Effects of Pressure on Microstructure and Residual Stresses during Hot Isostatic Pressing Post Treatment of AISI M50 Produced by Laser Powder-Bed Fusion

Siyuan Qin,Anke Kaletsch,Christoph Broeckmann,Simone Herzog

doi:10.3390/met11040596

Abstract

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.

Highlights

Additive manufacturing (AM), known as 3D-printing, describes technologies to fabricate three dimensional parts directly from computer-aided designed models [1].Laser powder-bed fusion (LPBF) is a layer-wise AM technology that utilizes a high-power laser to consolidate metallic powders [2]
The effect of pressure during Hot isostatic pressing (HIP) post treatment with integrated quenchHigh pressure of the HIP process retarded recrystallization and grain growth during ing on the microstructure evolution and residual stresses of high alloyed steel AISI M50 fabaustenitization, causing fine parent austenite grains, which led to small martensitic ricated by LPBF was analyzed
Cellular network-shaped carbon segregation tended to transform tensile stresses to compressive stresses at higher preheating temperareas could be observed in both as-built conditions

Summary

Introduction

Laser powder-bed fusion (LPBF) is a layer-wise AM technology that utilizes a high-power laser to consolidate metallic powders [2]. This process has the potential to fabricate tools or dies due to its capability of producing parts with complex geometry, for instance, internal cooling channel [3]. The characteristics of the LPBF thermal profile are rapid heating and cooling. Most of the lower layers experience reheating and remelting. Residual stress from this distinctive thermal profile leads to geometrical distortion and can negatively affect mechanical properties [4,5]. The quenching process generates residual stresses due to the temperature difference in the component leading to location- and time-dependent phase transformation [8]

Results

Discussion

Conclusion