A novel technology has been developed for the resistance spot welding (RSW) of magnesium to steel producing joint strength as high as 95 pct of that of Mg to a Mg joint. The mechanisms of the dissimilar joining have been investigated using mechanical testing and metallurgical examination employing scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The results show that the mechanisms of joining during RSW of a magnesium alloy to Zn-coated steel involve braze welding, solid-state bonding, and soldering. The joint formation in comparison of RSW of Zn-coated steel with steel, Au-plated Ni, and bare Ni sheets is discussed. A possible expansion of this technology also is suggested.
In this work, resistance spot welding of Mg alloy AZ31 sheets was investigated in as received and acid cleaned surface conditions. As received sheets had higher contact resistance which required lower current thresholds for weld initiation and for four root t nugget size (where t is sheet thickness). However, it also led to both serious expulsion and internal defects. The fracture mode of welds in as received sheets was interfacial failure while that of the acid cleaned specimens shifted from interfacial to nugget pullout and exhibited better strength. The acid cleaned sheets also produced less damage on electrode tip faces.
Resistance spot welding of AZ31 magnesium alloys from different suppliers, AZ31-SA (from supplier A) and AZ31-SB (from supplier B), was studied and compared in this article. The mechanical properties and microstructures have been studied of welds made with a range of welding currents. For both groups of welds, the tension-shear fracture load (F C ) and fracture toughness (K C ) increased with the increase in welding current. The F C and K C of AZ31-SA welds were larger than those of AZ31-SB welds. The fracture surfaces of AZ31-SB welds were relatively flatter than those of AZ31-SA. Microstructural examination via optical microscope demonstrated that almost all weld nuggets comprised two different zones, the columnar dendritic zone (CDZ), which grew epitaxially from the fusion boundary, and the equiaxed dendritic zone (EDZ), which formed in the center of the nugget. The nature and extent of the CDZ seemed to be critical to the strength and toughness of spot welds because of its position adjacent to the inherent external circular crack-like notch of spot welds and the stress concentration in this region. The width and microstructure of the CDZ were different between AZ31-SA and AZ31-SB. The AZ31-SA alloy produced finer and shorter columnar dendrites, whereas the AZ31-SB alloy produced coarser and wider columnar dendrites. The width of the CDZ close to the notch decreased with the increase of current. The CDZ disappeared when the current was higher than a critical value, which was about 24 kA for AZ31-SA and 28 kA for AZ31-SB. The microhardness of the two base materials was the same, but within the CDZ and EDZ, the hardness was greater in AZ31-SA than AZ31-SB welds. It is believed that the different microstructures of spot welds between AZ31-SA and AZ31-SB resulted in different mechanical properties; in particular, K C increased with the welding current because of the improved columnar-to-equiaxed transition.
DC magnetron sputtering was used to deposit titanium (Ti), titanium nitride (TiN), and Ti/TiN multilayer coatings on Ti6Al4V alloy substrates. The multilayer coatings have 1, 4, 10, 16, 32 modulation periods respectively. SEM were used to analyze the surface and cross-sectional micro-features of the coatings. Electrochemical tests were carried out 3.5 wt% NaCl solution at room temperature. The resistance of the coating reaches the maximum value, which is 6 times that of the substrate, 2.4 times that of a single layer of titanium, and 5 times that of titanium nitride. With the increase of the modulation period, the charge transfer resistance of the Ti/TiN multilayer coatings increase firstly and then decreases, and charge transfer resistance reaches the maximum at the period of 4, which is closely related to the surface morphology.
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