Date of Award

Summer 2025

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Lang Yuan

Abstract

AA6061 is a widely used aluminum alloy known for its excellent thermal conductivity, strength, and corrosion resistance. However, its application in Laser Powder Bed Fusion (LPBF) additive manufacturing is restricted by solidification cracking and porosity. This study examines defect formation, microstructural evolution, and precipitate behavior in AA6061 processed under various LPBF conditions, including room temperature and heated substrates (500°C), as well as pulsed wave laser configurations. Microstructural characterization was conducted using optical microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM).

A maximum relative density of 99.17% was achieved in fabricated cubes with room temperature substrate, which is the highest reported in literature. This study confirmed that solidification cracks propagated along grain boundaries and were mostly influenced by laser speed. A new type of pore was revealed under high energy density ( >150 J/mm3) conditions, owing to vaporization and denudation effects. Detailed TEM analysis revealed that nano-sized β (AlFeSi) precipitates form in all as-build process conditions. T6 heat treatment promotes uniform precipitate distribution with the presence of coherent Mg₂Si, and phase transformation of β to α (Al(Fe,Mn,Cr)Si), leading to a significant (>102%) increase in yield strength. With 500°C of substrate, crack formation is significantly suppressed with area fractions to below 0.7% from all the process conditions explored. Coarser Mg₂Si precipitates formed at both low and high power, with α-phase precipitates suggesting in situ heat treatment. T6 treatment dissolved incoherent Mg₂Si, forming coherent β" precipitates and increasing yield strength by 400%. Furthermore, to facilitate the quantification of process parameters, bead-on-plate experiments, guided by numerical simulations, were performed to correlate the printability of cubic samples. Increased hatch spacing reduced porosity but intensified cracking under high energy densities (>105 J/mm3), whereas low energy density conditions exhibited the opposite trend. A novel exploration of a pulsed wave laser revealed new mechanisms for engineering microstructures, including both grain size and subgrain microstructure, in LPBF. The presence of multiple melt pool boundaries under a low-duty cycle correlates directly with the degree of grain refinement achieved, which further facilitates uniform subgrain cellular cell size and segregation across the melt pool. By implementing a high frequency and duty cycle, the pulsed laser can exhibit behavior akin to the continuous laser with even higher energy input, yielding grain coarsening.

This study provides detailed characterization and comprehensive understanding of the microstructure and defect formation in AA6061 in LPBF. It explores viable strategies, including heated substrate and pulsed laser, to tailor microstructure and mitigate solidification cracking without altering alloy chemistry, advancing additive manufacturing processes to achieve desired material properties.

Rights

© 2025, Sivaji Karna

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