Corrosion Behavior of the S136 Mold Steel Fabricated by Selective Laser Melting
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The selective laser melting (SLM) method has a great potential for fabricating injection mold with complex structure. However, the microstructure and performance of the SLM molds show significantly different from those manufactured by traditional technologies. In this study, the microstructure, hardness and especially corrosion behavior of the samples fabricated by SLM and casting were investigated. The XRD results exhibit that the γ-Fe phase is only obtained in the SLM parts, and the α-Fe peak slightly moves to low diffraction angle compared with casting counterparts. Due to the rapid cooling rate, the SLM samples have fine cellular microstructures while the casting ones have coarse grains with obvious elements segregation. Besides, the SLM samples show anisotropy, hardness of side view and top view are 48.73 and 50.31 HRC respectively, which are 20% higher than that of casting ones. Corrosion results show that the SLM samples have the better anti-corrosion resistance (in a 6% FeCl3 solution for 48 h) but the deeper corrosion pits than casting ones. Finally, the performance of the SLM molds could meet the requirement of injecting production. Moreover, the molds especially present a significant decrease (20%) of cooling time and increases of cooling uniformity due to the customized conformal cooling channels.
KeywordsSelective laser melting S136 mold steel Corrosion behavior Conformal-cooling channels
Injection molds were widely used for mass production of thermoplastic parts with a high production efficiency . The quality of injection production is related to the mold steel materials  and the structure [3, 4]. Particularly, cooling channels in molds greatly affect the productivity, product deformation and die life [5, 6]. Conventional straight-line channels easily cause a heterogeneous heat dissipation and cooling due to the inconsistency between channels and mold cavities. Moreover, the heterogeneous cooling will increase the cooling time , even result in some defects such as part warping and sink marks [2, 8]. By contrast, the emerging conformal cooling channels that conform to mold cavities can bring into a steady and homogeneous heat transfer from the cavity surfaces to the coolant, which improves directly the product quality . However, it is difficult to fabricate molds with conformal cooling channels by traditional manufacturing techniques, such as machining, and electrical discharge machining (EDM).
Selective laser melting (SLM), one of the additive manufacturing (AM) technologies, is able to fabricate complex metal parts with a high density layer by layer from 3D CAD data [10, 11]. Recently, SLM has become one of the most promising AM routes for fabricating metal tools and molds due to its ability to create intricate structure, consequently attracted the attention of both industry and academia . Previous researches have successfully used SLM to manufacture conformal cooling channels for injection molds , forging dies  and die-casting molds . Meantime, many studies investigated the microstructure and mechanical properties of mold steel fabricated by SLM [15, 16, 17, 18, 19, 20, 21, 22]. Zhao et al. [15, 16] developed a high-dense AISI 420 steel by SLM for injection molds. The hardness and the tensile strength reached 50.7 HRC and 1045 MPa respectively, showing a good potential for practical application. Mertens et al.  fabricated an H13 mold steel by SLM using different powder-bed preheating temperatures to improve hardness and tensile properties. The more homogeneous microstructure and better mechanical properties were obtained at the preheating temperature of 400 °C comparable to those of wrought H13 counterparts. A TiC-reinforced H13 steel with enhanced wear resistance was in situ synthesized by SLM , which make it very attractive candidate materials for future tooling applications. Except the hardness, mechanical properties and wear resistance that mentioned above, the corrosion resistance is also one of the most important performances that affect the quality of die and mold. Because the injection molds are generally exposed to the acidic environment due to the decomposition of thermoplastics . Nevertheless, to the best of authors’ knowledge, the studies on the corrosion behavior of the SLM samples for molds are limited.
S136 mold steel modified from AISI 420, shows favorable mechanical properties and outstanding corrosion resistances . It currently becomes one of the most widely used materials for injection molds. However, few studies focus on the SLM S136 steel. Therefore, this study are to characterize the microstructure and hardness of S136 steel fabricated by SLM; to evaluate the corrosion resistance of as-SLM S136 steel through chemical corrosion; and to compare the differences in microstructure, hardness and corrosion resistance of as-SLM and casting S136 alloys.
2 Materials and Methods
2.1 Feedstock Materials
Chemical composition (wt%) of S136 powder
2.2 SLM Machine and Process
XRD measurements were conducted on a XRD-7000S machine (Shimadzu, Japan) with a Cu tube at 40 kV and 30 mA. The diffraction angle of 2θ varied from 30° to 100° with a continuous scan mode at 10°/min. The polishing surfaces of the samples were characterized by a digital optical microscope (OM, VHX-1000C, KEYENCE, Japan). The micro morphologies were observed using a JSM-7600F Field Emission Scanning Electron Microscope (JEOL, Japan). The prepared samples were hot mounted. Then the surfaces of samples were grounded and polished by an automatic polishing machine (Automet300, Buehler, America). A reagent consisting of 5 g FeCl3, 15 mL HCl, and 60 mL H2O was used as corrosive liquid. The Rockwell hardness was measured using a hardness tester (600MRD, Wolpert, America). For each sample, ten points at random positions were measured to obtain the average value of hardness.
