of direct laser radiation (due to the equipment of the LRESS) and, in addition, from the impact of side negative phenomena: reflected and diffusely scattered laser radiation on personnel outside the protective cabin when performing specific technological laser processing processes.
CONCLUSION
For a comprehensive assessment of the operational reliability of APMC, a two-level design and experimental technique has been developed and implemented in practice. This study presents the results of experiments performed using a two-level technique.
At the first level of the methodology, the experimental determination of the time of the «response» of the LRESS on the APMC during the implementation of the technological process of laser cladding. Evaluation of the efficiency of emergency automatic shutdown of laser radiation was carried out when operating a continuous laser generator with a fiber optic system with a maximum power of 6 kW. At the second level, the effect of laser radiation on the structural and physical-mechanical characteristics of the metal, the stress state, as well as the operational properties of the APMC elements was investigated.
It is established by means of calculation and experiments:
The activation efficiency of the emergency shutdown system of laser radiation with a working laser source with a power of 6 kW at a serial laser processing complex has been experimentally proved. LRESS provides «triggering» of sensors configured for emergency disconnection of the laser generator from the mains supply for a specified minimum period of time.
The increase in the level of stresses in the elements of the metal structure under the influence of the investigated factors caused by LR are not so critical in order to contribute to the accumulation of damage in the metal of the panels during the operation of the APMC.
The thermal effect of LR on the structure, physical and mechanical properties of the metal and the stress state of the investigated element of the protective cabin with an installed sensor sensitive to LR does not reduce the performance of the sensor itself and the metal structure of the panel, which indicates the reliability of the cab protection system as a whole.
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AUTHORS
A. B. Lyuchter. Cand. of Science. Vladimir State University named after A.G. and N. G. Stoletovs, 3699137@mail.ru, Vladimir, Russia.
ORCID: 0000-0003-1523-0637
V. I. Krivorotov. Cand. of Science. NTO IRE-Polyus LLC, vKrivorotov@ntoire-polus.ru, Fryazino, Moscow region, Russia
K. V. Skvortsov. Cand. of Science. Vladimir Law Institute of the Federal Penitentiary Service of Russia (VLI of FPS of Russia), k-skv@yandex.ru, Vladimir, Russia.
ORCID 0000-0001-8611-3353
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