Study of the Effect of Growing Modes by Selective Laser Melting Method on Porosity in Copper Alloy Products
Article accepted for publication on 24.12.2018
The process of selective laser melting (SLM) is used to create complex structures and components based on three-dimensional CAD models. The SLM technology, firstly, allows you to directly create finished parts, and secondly, it is cost-efficient for the production of single or small-scale batches of products. Copper and copper-based alloys are of great interest to manufacturers due to their high thermal conductivity and electrical conductivity. But due to the low absorption of laser radiation by these materials and their high thermal conductivity, it turns out in practice that to obtain a stable and dense structure, it is necessary to use high-power laser sources at wavelengths Δλ = 1–10 µm and to use a low beam scanning speed. When creating products, problems arise in the formation of single fused paths, and this often limits the ability to grow quality products.
The analysis of the results of foreign studies showed that selective laser melting of copper is carried out using laser sources with a power of 800–1200 W . Therefore, industrial complexes for the SLM process require the setting of high-power laser sources up to 1500 watts. In some cases, even two lasers with a power of 400 and 1000 W are installed. This set-up leads to a multiple increase in the price of the final product. The works are known, where the process of solid phase [2–4] and liquid phase sintering of the copper powder compositions, including with the addition of the fusible element [5–7], was conducted using low power laser light. In [8, 9], the influence of the parameters of the SLM process on the formation of the microstructure and the density of samples from copper (C184000) was analyzed. When using two 400 and 1500 W lasers in the process of selective laser melting, the samples with a density of 96% were obtained. It was found that a laser source with a beam having a uniform cross-section distribution of energy density provides for the creation of products with a higher relative density than the source, the distribution of the energy density in the cross section of which obeys the normal Gauss law. The experiments headed by Z. Mao  showed that the laser power has a strong influence on the relative density of Cu‑4Sn samples and their Vickers hardness. In , the authors used a CO2 laser with a power of 200 W and gas-sprayed pure copper powder with an average diameter of 35.52 µm for selective laser melting of copper. By varying the growing modes, the researches received a sample with a relative density of the material over 88%.
The goal of the study was to examine the dependence and porosity of the samples on the parameters of the growing process from various materials based on copper. It was decided, on the basis of the results obtained, to identify the limits of variation of parameters on the SLM set-up with a laser power not exceeding 100 W to create the samples with high relative density
Growth experiments were carried out on the installation for selective laser melting SLP‑110 . The set-up is equipped with a continuous fiber ytterbium laser with a maximum power of 100 W, the spot diameter at the focus is 50 µm, the treatment field is 110 Ч 110 mm (Fig. 1). The growing process was carried out in a chamber with a controlled atmosphere, from which air was preliminarily evacuated and then protective gas (argon) was pumped in. Pre-evacuation allows for clearing not only the working volume of the chamber, but also the space between the metal powders in the feed hopper. The movement of the growing bunker and the feeding bunker was implemented using a ball screw with a stepper motor (5 µm pitch).
The granulometric analysis of the powders 99.7% Cu and PR-BrH was carried out using a particle size analyzer (HORIBALA‑350). The measurement range of particles extends from 100 nm to 1000 µm. The analyzer is based on the laser diffraction method; the computational algorithm is designed in accordance with the Mie scattering theory. The radiation source is a laser diode (λ = 605 nm, P = 5 mW), the detector is 64 photodiodes located on a logarithmic spiral, 6 silicon photodiodes are used for backscatter analysis.
Microstructure studies were performed using metallographic analysis of transverse thin sections, for which a combined machine for automatic and manual cutting (StruersDiscotom‑6) was used (Fig. 2b). An automatic electro-hydraulic press (StruersCitoPress‑20) was used for hot pressing of the obtained samples (Fig. 2a). Grinding and polishing of the samples was carried out on an automatic grinding and polishing system (StruersTegramin‑30) (Fig. 2c).
The morphological analysis of PR-BrHi powders of pure copper were performed on an inverted metallurgical microscope (OlympusGX‑51). The microscope is equipped with a digital camera Altra 20 (2 MP resolution; 10 bits color depth), image processing was carried out using the system of automated image analysis (SIAMSPhotolab).
To create dense samples in an installation with a 100 W laser, heating of the substrate was carried out, which made it possible to compensate for the effect of high thermal conductivity and reduced thermal stresses. It is obvious to those skilled in the art that increase in the temperature of the material leads to an increase in its absorption coefficient. Therefore, heating of the growth substrate was carried out up to 250 °C using a flat ceramic heater. The heating temperature was controlled using a chromel / alumel thermocouple with a sensitivity of 41 mV / °C.
