Effects of Heat Treatment Temperature on Microstructure and Mechanical Properties of M2 High-Speed Steel Selective Laser Melting Samples

At different heat treatment temperatures, the hardness and flexural strength of M2 high-speed steel selective laser melting (SLM) parts show mixed trends. When the heat treatment temperature is 260°C, the hardness and flexural strength of the M2 high-speed steel SLM part are decreased, but the hardness difference between the upper and lower surfaces of the M2 high-speed steel SLM part is also reduced. When the heat treatment temperature is 560°C, the hardness and flexural strength of the M2 high-speed steel SLM part are almost close to that of the original M2 high-speed steel SLM part, and the performance gradient in the sample is improved, and the overall structure is uniform. When the subsequent heat treatment temperature is 860 °C, the hardness of the SLM parts reaches a minimum, with an average value of 24 HRC. However, the flexural strength exceeds that of the original SLM parts. Moreover, the microstructure of the sample is uniform, which significantly improves the anisotropy of performance.


Introduction
Additive manufacturing (AM) has become the most popular manufacturing method in recent years. We can expect that AM technology will play an important role in manufacturing complex shapes in the future. Besides, many scientific publications in recent years have emphasized the primary importance of AM technology in this century. In addition, AM technology is fundamentally different from traditional manufacture or subtractive manufacturing processes. One of the most significant differences is that the AM technology does not require the use of molds.
Traditional subtractive manufacturing processes always need molds to form parts. In this regard, it cannot directly create complex parts and relies on post-processing-this type of manufacturing wastes too much materials. In contrast, additive manufacturing without any mold is required. It can directly form parts of any shape. However, many studies have shown insufficient results. Due to the high content of carbon and other alloying elements (high cold-crack tendency), the HSS materials are considered difficult to process by SLM. Besides, the HSS materials exhibit complex processability due to phase transformation, such as martensitic transformation and precipitation of solid carbides. It is associated with the change of a specific volume during the phase transformation. It may also cause additional stresses, thereby supporting the formation and propagation of cracks and the distortion of the specimen [16]. Through the author's previous work [17], i.e., the studies of the basic microstructure and mechanical properties of M2 HSS based on SLM technology, it shows that with the increase of the substrate temperature, the microstructure of HSS becomes more uniform, the columnar crystals grow from single direction to multi-direction, the carbide content increases, and the solid solution of the alloy elements increases. All these results indicate that heat treatment should be performed on the as-printed parts to make them homogeneous. Therefore, the primary purpose of this work is to investigate the effects of heat treatment on the microstructure and mechanical properties of M2 HSS as-print parts.

Materials and Method
In this study, the AM samples were prepared by the FS-271M system (Farsoon Tech, China).
The SLM machine is equipped with a 500 W Gaussian beam fiber laser. The laser beam is 80 μm in diameter. M2 HSS powder was supplied by AMC Powders (China). The composition of the powder is shown in Table 1, the bulk density of the powder is 4.16g/cm 3 , and the morphology and particle size distribution of the gas atomized powder is shown in Fig.1. Table 1 Nominal chemical composition of gas atomized HSS powder The powder was sieved with a 250-mesh sieve to remove large powder particles and ensure that the particle size was less than 58 µm. After sieving and drying, the powder was loaded into a powder drum of the SLM machine. The printing substrate was sandblasted to increase roughness to prevent the bottom powder from sliding during printing. After the substrate temperature raised to the set temperature, the leveling process was carried out. Finally, the SLM experiment was

