Bonding Strength and Grain Size Control of Dissimilar Joints of Al-Mg Alloys with Forge Welding
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MULTIMATERIALS based on aluminum (Al) alloys are being increasingly used in automobiles to reduce their weight.[1,2] However, conventional resistance spot welding, which is used in most automobile production lines, is difficult to apply with Al alloys due to the formation of brittle intermetallic compounds (IMCs) during dissimilar joining and rapid electrode consumption even when joining the same material. Thus, alternative methods to fusion-welding have been investigated, including friction stir welding,[3,4] mechanical fastening,[5] and laser brazing.[6,7] Forge welding, the method developed in this study, targets dissimilar joining. It meets the requirements for transportation equipment in terms of productivity and reliability, can bond press-molded materials within seconds, and is not limited to forged materials. A next-generation spot forge-welding multi-articulated robot with a c-type arm has been envisaged to resolve
HIDEKI YAMAGISHI, MASARU SATO, and SHIGEKI KAKIUCHI are with the Toyama Industrial Technology Research and Development Center, Takaoka, 933-0981, Japan. Contact e-mail: [email protected] Manuscript submitted July 20, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the problems with ordinary resistance spot welding.[8] Furthermore, even a joining process, such as with the non-heat-treatable magnesium (Mg) alloy used in this study, could achieve high strength via dynamic recrystallization and grain refinement. Previous studies [8,9] examined the processing conditions of forge welding of Al and Mg alloys using tensile strength as an indicator. Insertion of a pure titanium (Ti) sheet has been used to inhibit the generation of brittle Al-Mg IMCs, which have almost no solubility in Mg and have a tensile strength that is significantly lower at high temperature, and thereby achieve high-strength bonding.[9] This improved bond strength is attributed to the formation of a high-quality thin (several-nanometers-thick) reaction layer (RL) within the Ti-Mg alloy-bonded interface, which results from the removal of the oxide layer by plastic flow, and the reaction between elemental Al in the Mg-Al alloy and Ti. However, that study used a static hydraulic press and a low slide velocity. Furthermore, the pressure holding time of 20 seconds and cycle time of approximately 1 minute are much too slow for typical automobile production lines. Improved productivity was achieved with high-cycle processing by hammering using an alternating current (AC) servo press, which could be quickly crank-operated, and finely and flexibly controlled by a servo motor.[8] In this case, the pressure holding time was
0.1 seconds and the cycle time was less than 2 seconds. The processing latitude of forge welding using an AC servo press was examined; parameters included the preheat temperature, insert thickness, and pressure holding time. The RL and variations in grain size and hardness of the Al and Mg alloys before and after welding were studied under the optimized conditions, as indicated by the highest tensile strength.
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