Date of Award

10-17-2019

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Additive Manufacturing, Laser cladding, Martensitic stainless steel, Mechanical Behaviour, Microstructural evolution

Supervisor

Ruth Jill Urbanic

Rights

info:eu-repo/semantics/openAccess

Abstract

Laser cladding is an additive manufacturing (AM) process that uses lasers to melt and deposit metallic powders in layer by layer to coat a substrate or to build three dimensional object. However, the AM industry encounters problems in handling residual stresses in the cladded parts or coating that lead to high hardness and distortion. Also, anisotropic properties developed in the laser-cladded AM parts are a challenge to use them as a functional component. This study aims to understand those problems with the laser-cladding AM process using AISI 420 martensitic stainless steel (MSS) powder in a coaxial direct powder deposition method. Primarily, this study focuses on the effect of process parameters, microstructural evolution, and associated residual stress development in the single bead of laser-cladded 420 MSS. Subsequently, the study was expanded to analyze the mechanical behavior of additive manufactured 3D samples using systematic approaches with X-ray diffraction, scanning and transmission electron microscopy (SEM/TEM), electron backscattered diffraction (EBSD) and MTS mechanical testing frame. This study revealed that laser speed has the most significant effect on the microhardness, while the powder feed rate has the most significant effect on the bead geometry. A detailed TEM study discovered various morphologies of martensitic phases that explained the reason behind the development of residual stress throughout the three zones, such as bead zone (BZ), dilution zone (DZ), and heat affected zone (HAZ) in a single bead clad. A high profile tensile residual stress (310–486 MPa) was observed in the upper BZ, while compressive stress (420–1000 MPa) was seen in the rest of the BZ and the DZ. This laser-cladded stainless steel vi showed a ~16% increase in yield strength (YS ~ 521 MPa), ~ 63% increase in tensile strength (TS ~ 1774 MPa), and a ~ 22% increase in ductility in terms of percentage of area reduction when compared with a similar 420 commercial grade MSS (YS - 483 MPa, TS - 1087 MPa), in the rolling direction with pre-hardened condition. The study showed that a post-cladding heat treatment at 565 °C for an hour reduced the tensile residual stress substantially in a single bead clad. A similar heat treatment also improved the fracture mode of 3D AM sample from brittle to ductile fracture and changed the anisotropic properties of the as-cladded sample in the transverse direction. This indicated that for design purposes, a simple post-cladding heat treatment (at 565 °C for an hour) is very important to minimize the anisotropy in the mechanical properties of as-cladded transverse sample. Also, it showed that a parts building technique with 30° angle to the base improved the ultimate tensile strength and partially eliminated the directionality issue. These findings could be important information for the designers with respect to “design for AM strategies.” It is expected that the above findings will be useful for the laser-based additive manufacturing application of AISI 420 martensitic stainless steel in designing functional components. However, the ratio of the yield strength vs. tensile strength of as-cladded AM sample needs to be improved to use this AM alloy in potential automotive applications.

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