Liquid phase surface nitriding of aluminium using TIG process

Abstract

Liquid phase surface nitriding of AA5052 aluminium was performed using the heat of a TIG (tungsten inert gas) torch in a gas shielding which was a mixture of argon and nitrogen. The feasibility of obtaining nitride compounds at various TIG processing parameters and nitrogen contents in the shielding gas were studied. The presence of AlN phase was proved by X-ray diffraction analysis. Scanning electron microscopy with EDS analyser was carried out to study the morphology and chemical composition of the nitride phase. The microhardness test was also performed on cross sections of treated layers. This measurement demonstrated that the surface hardness of the untreated aluminium was increased from 52 HV for the untreated aluminium alloy to as high as 1411 HV for the nitrided sample. It was also noticed that liquid phase surface nitriding reduced the wear rate to less than quarter of that of the untreated substrate.

Tribology of Si-rich TIG-deposited coatings on Zn–40Al–2Cu alloy

“…As seen, compared to their as-cast microstructures (Figure 2), the dendritic network, eutectoid regions, and Si particles have been significantly refined where the average size of Si P particles has reduced by 77 and 68%, respectively (Figure 4). The high efficiency of TIG re-melting in surface microstructure modification of alloys has been previously demonstrated and attributed to rapid solidification (high degree of under cooling) of the surface molten layer mediated by steep thermal gradient generated on the surface [22,23].

Figure 3 SEM microstructures of (a) 4Si-D alloy, (b) enlarged view of (a), (c) 8Si-D alloy, and (d) enlarged view of (c).

Figure 4 Average size of Si P particles in the experimental alloys.

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“…The improved mechanical properties of 4Si-D and 8Si-D coatings compared to 4Si and 8Si alloys, respectively, can be explained by the chill modification effects of TIG remelting process on the coating microstructure [22,23], especially the refinement and even distribution of second-phase particles/microporosities, refinement of grains, and reduction of the secondary dendrite arm spacing (SDAS) Si and ε-phase are inherently brittle and due to high melting entropy [26,27], exhibit faceted-type interface with the matrix. Therefore, they are quite prone to microcracking under tensile/shear stresses.…”

“…In this study, Si-rich Zn–40Al–2Cu–xSi ( x = 4 and 8 wt-%) coatings were deposited on the surface of Zn–40Al–2Cu alloy via a TIG process with the aim of taking the benefits of Si addition to the hardness and tribological properties while retaining the appropriate bulk mechanical properties of Zn–40Al–2Cu alloys. TIG is capable of producing surface coatings with fine microstructure and superior wear properties provided that the process-related defects (like pores and microcracks) are kept to a minimum [22,23]. Therefore, given the ability of FSP in surface modification/densification of metallic alloys, the as-deposited coatings were subjected to FSP to modify their microstructure, decrease their defect density, and enhance their tribological properties.…”