Increasing the strength of an automobile frame enables a reduction in weight and an increase in passenger protection. Steels that produce the filler must have a high strength. Recently, additive manufacturing (AM) is gaining momentum in diverse industrial applications. Among AM technologies, directed energy deposition (DED) technology is capable of achieving 3-dimensional molding to obtain diverse shapes and local strengthening of metallic surfaces; hence, this technology is receiving significant attention. The DED technology can be used for hardfacing tool steel surfaces and to perform repairs because it exhibits a superior refined microstructure and strong fusion bonding between the substrate and the metal powder. Hashemi et al. [1
] reported an improvement in the wear resistance and tool life by laser cladding a thin layer of high-speed steel (HSS) powder on the target materials. However, despite its superior performance, HSS is extremely susceptible to cracking during laser cladding when there is a high amount of carbon [2
]. Fallah et al. [3
] studied the effect of localized surface preheating on the microstructure of the laser-direct-deposited samples as well as crack formation in the samples. High-speed tool steel M4 was useful for effectively improving the wear resistance through preheating with no deposition defects such as cracks and pores [4
]. Nevertheless, the M4 deposited layer showed poor mechanical properties such as low toughness and vulnerability to cracks despite its high hardness [5
]. Furthermore, although the thermal stress was reduced, residual stress remains after cooling the metal to room temperature in the deposition process. Post-heat treatment is a conventional method of combining heating and cooling operations that is used to improve the mechanical properties, such as the hardness, wear resistance, and impact resistance, of metals or alloys. Sola et al. [6
] assessed the influence of the heat treatment on the microstructural and mechanical properties of high-speed tool steel (AISI M2). Through post-heat treatment, recrystallization of metal structures, the diffusion of atoms and phase transformation can occur, and the residual stress is relieved. Post-heat treatment alters the microstructures of the deposited layer that are produced via the direct energy deposition (DED) process, and it changes its mechanical properties. However, most studies on post-heat-treatment processes have focused on bulk or welded materials. As the microstructures of the deposited layer produced via the DED process generally form fine dendritic tissues, differences in the properties of the metals that are used for welding with a bulk material are expected. Therefore, the effects of the post-heat-treatment process on the microstructure and the mechanical properties of an M4 deposited layer produced via the DED process should be studied.
Shim et al. [7
] determined that the post-deposition heat treatment significantly influences the mechanical and metallurgical characteristics of M4. In the experiment, it shows post-deposition quenching and tempering, which led to a reduction in the hardness. This was attributed to the relieved residual stress, the tempered martensite, and the removal of carbon from the martensite during tempering. Wang et al. [8
] compared the abrasive wear behavior of a variety of tool steel coatings (CPM 9V, CPM 10, and CPM 15V) in the laser cladding process. They reported that when large amounts of residual austenite exist in the deposited layer, the mechanical properties can be improved via transformation into martensite, which is promoted by the post-heat treatment. Telasang et al. [9
] improved the mechanical properties of the H13 powders through laser cladding and the post-cladding heat treatment. In addition, Telasang et al. investigated the cause through the observation of changes in the microstructural properties and the residual stress. They reported that the mechanical properties changed according to the volume fraction of carbide as the martensite or the residual austenite phase-transformed to pearlite/bainite through the post-cladding heat treatment. Park et al. [10
] examined the effect of the heat treatment on the properties of the tool steels (H13 and D2) that are deposited via the DED process. According to their study, the hardness of the deposited H13 decreased after the heat treatment, but the hardness of the deposited D2 increased after the heat treatment. Moreover, the microstructure of the deposited D2 steel after the heat treatment consisted of fine carbides of tempered martensite; the microstructures of the bulk material D2 and the deposited layer showed differences. Sun et al. [11
] used laser cladding for repairing the damaged part in accordance with AISI 4340 and they applied the post-heat-treatment process to maintain or improve the fatigue characteristics, which improved the tensile property and the fatigue life. In addition, Lourenço et al. [12
] deposited the AerMet®
100 powder on a substrate of the same material and observed the fatigue life after post-heat treatment, which was not effective for improving the fatigue life after deposition. Jo et al. [13
] reported that this method, based on multi-layer cladding with a combination of functional metal powders, increased the tool steel life.
Most of the abovementioned studies used laser cladding and they aimed to improve the mechanical properties (e.g., hardness, wear resistance, fatigue life) through the post-heat-treatment process. Furthermore, by treating the laser melting state as hardened tissue, they focused on the changes in the microstructure of the tool steel that is post-treated with tempering only. Thus far, little research has been conducted on manufacturing M4 powder multi-layers via laser deposition and applying them to the post-heat-treatment process. Almost no studies have focused on the effect of quenching processes, including tempering the M4 tool steel that is deposited on a conventional die steel (D2). Moreover, tool steels that are used in high-hardness cold presses require a high toughness in addition to a high hardness and wear resistance, and the durability (i.e., hardness, wear behavior, and toughness) of the tool steels deposited through DED should be evaluated; however, no study in this regard has been conducted.
Therefore, our study focused on the mechanical properties of the tool steel required for the hardfacing of cold press dies (e.g., a press die of UHSS). This study also investigated the changes in the mechanical properties that are caused by different heat treatment conditions that are applied after depositing the M4 powders using DED technology. Hence, AISI M4 powders were deposited on an AISI D2 substrate through preheating, and a specimen for evaluating the impact toughness was fabricated through the post-heat treatment. The toughness changes that were incorporated via post-heat treatment were evaluated using the Charpy impact test, and the hardness and the microstructure of the deposited layer were also examined.