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Access Type
WSU Access
Date of Award
January 2024
Degree Type
Dissertation
Degree Name
Ph.D.
Department
Mechanical Engineering
First Advisor
Xin Wu
Second Advisor
Yara Almubarak
Abstract
Aluminum (Al), for its excellent strength-to-weight ratio, offers lightweight material replacement in many applications such as automotive, aerospace, and many other industrial applications. In the automotive industry, the demand for Al alloys has increased since the introduction of electric vehicles and the need for more fuel-efficient cars. In addition, the demand for Additive Manufacturing (AM)-specific Al alloys was projected to overtake the demand for Die-Cast and other types of alloys within the next five years. However, in the 2020 annual Aluminum Association report, only 22 Al alloys are registered for AM compared to 560 wrought Al alloys. Therefore, the need for new alloys, developed primarily for AM, has increased. 55% of research on the Al alloys fabricated by the AM process has been limited to binary Al-Si alloy systems due to challenges inherited within the AM process and alloy chemistry. Current traditional alloy design and development cycles require extensive iterations and resources during the initial stage, which are deemed to be inefficient. In addition, most medium and high-strength Al alloys consist of at least two or three elements as the major alloying elements, which adds to the complexity of the alloy design and development cycles. This research employed Laser Metal Deposition (LMD) as a versatile technique for alloy development. A new high-throughput alloy design and development methodology using LMD was presented and used for more efficient full-cycle data collection, feedback, and analysis.The high cooling rate and rapid solidification during the AM process produce a non-homogeneous microstructure that is substantially different from the equilibrium v microstructure. Therefore, binary alloy systems are more suitable for studying the effect of the AM process while changing the composition. A new fundamental understanding of the effect of rapid solidification on the non-equilibrium phase transformation was revealed for hypo-eutectic binary Al-xSi alloys using the newly proposed high-throughput alloy design and development methodology. It was found that the volume fraction of the Al-Si eutectic phase during the non-equilibrium solidification in Al-xSi alloys decreased compared to the calculated equilibrium phase, resulting in a shift of the eutectic point in the phase diagram. The eutectic point in the non-equilibrium Al-Si phase diagram is estimated to be around Al-23wt.%Si compared to Al-12.6 wt.%Si wt.% in the equilibrium phase diagram. In addition, the size and morphology of Si particles observed in the microstructures suggested that the formation of Si particles occurred in two stages during the solidification of the alloy: directly precipitate from the liquid before reaching the eutectic temperature, and Si particles formed from the eutectic reaction and the supersaturated Al matrix. Moreover, from the mechanical properties study of Al-xSi alloys in as-deposited conditions, the contribution of matrix (Al) in the overall ultimate tensile strength of these alloys was found to be 77% (MPa). This knowledge was extended from a binary Al-Si alloy system to a ternary Al-Mg-Si (Cu) alloy system that is widely used in automotive applications. Defect-free Al6000’s (AA6111) alloy was successfully deposited for the first time by the LMD process. In addition, an investigation of two deposition strategies, hatch and circular patterns, was conducted to identify the effect of the cooling rate. The circular pattern offered a more consistent cooling rate that eliminated the cracking issues in Al6000’s series during the vi AM process. A comprehensive comparative study for AA6111 was conducted for AM fabricated samples and the conventional Direct Chill (DC) Casted samples during a multi-stage rolling and heat treatment procedure. The thermodynamic simulation of the phase diagram for AA6111 suggested that the freezing range of the alloy is extended by 100 oC non-equilibrium condition for the AM process compared to the DC cast material. Moreover, from the microstructure evolution study, a secondary intermetallic phase that is rich in Fe and Mn was observed for both AM and DC cast material. This intermetallic phase contributes to the solid solution's strengthening of the AA6111. However, the size and morphology of these particles vary from one material to the other during the rolling and heat treatment procedure. The mechanical properties of AM AA6111 were similar to DC cast in wrought conditions. However, the DC cast material was better by around 10% in yield and ultimate tensile strength due to the variation in the Fe and Mn between DC and AM compositions. This variation led to a lower concentration of intermetallic phases in the AM sample. Overall, the proposed high-throughput alloy design and development methodology using LMD allowed for more insight into the behavior and overall performance of the materials. A new fundamental knowledge of the effect of the AM process was revealed and studied from the manufacturing process and alloy chemistry perspectives.
Recommended Citation
Alrehaili, Husam Mohsen, "Investigation Of Medium And High Strength Aluminum Alloy Via Direct Laser Metal Deposition" (2024). Wayne State University Dissertations. 3987.
https://digitalcommons.wayne.edu/oa_dissertations/3987
