INVESTIGATION OF MEDIUM AND HIGH STRENGTH ALUMINUM ALLOY VIA DIRECT LASER METAL DEPOSITION
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Date
2024-03-06
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Wayne State University
Abstract
Aluminum (Al), for its excellent strength-to-weight ratio, offers lightweight material replacement in many automotive, aerospace, and other industrial applications. The demand for additive manufacturing AM-specific Al alloys is expected to overtake the demand for Die-Cast alloys and different types of alloys within the next five years. However, the processing of medium and high strength Al alloys by additive manufacturing (AM) has been limited due to challenges inherited within the AM process and alloy chemistry. As a result, in the 2020 annual Aluminum Association report, only 22 Al alloys are registered for AM compared to 560 wrought Al alloys.
The conventional alloy design and development cycles require extensive iterations and resources during the initial stage, which are deemed to be inefficient. 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. In addition, this methodology was used to investigate the microstructure evolution during the high cooling rate caused by the rapid solidification during AM.
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 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-24.8 wt. %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 varied based on the location of the microstructures. In the second part of this dissertation, the microstructural evolution, and mechanical properties of an alloy from the ternary Al-Mg-Si (Cu), Al6000’s series, alloy system that is widely used in automotive applications was studied in detail. Two deposition strategies, hatch and circular patterns, were investigated to identify the effect of deposition strategy on crack formation. The cracking in Al6000’s series processed by the LMD process was successfully mitigated for the first time for AA6111 using a circular pattern deposition strategy. This strategy offered a more consistent cooling rate, resulting in less residual stresses accumulating in the material upon solidification. Furthermore, a comprehensive comparative study for AA6111 was conducted for AM fabricated samples and the conventional Direct Chill (DC) Casting component during a multi-stage rolling and heat treatment procedure. From the thermodynamic simulation, the non-equilibrium phase diagram for the AM processed AA6111 suggested that the freezing range of the alloy is extended by 100 oC compared to the equilibrium phase diagram of DC cast material. In addition, a secondary intermetallic phase rich in Fe, Si, and Mn was predicted for both AM and DC cast material, which was confirmed from the observation during the microstructure evolution study. 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 rolled condition. The DC cast material showed around 10% more than AM regarding yield and ultimate tensile strength of 226.7 and 270.6 MPa, respectively. The AM AA6111 yield and ultimate tensile strength were 204.4 and 249.3 MPa, respectively. The discrepancy in the mechanical properties of Am and DC cast AA6111 is believed to be due to the difference in the Fe and Mn concentration 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.
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Keywords
Aluminum Alloys, Additive Manufacturing, Laser Metal Deposition, Phase Diagram