A Study on Dissimilar Metal Functionally Graded Material via Multi-material Selective Laser Melting

Abstract

Functionally Graded Materials (FGMs) are composite materials with a deliberate, engineered variation in composition and properties, enabling seamless transitions of characteristics. This unique property enables a smooth shift from one material to another, facilitating the tailored design of desired properties and compositions. In contemporary manufacturing, additive technologies like Selective Laser Melting (SLM) have gained prominence for fabricating FGM. In the realm of multi-material SLM, the distinct material properties often lead to defects at material interfaces. However, the concept of FGM mitigates this concern by sidestepping the obvious interfaces through gradual structural and compositional transitions. This study aims to harness these advantages to fabricate defect-free dissimilar metal FGM samples through multi-material SLM technique. It also seeks to comprehensively explore aspects like material design, material characterization, and mechanical properties within these samples. The study firstly focuses on fabricating vertical dissimilar metal FGM by employing Invar, Pure Copper, and Ti6Al4V powders using a multi-material SLM system. The presented work endeavours to establish a transition from Invar to Ti6Al4V through innovative interlayer strategies. Microstructural evolution, elemental distribution, phase composition, and mechanical characteristics are rigorously examined through various analytical techniques. Findings from the study illuminate key insights into this complex process. A gradient zone formation between regions formed by pre-prepared powders has a fluctuating composition, necessitating meticulous optimisation of laser processing parameters for this area. The presence of various intermetallic compounds—FeNi, CuNi, FeCu, TiFe, TiNi, TiCu, Ti2Cu, and Ti2Cu3—has profound implications for microstructure and mechanical performance. The emergence of cracks between the interlayer gradient zone and the Invar regions was attributed to the coexistence of FeNi and CuNi intermetallic phases, characterised by discrepant thermal expansion coefficients and elastic modulus. Lack of Fusion defects were observed at the position of pure Cu interlayers, attributed to poor wettability of Cu-Fe system and discrepancies between laser parameters and the local material composition. This situation can be improved by introducing mixed powder. In addition to its primary focus on vertical FGM, the study also highlights a distinct exploration of horizontal FGM—a less explored domain. The study also explores the creation of a horizontal FGM transitioning from Ti6Al4V to Cu10Sn, utilizing a multi-materials SLM system. Computational calculation and experimental characterization shed light on the microstructure, composition, phases, and microhardness across the gradient. The experimental findings showcase that the composition transition within the FGM sample adheres to a linear-staged path design, effectively progressing from one end to the other. The composition of the gradient zone formed at the junction area of the pre-prepared powders resides between those of the adjacent powders, contributing to a smoother compositional change, though with fluctuations. Thermodynamic calculations aptly predicted all main intermetallic compounds (Ti2Cu, TiCu, Ti2Cu3, Ti2Sn, Ti3Sn, and Cu41Sn11) identified in the experimental characterization of the FGM samples. However, the specific content and distribution of each compound deviated from the experimental results. This divergence can be attributed to the inherent heterogeneity of the multi-material SLM process itself, coupled with the complex nature of the thermal cycles involved. Notably, distinct phase formation within various regions led to substantial variations in element distribution and microhardness. Cracks were detected in the region boasting the highest hardness. This occurrence arises from the coexistence of TiCu, Ti2Cu and Ti2Cu3, which, present in comparable quantities, exhibit

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andrew Thomas (Supervisor) & Lin Li (Supervisor)