Linear Friction Welding of an AZ91 Magnesium Alloy and the Effect of Ca Additions on the Weld Characteristics

Abstract

Solid-state welding offers distinct advantages for joining reactive materials, such as magnesium (Mg) and its alloys. This study investigates the effect of linear friction welding (LFW) on the microstructure and mechanical properties of cast AZ91 (Mg–9Al–1Zn) and AZ91–2Ca alloys, which (to the best knowledge of the authors) has not been reported in the literature. Using the same set of LFW process parameters, similar alloy joints—namely, AZ91/AZ91 and AZ91–2Ca/AZ91–2Ca—were manufactured and found to exhibit integral bonding at the interface without defects, such as porosity, inclusions, and/or cracking. Microstructural examination of the AZ91/AZ91 joint revealed dissolution of the Al-rich second phase in the weld zone, while the Mn containing phases remained and were refined. In the AZ91–2Ca/AZ91–2Ca joint, the weld zone retained Ca- and Mn-rich phases, which were also refined due to the LFW process. In both joint types, extensive recrystallization occurred during LFW, as evidenced by the refinement of the grains from ~1000 µm in the base materials to roughly 2–6 µm in the weld zone. These microstructural changes in the AZ91/AZ91 and AZ91–2Ca/AZ91–2Ca joints increased the hardness in the weld zone by 32%. The use of digital image correlation for strain mapping along the sample gage length during tensile testing revealed that the local strains were about 50% lower in the weld zone relative to the AZ91 and AZ91–2Ca base materials. This points to the higher strength of the weld zone in the AZ91/AZ91 and AZ91–2Ca/AZ91–2Ca joints due to the fine grain size, second phase refinement, and strong basal texture. Final fracture during tensile loading of both joints occurred in the base materials.

Corrosion of Linear-Friction-Welded AZ91 and AZX912 Mg-Al Alloys

“…[24][25][26][27] The corrosion behavior of the LFWed Mg alloy joint was characterized by mass loss measurements and several electrochemical techniques, such as potentiodynamic polarization, galvanic current/potential analysis, and the scanning vibrating electrode technique (SVET). The microstructures and mechanical properties of the weldments, which were previously reported by our team, 8,9 are invoked to reinforce the experimental results described herein.…”

“…It appears that the WZ is wider in the AZ91/AZ91 and AZ91/AZX912 welds than in the AZX912/AZX912 and this may be related to the effect of Ca on the thermal conductivity 29 and high temperature strength 30 which in turn affect the heat flow distance and deformation zone. 8 Mass loss analysis of LFWed Mg-Al alloys.-The Mg-Al alloy welds and their surface profiles after 24 h of immersion in the test solution are shown in Fig. 4 with the WZs clearly visible for all three joint combinations (AZ91/AZ91, AZX912/AZX912 and AZ91/ AZX912).…”

“…The AZX912 alloy was synthesized with master alloys and pure elements using an in-house melting furnace at McGill University (Montreal, QC, Canada) as described previously. 8 The chemical compositions of these alloys are listed in Table I.…”

“…The details of the LFW protocols have been reported elsewhere. 6,8 Figure 1 shows images of a LFWed AZ91/AZX912 sample. The as-welded joint (Fig.…”

“…This process consists of joining two surfaces through the frictional heat generated by the displacement of one workpiece against the other. 6 The existing literature on the application of LFW for Mg alloys [7][8][9][10] focuses predominantly on the effects of the process parameters and the resultant microstructures.…”