Abnormal thermal expansion behaviour and phase transition of laser powder bed fusion maraging steel with different thermal histories during continuous heating

Abstract

Although maraging steel materials have been widely used in the laser powder bed fusion (LPBF) process, there are few reports on the behaviour of thermal expansion, which is an important indicator for materials used in machinery, moulds, aerospace, new material development, etc. The unique process of LPBF causes the difference in performance between LPBFed parts and their traditional counterparts. In this regard, thermal expansion is an important factor for phase transition investigation. In this study, a highly sensitive computer-controlled dilatometer was used to systematically study the thermal expansion behaviour along different orientations during continuous heating and corresponding phase transition. The scanning electron microscope, electron backscatter diffraction and microhardness testing were used to study the fine cellular/columnar structures, martensite lath, textures and formation mechanism of precipitates in the as-built and heat-treated samples. The results show an abnormal thermal expansion behaviour and phase transition during continuous heating for both samples with different building directions. For the as-built and direct ageing-treated OX samples (parallel to the building direction), the first step reversion of martensite to austenite is much stronger than the second step. After solution treatment and solution+ageing treatment, both OX and OZ (perpendicular to the building direction) samples display similar thermal expansion curves and a typical two-step martensite-to-austenite reversion. The increase of the austenite phase during the heating process leads to a quick shrinkage with a negative coefficient of thermal expansion (CTE) in the solution-treated and solution+ageing-treated OX samples. However, a very small shrinkage with a positive CTE appears in the as-built and direct ageing-treated OZ samples. Overall, the as-built and ageing-treated samples show differences in phase transition due to precipitation behaviour.