2.4 Corrosion Tests
All surfaces of the samples were grounded using SiC papers from 400 to 2000 grits, and cleaned with ethanol liquid. A Mettler Toledo AL204 analytical balance (Switzerland) with the smallest increment of 0.01 mg was used for all weight measurements. Then, the samples were immersed in a 6% FeCl3 solution  at 50 ± 2 °C for 48 h, get out every 2 h in first 12 h and then every 12 h. Subsequently, they were rinsed with distilled water, dipped in acetone, dried in hot air, weighed, and re-immersed . The surfaces of samples after corrosion were characterized with optical microscopy (OM) equipped a digital microscopy (VHX-1000, Keyence, Japan).
3 Results and Discussion
3.1 Phase Analysis
Volume fraction of the detected phase
α-Fe phase (%)
γ-Fe phase (%)
Besides, it can be seen that the bases of the peaks in SLM part are comparatively wider than that in casting. This suggests that the crystal lattice structures of SLM part are experiencing certain level of internal stresses that are thermally induced during the rapid solidification SLM process [29, 30]. Moreover, it can be found that the α-Fe diffraction peak of as-SLM S136 slightly moved left compared with that of casting, as depicted in Figure 3(b). It might be associated with the generation of residual stresses , the existence of retained austenite as well as the martensitic transformation that often occurred in the process of SLM [32, 33].
3.4 Corrosion Behaviors
Different from those on as-SLM samples, many fine corrosion pits are evenly distributed on the surfaces of casting samples (Figure 8(e)). The size of the corrosion pits is just a few microns, and the maximum corrosion depth is only 75.51 μm (Figure 8(f)). However, the number and the density of corrosion pits are much higher than those of as-SLM samples, resulting in a greater degree of corrosion. This kind of difference could be explained by the differences in microstructures of SLM and casting. The corrosion in as-SLM samples mainly initiates at melt track boundaries, having a feature of locality and anisotropic because of the special metallurgical process of SLM. Differently, the corrosion in casting samples mainly starts along the grain boundaries. Therefore, the corrosion pits in casting samples distribute more uniformly because of those uniform equiaxed grains.
3.5 Case Study
Production efficiency of the two kinds of mold inserts
Filling and holding time (s)
Cooling time (s)
Production cycle (s)
The α-Fe phases are obtained both in SLM-processed and casting samples while γ-Fe phase only exists in the SLM-processed S136. Compared to the casting S136, the SLM-processed specimen exhibits very fine grains about 1 μm.
The top view of SLM-processed parts shows the highest hardness of 50.31 HRC, while the side view is 48.73 HRC. The hardness of casting samples is 41.23 HRC, which is obviously less than the SLM-processed parts.
In the 6% FeCl3 solution immersion test, casting samples have a greater mass loss (4.16 ± 0.03 mg/cm2) than the SLM-processed parts (3.47 ± 0.2 mg/cm2), but the surfaces of SLM-processed specimens have a much worse destroy than the castings.
A case of SLM-processed S136 mold with conformal cooling channel is used, which can reduce cycle times and increases cooling uniformity during the injection process.
S-FW and X-TJ were in charge of the whole trial; YZ together with Q-SW and Y-SS wrote the manuscript; C-JH assisted with sampling and laboratory analyses. All authors read and approved the final manuscript.
Shi-Feng Wen, born in 1979, is currently a lecturer at State Key Laboratory of Materials Processing and Die & Mold Technology, Huazhong University of Science and Technology, China. His research interests include additive manufacturing.
Xian-Tai Ji, born in 1992, is currently a master candidate at State Key Laboratory of Materials Processing and Die & Mold Technology, Huazhong University of Science and Technology, China.
Yan Zhou, born in 1987, is currently an associate professor at Faculty of Engineering, China University of Geosciences, China. Her research interests include selective laser melting.
Chang-Jun Han, born in 1991, is currently a doctoral candidate at State Key Laboratory of Materials Processing and Die & Mold Technology, Huazhong University of Science and Technology, China.
Qing-Song Wei, born in 1975, is currently a professor and a PhD candidate supervisor at State Key Laboratory of Materials Processing and Die & Mold Technology, Huazhong University of Science and Technology, China. His research interests include additive manufacturing.
Yu-Sheng Shi, born in 1962, is currently a professor and a PhD candidate supervisor at State Key Laboratory of Materials Processing and Die & Mold Technology, Huazhong University of Science and Technology, China. His research interests include additive manufacturing.
The authors sincerely thanks to Professor Chun-Ze Yan of Huazhong University of Science and Technology for his critical discussion and reading during manuscript preparation.
The authors declare that they have no competing interests.
Supported by National Natural Science Foundation of China (Grant No. 51605176), National Hi-tech R&D Program of China (863 Program, Grant No. 2015AA042501), Hubei Provincial Natural Science Foundation of China (Grant No. 2018CFB502), Guangdong Provincial Technology Major Project of China (Grant No. 2017B090911007), State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (Grant No. P2019-006), and Engineering Research Center of Rock-Soil Drilling & Excavation and Protection, Ministry of Education (Grant No. 201804).
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