| Hybrid structures of Al/Ti and Al/Steel have great potential for applications in the aerospace and automobile industry, which can take full advantage of the excellent performance of each material. However, welding of dissimilar metals successfully is still a great challenge due to great differences in physical, chemical and metallurgical properties of the base materials used, and the tensile strength of the joint could also be degraded seriously for the large produce of the brittle intermetallic compounds. Thus, it is necessary to control effectively the formation and growth of intermetallic compounds during the welding process.AA6061-T6and Ti-6A1-4V was joined in the lap shear joint configuration by cold metal transfer (CMT) technology using filler metal of AlSi5wire. Two different configurations of Al/Ti lap shear joints with an overlap distance of10mm were adopted, and when the Al sheet was placed on top of the Ti sheet, poor welding appearances with unstable arc and large spatters were produced. In contrast, when the Ti sheet was placed on top of Al sheet, continuous and uniform welded joints could be achieved, and good wettability and spreadability of the weld metal on the surfaces of both sheets were presented. In addition, there was almost no deformation after welding for the latter joint configuration, so the tensile strength and mechanisms of these joints were analyzed. It was found that typical CMT welding-brazing joints were obtained for the Ti/Al dissimilar alloys. Among them, the Al base metal melted because of the lower melting point and merged with the molten Al wire, leading to the formation of the welding part of the joints. Whereas the solid Ti base metal interacted with the weld metal through atomic diffusion, thus, three brazing interfaces were formed between weld metal and Ti base metal. Microstructures of fusion welding zone consisted mainly of a-Al grains and reticulate Al-Si eutectic phases. The intermetallic compounds at the brazing interface were complicated. Owing to different reaction time and temperature at each brazing interface, the thicknesses of various interface layers were different, yet the phases at each interface were same bascially. The microstructural characteristics consist of thin continuous layer close to Ti alloy matrix, and discontinuous serrate shape layer next to the weld metal. Various Ti-Al serious intermetallic compounds were formed at the brazing interface, which were mainly included TiAl3, TiAl and Ti3Al from the weld metal to Ti base metal. In addition, it was found that Si gathering phenomena was produced at the brazing interface layer, which may prevent the growth of brittle Ti-Al intermetallic compounds layer. The deviation distances have great effect on the tensile strength of the joints, and the tensile strength increases with increasing deviation distances from0mm to1.5mm. Two different fracture modes were observed for the joints:one at the welding-brazing interface and the other at Al HAZ, and it is obvious that the latter joints had higher tensile strength than the former joints. The tensile strength of the joints is high up to194N/mm.CMT welding method was adoped to join AA6061-T6and Usibor steel using AlSi5filler metal for the purpose of investigating the effect of various coatings on CMT joining of Al to steel. Pure Zn coating on galvanized boron steel was melted under the arc, and it enhanced the wetting of molten aluminum filler onto the solid steel substrate. Consequently, the welds with smooth appearance and low reinforcement were produced. Based upon a design-of-experiments, the optimum range of welding variables are:a wire feed speed of3m/min, a correction of the arc length of-20%, a deviation distance of2mm and100%argon shielding gas with a flow rate of151/min. A welding-brazing joint was composed of the rich zinc, reaction layer interface, and fusion region. The reaction layer that is mainly composed Fe-Al intermetallics formed and its thickness of the reaction layer is the range of4to8μm. Fe-Zn coating from galvannealed boron steel was barely melted under the arc due to its high melting point, and consequently it barely enhanced the molten aluminum filler to wet the steel substrate. As a result, the welds with poor quality were produced. During CMT welding processes of Al to Al-Si coated boron steel and bare boron steel, poor welding appearances with low tensile strength were produced due to large formation of brittle Fe-Al intermetallics at the interface and welding metal of the joints. |
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