Laser powder-bed fusion (LPBF) enables the production of difficult-to-machine materials with near-net shape and complex geometries. Components made of tool steels produced by LPBF, even using high preheating temperature, tend to show residual porosity, cracks, and high residual stresses. Hot isostatic pressing (HIP) is able to densify components and modify their microstructure. Moreover, compared to conventional heat treatment at ambient pressure, rapid cooling within the HIP vessel can alleviate thermal stresses, warping or cracking during quenching. In this study, the effects of isostatic pressure on microstructure evolution and residual stresses are investigated. Samples were produced by LPBF. Partly, they were conventionally heat treated by austenitizing, quenching, and tempering, partly using a HIP-device with an integrated quenching facility. The microstructure was characterized by optical microscopy, scanning electron microscopy employing energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis. The results showed that besides the densification of the material to the porosity of 0.001%, HIP influenced the microstructure evolution by retarding recrystallization during austenitization due to the pressure and led to slight compressive residual stresses around 11 MPa on the surface of components.

Laser powder bed fusion is an attractive manufacturing technology promising novel components for the aircraft, automobile, and medical industries. However, depending on the material, some defects in the parts, especially pores or microcracks, cannot be avoided in the LPBF process. To achieve a part with low defect density, the optimal parameter sets must be determined. Many investigations have focused on how laser speed and laser power influence the melting process and the relative density of as-built parts. In this study, we considered laser and heated powder beds as two energy input sources, represented by volume energy density and preheating temperature, respectively. The interaction of these two energy inputs for the fabrication of AISI H13 was investigated. It was found that high preheating temperatures shifted the optimal parameter sets from the low energy density area to the high energy density area. In addition, high preheating also led to hot cracking, which was confirmed with Scheil solidification simulations.

In this study, the microstructure and mechanical properties of martensitic hot-work tool steel AISI H13 produced by laser powder bed fusion (LPBF) under argon atmosphere in different LPBF process times were analysed in the as-built, hardened and tempered (HT) and hot isostatic pressed, hardened and tempered (HIP+HT) condition and compared to conventionally manufactured reference material. Three parameter sets were selected to adjust an initial defect density of 0.6%, 0.9% and 2.2% and thus process time reductions of 17% and 29%. We observed a change in the defect type from almost argon-free microcracks to argon-filled pores with higher scan velocities. Argon embrittlement was evident also in the results of tensile, notched impact toughness and fatigue tests. After HIP+HT a fatigue strength of 972 MPa was obtained for the set with 0.6% microcracks, but only 775 MPa and 718 MPa were obtained for samples with argon porosity.

Gadolinium-Doped Ceria (GDC) is a prospective material for application in electrochemical devices. Free sintering in air of GDC powder usually requires temperatures in the range of 1400 to 1600 °C and dwell time of several hours. Recently, it was demonstrated that sintering temperature can be significantly decreased, when sintering was performed in reducing atmosphere. Following re-oxidation at elevated temperatures was found to be a helpful measure to avoid sample failure. Sintering temperature and dwell time can be also decreased by use of Spark Plasma Sintering, also known as Field-Assisted Sintering Technique (FAST/SPS). In the present work, we combined for the first time the advantages of FAST/SPS technology and re-oxidation for sintering of GDC parts. However, GDC samples sintered by FAST/SPS were highly sensitive to fragmentation. Therefore, we investigated the factors responsible for this effect. Based on understanding of these factors, a special tool was designed enabling pressureless FAST/SPS sintering in controlled atmosphere. For proof of concept, a commercial GDC powder was sintered in this tool in reducing atmosphere (Ar-2.9%H₂), followed by re-oxidation. The fragmentation of GDC samples was avoided and the number of micro-cracks was reduced to a minimum. Prospects of GDC sintering by FAST/SPS were discussed.

The combination of Additive Manufacturing (AM) and Hot Isostatic Pressing (HIP) offers great potential for industrial applications. Skilfully connecting AM and HIP creates a manufacturing process that unites the advantages of the two approaches. We demonstrate a production route for composite components by combining AM and HIP: a HIP capsule made of corrosion resistant steel is additively manufactured using Powder Bed Fusion with Laser Beam (LPBF). Before LPBF, the distortion and shrinkage of the capsule due to HIP is compensated by optimising the capsule’s geometry using Finite Element (FE) simulation of the HIP process. The capsule is filled with steel powder and manufactured by a conventional powder HIP. After HIP, the capsule remains as an outer layer on the component, so that a functional net-shape composite component is produced. Subsequent investigations show that a complex extruder screw segment can be successfully produced with a geometrical deviation smaller than 1.5%.