Surface modification of AZ91 magnesium alloy using GTAW technology

Abstract

Abstract. In this study, surface remelting treatment of the AZ91 magnesium alloy by means of welding using a non-consumable electrode in an inert gas shield was carried out. Three variants of surface treatment were used, i.e. the single torch variant with a single heat source without cooling down the samples, the single torch variant with a single heat source and a cooling system with liquid nitrogen, and the double welding torch variant with a double heat source in the torches operating in a tandem configuration. Experimental verification of the applied apparatus solutions was based on both macro-and microstructural assessment of the obtained effects. Comparative analysis of the variants used and the obtained microstructural results allowed the authors to indicate the deficiencies and limitations of particular solutions and to single out the best solution that would be useful for modifying the surface layers of magnesium alloys, as well as other materials having a strong oxygen affinity.Key words: remelting treatment, GTAW technology, AZ91 magnesium alloy.Surface modification of AZ91 magnesium alloy using GTAW technology [14][15][16][17][18][19]. To this end, both lasers with continuous operation characteristics and pulse lasers are used. Among the advantages of using the laser beam one can mention is the short time of process execution, the possibility of precise concentration of the beam on a particular surface, as well as obtaining high temperature gradients in the material. The use of methods that are alternative to laser technologies, e.g. conventional welding arc heat sources, is sporadic. Difficulties may occur during the remelting treatment carried out by means of welding methods. Those difficulties are often the effect of the physical and chemical properties of magnesium and magnesium alloys, and particularly their strong oxygen affinity, whose consequence is the presence of a compacted, non-conductive MgO oxide layer on the material surface. Until recently, welding heat sources were used almost only for joining materials and their regeneration. This situation has been systematically changing in recent years. The GTAW method has been used, among others, by the authors of study [20], who subjected the AK7 aluminium alloy to surface treatment. The favourable microstructural changes triggered by remelting of the alloy resulted in an increase in hardness and resistance to abrasion. In turn, in study [21] iron castings were subjected to GTAW method treatment, and equally satisfactory treatment results were obtained. The GTAW method was used also by D. Wenbin et al. in study [22] to form a composite structure in the surface layer of the AZ31 magnesium alloy. Within this study assessment of the GTAW method effectiveness in the remelting treatment of the AZ91 magnesium alloy surface layer was conducted, as well as the comparative analysis of various apparatus and parameter variants.

Microstructure and Corrosion Resistance of AZ91 Magnesium Alloy after Surface Remelting Treatment

“…To obtain satisfactory effects of remelting, it was necessary to use an unconventional solution. The undertaken work resulted in the development of an original two-burner system working in a tandem configuration, where the task of the first welding torch, powered with an alternate current, was to remove the layer of oxides by cathodic cleaning, and the task of the second, connected to the power supply of a one-directional pulsating current, was to remelt the thus cleaned surface [24,25]. The developed solution has made it possible to combine the process of removing oxides from the surface with the remelting process.…”

“…An illustrated model of the two-burner tandem system is shown in Figure 1. Detailed data concerning the developed solution are presented in earlier works [24,25].…”

Archives of Metallurgy and Materials

“…Generally preplaced reinforcements are packed into the substratum by running with a pinless shoulder tool or by using a thin Al foil before the final FSP to originally cover the particles filled grooves or holes to avoid sputtering. Interestingly, in multichamber technique [29], the surface blind holes are processed by avoiding the particle sputtering using single tool by performing FSP in two steps. In the DFSP method, there is facility of simultaneous reinforcement's addition through tool itself while performing FSP [30].…”

Surface remelting treatment of 7075 aluminum alloy—microstructural and technological aspects

“…This is an inappropriate approach as electric arc treatment can lead to similar effects as those of a laser beam or plasma jet. As shown by the results of works [15][16][17][18][19], GTAW surface remelting treatment allows one to bring about favorable changes in the surface layer of the modified material. The advantages of welding technologies lie primarily in the availability of welding equipment, ease of implementation, low equipment cost and low cost of machining.…”