Laser Powder Bed Fusion of AISI H13 Tool Steel for Tooling Applications in Automotive Industry

Abstract

Owing to good combination of high hot hardness, wear resistance, toughness, and fairly low material cost in comparison to its counterparts, AISI H13 tool steel is commonly used in the manufacture of tooling and dies in different industries. Due to the layer-wise nature of the additive manufacturing (AM) techniques such as Laser Powder Bed Fusion (LPBF), they offer substantial geometric design freedom in comparison with conventional subtractive manufacturing methods, thereby enabling construction of complex near-net shape parts with internal cavities like conformal cooling channels. In the context of AM, processing H13 tool steel is quite challenging, which in turn hurdles its implementation in industrial practices to manufacture reliable tools and dies. Because of the presence of martensite phase (as a brittle phase) and the considerable thermal stresses during the process, H13 tool steel has a high propensity to developing cracks in the final parts. Porosity, and surface quality are other major issues that must be appropriately dealt with. On the other hand, the presence of the residual stresses in the final part begs the question whether the costly stress-relief post-processing is mandatory. After meeting these challenges to get a defect-free parts, it would be of great interest to know if it is possible to utilize the capabilities of AM to design functional materials to enhance wear resistance of the monolithic H13. The following is the synopsis of the obtained results in regards to the abovementioned discussion. First, the relative density, surface roughness, crack formation, microstructure, and hardness were evaluated. The relative density is shown to increase with increasing the volumetric energy density and then no significant increase could be pointed out; the maximum relative density of 99.7% was obtained. A preheating of 200°C generally aided to increase the relative density and eliminated the crack formation in the final parts. The microstructure of as-built samples showed a fine cellular-dendritic structure composed of martensite as the predominant phase, and some retained austenite. The microhardness of the as-built samples was found to vary from 650 to 689 HV 0.2, which is superior to a conventionally produced H13 tool steel. Second, residual stresses, as the other important defect developed during the laser powder bed fusion processing of H13 tool steel was thoroughly examined via experiment and part-level simulations. Twin-cantilever beams and cubic coupons were printed in a wide range of process parameters to characterize the nature of residual stresses and contributing factors under preheated and non-preheated conditions. The residual stresses were assessed using X-ray diffraction method (XRD) as a direct way of getting the residual stress figures at distinct spots and along the depth of cubic coupons. Moreover, the level of beam deflections, as an indication of the magnitude of residual stresses, were both measured and simulated to gain an insight into the accuracy of the part-level simulation. The XRD and beam deflection measurement results revealed the significant role of martensitic phase transformation, process parameters, densification level, and preheating condition on the final residual stress regime and consequently beam deflections. Neglecting the martensitic phase transformation and defects in the deflection modelling introduced a discrepancy between the predictions and experimental measurements. The knowledge gained from the microstructure and residual stress characterizations unraveled that 200°C of preheating eliminates not only the cracks but also the need for stress-relief post-processing. At last, the feasibility of fabricating defect-free functionally graded bi-materials (FGMs) with enhanced wear resistance via incorporation of vanadium carbide (VC) into H13 tool steel was investigated. Three distinct composite powders containing 1, 3, and 5wt.%VC were prepared through ball-milling and subjected to laser powder bed fusion (LPBF) process to print different composites on top of monolithic H13 in a wide range of process parameters. Almost fully-dense parts were achieved (maximum of 99.8, 99.8, and 99.5% for 1, 3 and 5wt.%VC composite systems, respectively); however, the increase in VC content narrowed down the processability window range from 60 J/mm3 for 1, and 3wt.%VC systems to 30 J/mm3 for 5wt.%VC system. The mechanical properties of optimum samples were characterized through microhardness, nanohardness, and wear tests. The incorporation of VC significantly improved the mechanical properties, 17-40% in microhardness, 10-40% in nanohardness, and 20-53% in wear resistance. The underlying reasons behind such an improvement were correlated to the dissolution of VC during the heating stage of the LPBF process and the formation of (V+C)-supersaturated solid solution in large extents as a result of extremely high cooling rates. This study introduces LPBF-processed FGMs as promising candidates for applications in which wear resistance is paramount.