Growing demand for weight reduction in numerous industrial fields has significantly expanded magnesium alloy applications in recent years. Unfortunately, the use of magnesium wrought products is still limited, mainly because of their intrinsic disadvantages, such as low formability and strength, poor corrosion resistance, and high cost. This is why magnesium/aluminum (Mg/Al) alloy composites are highly desirable lightweight structural materials that come with the benefits of both Al and Mg alloys, which find numerous applications in the transportation and aerospace industries.
Cast bonding and vacuum diffusion bonding techniques can be used to manufacture Al clad Mg products. However, the methods take a long time and are difficult to develop a diffusion layer around Al/Mg interfaces. Therefore, several solid bonding techniques have been developed to manufacture Mg/Al composites through severe plastic deformation techniques, including accumulated roll bonding (ARB), equal channel angular pressing (ECAP), and co-extrusion. Although previous studies found that these techniques could obtain Al/Mg alloy composites with excellent interfacial bonding, their high-temperature preheating requirement and complicated processes seriously compromised the composites’ mechanical properties due to intermetallic compounds’ formation at interfaces. Therefore, there is an urgent need to develop a new technique to manufacture Al/Mg composites without preheating.
Continuous extrusion is a severe plastic deformation method that is short and doesn’t require an external heat source. It has been successfully studied before, but little research effort has been put to analyze its feasibility until now. Researchers Dr. Lili Guo, Jianqiang Wang, and Xinbing Yun from Dalian Jiaotong University, China, and Professor Zhongchun Chen from Tottori University, Japan, developed Al clad Mg composites by this account continuously extruding AZ31 Mg alloy rod and 6063 Al alloy tube with and without preheating the magnesium alloy. The authors investigated the Al/Mg interfacial diffusion, texture, microstructure, and flexural characteristic. Their work is currently published in the journal Materials Science & Engineering A.
6063 Al alloy tubes and AZ31B Mg alloy rod were continuously extruded under two different conditions to understand the effects of preheating of Mg alloy on the mechanical properties and microstructure of the composites. Under condition one 1, there was no preheating of both the Mg rod and Al tube. Under condition 2, the Mg rod was preheated, but the outer Al tube wasn’t. The extruded samples’ crystal orientations were investigated, and three-point bending tests were conducted on the samples. The authors also characterized the microstructure near the Al/Mg interfaces in the extruded samples.
Samples that were not preheated had smoother surfaces indicating that sound extrudates could be obtained without preheating both Al tube and Mg rod. Analyzing the microstructure near the Al/Mg interfaces in both samples revealed no lack-of-fusion or any other defects at the interfaces. Gradual composition transitions of magnesium and aluminum across the interfaces were reported. A thicker diffusion layer was, however, reported in the preheated sample. This was because preheating of the magnesium alloy resulted in improved diffusion rates of magnesium and aluminum atoms.
The authors’ findings show that interdiffusion of atoms occurred during the continuous extrusion process even without preheating either the Al tube or Mg rod. Still, good metallurgical bonding between the magnesium core and aluminum shell was reported. The heat generated between the tools and materials and plastic deformation during the continuous extrusion promoted the diffusion of atoms, making Al and Mg bond well. This is why good interfacial bonding between Mg and Al alloys was obtained in the sample that wasn’t preheated.
Preheating the magnesium rod caused complete dynamic recrystallization (DRX) of both the aluminum shell and magnesium core. Complete dynamic recrystallization, which didn’t occur in the sample not preheated, caused grain refinement in the Mg alloy core during the continuous extrusion, which led to improvement in the bendability of the extruded Al/Mg composite. Extruded samples in the current work showed much higher flexure strength and stress levels. Sample without preheating recorded higher ultimate flexural strength than the preheated sample mainly because of the fine-grained microstructure in both Mg core and Al shell.
The research team successfully fabricated Al/Mg alloy clad composites by continuous extrusion without preheating the Al tube and Mg rod. The findings of this study provide a technique to fabricate Al/Mg clad composites through solid bonding by continuous extrusion at room temperature.
Lili Guo, Jianqiang Wang, Xinbing Yun, and Zhongchun Chen. Fabrication of aluminum-magnesium clad composites by continuous extrusion. Materials Science & Engineering A, issue 802 (2021), 140670.