Microstructure
SLM parts under different heat treatment conditions were ground and polished to make the surface smooth and bright. Then, the random areas on the surfaces of the SLM parts (including asbuilt SLM parts and the SLM parts with different heat treatment temperatures) were observed under SEM and electron microscope, as shown in Figure 2. As shown in Figure 2 (a), the surfaces of the as-built SLM parts are almost uniformly distributed with reticular carbides. From previous studies, we can define that this special structure is mainly alloyed carbides, but the content of the alloying elements on the carbides is low. After heat treatment at 260℃, the continuous network of carbides on the surface was cracked and distributed unevenly. The grain size also increased slightly.
After heat treatment at 560℃, the surface morphology of the SLM parts was restored to the surface morphology of the as-built SLM parts, showing a network structure again. Although the grains in the alloy returned to equiaxed grains after the heat treatment at 560℃, the grain size was larger than those in the original SLM parts after the heat treatment. After the SLM sample was heattreated at 860℃, a large number of granular carbides were precipitated on the surfaces of SLM parts. And these spherical particles were unevenly distributed on the surfaces, which proves the law of colloidal equilibrium, i.e., the metallic matrix dissolves small carbides particles and large carbides particles will grow. To further confirm the analysis results, an EDS energy spectrum analysis was carried out for the above-mentioned carbides. The results are shown in Table 3.  Generally, we know that rapid cooling promotes the transformation of austenite to martensite in HSS [11]. During the SLM process, the cooling rate is very fast. In this regard, there are lots of  showed only three peaks of the α-Fe phase. At the same time, alloy carbides precipitated again during the cooling process; it precipitated again at the grain boundaries and presented a network structure in the matrix. When the SLM formed parts were heat-treated at 860℃, the amount of free carbon in the matrix increased, and it is easy to combine with alloy elements to form carbides.
According to the XRD pattern, the carbides formed in the matrix are mainly Fe3W3C carbides.
These carbide aggregates grow and nodule according to the colloidal equilibrium law of small particle dissolution and the large particle growth. The carbides are unevenly distributed on the surface of the matrix, which weakens the strengthening effect of the carbides on the matrix and reduces the hardness of the SLM parts but improves the toughness of the part.    Table 5 shows the flexural strength of the SLM parts at different heat treatment temperatures.
The average flexural strength of the as-built parts is 2417.33 MPa, which is higher than that of the traditional powder metallurgy M2 HSS (2100 MPa) [19]. However, the elongation of the SLM parts is low, and the fracture form is a mainly brittle fracture. After heat treatment, the tendency of the flexural strength of SLM parts is similar to that of the hardness change. After the heat treatment at 260℃, the flexural strength of the sample decreased, and the elongation increased. After heat treatment at 560℃, the flexural strength of the sample is the same as that of the as-built part.
However, the elongation of the 560℃ specimen is higher than that of the as-built sample. For the 860℃ samples, the flexural strength of the part exceeded that of the as-built sample, and the elongation reached its peak. The as-built parts have a high flexural strength but a low elongation. It shows obvious brittle fracture characteristics. The fracture characteristics of the SLM parts changed after heat treatment at different temperatures. Fig. 4 shows the SEM fracture morphology of the SLM parts before and after heat treatment at different temperatures.
Flexural strength is the ability of a material to resist bending without breaking under external forces. This mechanical property is related to the grain size, the morphology and distribution of carbides, and the preferred texture in the microstructure. It can be found from the fracture morphology diagram shown in Fig. 4  columnar crystal structure were significantly reduced. After heat treatment at 560℃, no apparent columnar crystal structure was found in the fracture structure, but there was still a small amount of cleavage steps on the surface, and the fracture form was a brittle fracture. For the 860℃ samples, the flexural strength of the SLM parts also reached and exceeded that of the original SLM parts.
At this point, a large number of dimples were observed in the cracks, but the size and distribution of the dimples were uneven.

Conclusions
In this paper, the effects of heat treatment temperature on the density and properties of the M2 HSS parts by the SLM process were studied. The conclusions are summarized as follows: • At different heat treatment temperatures, the hardness and flexural strength of the SLM parts show mixed trends. When the subsequent heat treatment temperatures were 260℃ and 860℃, respectively, the hardness and flexural strength of the SLM molded parts decreased. At 860℃, the hardness of the SLM molded parts reached the lowest, with an average value of 24HRC, and the microstructure of the sample was uniform. When the subsequent heat treatment temperature was 560℃, the hardness and flexural strength of the SLM molded parts were very close to that of the original SLM molded parts, and the performance anisotropy of the samples was improved, and the overall structure is uniform without apparent preferred direction.
• After heat treatment at 860℃, the fracture mode of the SLM parts is transformed into a ductile-brittle composite fracture. Although the hardness is greatly decreased, the flexural strength is significantly increased. Subsequent heat treatment can improve the toughness performance of the SLM parts to a certain extent, but the heat treatment temperature is required to be higher than 860℃.