A relatively pure copper powder (99.7% Cu) and high-temperature bronze powder PR-BrH (the chemical composition of the powders is given in Tables 1 and 2) were used in the study. The powders differed in average particle diameter: PR-BrH had a diameter of 31.84 microns; 99.7% Cu powder – 48.67 microns. The distribution of powder particles of 99.7% Cu and PR-BrH by the average size of the diameter is shown in Fig. 4.
The morphological analysis showed that the granules of PR-BrHi powders with respect to pure copper have a spherical shape (Fig. 5 and 6).
In the course of the experiment, cubic samples of 8 Ч 8 Ч 8 mm were obtained from the two types of powders in modes differing in scan rate values (Fig. 7).
In order to build samples with high relative density it is important that the bath melt formed a stable single path – without drops and bare spots. In the experiments, the spot diameter, the distance between the paths and the power were left unchanged. The results of the experiments were brought to a single parameter of specific energy E, which summarizes the main parameters of laser processing. This parameter is convenient for comparing the results of experiments conducted on various types of equipment with different operating parameters characterizing a particular installation:
Е = Р / v · l · h,
where P is the laser emission power, v is the scanning speed, l is the the distance between the paths, h is the height of the layer.
On the one hand, the specific energy decreases with increasing the scanning speed. On the other hand, the scanning speed determines the performance of the growing process. However, an increase in laser power with a constant spot diameter at the focus increases the power density, which leads to overheating and splashing out of the material from the treatment area.
RESULTS AND DISCUSSION
It was not possible to grow all the samples at the selected speed values during the experiment. In the case of using pure copper powder at a scanning speed of 100 mm / s at a height of about 2 mm, the forming layers lost their stability. This led to the cessation of further growth. High values of specific energy led to overheating of the material, which violated the stability of the layer formation process. Disturbance of the growing process was also caused by a significant level of appearance of stress concentrators in the sample. They led to the formation of cracks at the sample base (Fig. 8). The resulting distortions and deformations created an excessive contact between the powder application knife and the sample. Growing in these modes was stopped ahead of schedule. The same pattern was observed in PR-BrH samples at speeds in the range of values from 100 to 300 mm / sec. The presence of alloying elements increased the efficiency of the process, which led to overheating of the material at lower values of speed in comparison with pure copper. The porosity of these samples was measured, which increased the degree of reliability in the metallographic analysis of the results.
The transverse thin sections of the grown samples were examined under a microscope for the presence of pores. The results of the study of the dependence of porosity on the specific energy are shown in the graph (Fig. 9).
The results of the research processes for the two types of powders showed a significant difference in this area of heat output. The porosity of pure copper was higher compared with PR-BrH in the areas with low specific energy values. A relatively large particle diameter of pure bronze powder has a definite effect on the growth of porosity. However, in both types of powders, a different character of the dependence of porosity on the specific energy was observed.
Having considered the obtained porosity values of pure copper, we can note the section from 100 to 140 J / mm3 where the porosity increases (Fig. 10a), then there is a gradual decrease in the porosity values with an increase in the specific energy (Fig. 10 b). The decrease in porosity is associated with an increase in specific energy. The reason lies in the fact that larger energy input allows for melting of larger powder volume and increases the lifetime of the melt bath at a fixed area, which allows the liquid metal to fill the pores.
The values of porosity of samples from PR-BrH at th area from 100 to 160 J / mm3 decrease, which is consistent with previously made assumptions. Porosity reduction at specific energy values of 200 J / mm3 can be associated with the achievement of values of the zinc boiling temperature (tZn = 907 °C), which fumes disrupt the continuity of the liquid bath, creating voids during cooling (Fig. 11a). A further increase in the specific energy allows to increase the lifetime of the liquid bath, which fills the voids formed by zinc fumes. To a level of specific energy of 320 J / mm3, a decrease in porosity is observed (Fig. 11 b). Exceeding this specific energy value leads to overheating of the material and the impossibility of growing the samples. The number of pores on these samples is growing, which can be associated with overheating of copper and chromium (Fig. 11c). Since there is no overheating in pure copper in this area, and the porosity decreases, it is necessary to work in the heat input energy range of 500–700 J / mm3 when creating samples using the SLM method from pure copper powder. When growing products from heat-resistant PR-BrH copper, the risks of evaporation of a part of alloying elements and the chemical composition of the product should be taken into account. To prevent such processes, it is necessary to limit the range of operation to specific energy values of 150–180 J / mm3. The results obtained give all grounds for concluding that installations with a 100 W laser source can ensure the creation of products with low porosity (less than 5%). This is an acceptable result for the products obtained during additive growth. The required power densities and heat input levels are achieved by using a small diameter of the laser spot in focus and additional heating of the treatment area.
The authors are grateful to the Russian Foundation for Basic Research, the financial support of which helped to carry out the study as a part of the research project of the Russian Foundation for Basic Research (RFBR) No. 18-38-00940.