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In this chapter, an overview of welding as a technology for surface engineering is explored. According to literature, all types of welding techniques are appropriate for coating applications. However, as a result of process characteristics, some welding processes stand out. The most used welding techniques in the metal coating are arc welding (MIG, TIG, and PAW) and oxyacetylene welding. In the coating of metals using welding techniques, the coatings produced usually have a thickness that ranges between 1 and 6 millimeters. Applications of surface coating have been studied extensively. Such applications include aeronautic industry, sports, transport industries, petroleum and chemical industries, mining, food, and in the electronic industry. Plasma MIG welding is an advanced plasma process that combines the advantages of both MIG and plasma welding. Applications of plasma MIG welding in the surface coating of metals are expected to be explored extensively in the future.

In this paper, the influence of varying DC current on microstructures of mild steel substrates coated with stainless steel using TIG welding technique was investigated. To achieve the desired results, mild steel substrates of sizes 40 mm × 10 mm × 1.5 mm were used. The mild steel substrates were ground using Silicon Carbide (SiC) papers to achieve smooth surfaces and to remove oxides that might be present on their surfaces. Then the substrates were coated using stainless-steel rods on TIG 2200i AC/DC machine as a welding power source. The process was undertaken under six varying currents of 40 A, 50 A, 55 A, 60 A, 65 A and 70 A. For purpose of characterization, microstructures of the coated samples were studied using optical microscopy and quantitative image analyses. The depth of penetration of the melt pool below the substrates and the coating thickness were measured for all the samples. Based on the optical microscopy, four zones were identified on the TIG coated samples; the heat-affected zone (HAZ), fusion, coating and a skirt (the region that formed between the substrates and the coatings). Minute porosities were also observed in the coated samples. It was observed that the maximum coating thickness was 5.345 mm and the highest average coating thickness was 3.522 mm, which both occurred at 40 A. The maximum length of penetration was 3.864 mm and was observed when 60 A DC was used. Similarly, the highest average depth of penetration was 2.422 mm and occurred at 65 A. The results of this study showed that for direct current, as the current increased from 40 A to 50 A, the porosities in the coated samples increased. However, at 55 A, the number of porosities decreased, but continued to increase from 60 A to 65 A, but the porosities decreased again at 70 A.

Structural steel components tend to wear when exposed to corrosive and cyclic loading environments. These components can be repaired by welding on failure. This study studied the weld parameters and weld quality (porosity, depth of penetration, and coating thickness), hardness, corrosion, and wear resistance during repair welding. Mild steel samples were weld coated by varying alternating (AC) and direct (DC) currents: 40 A, 50 A, 55 A, 60 A, 65 A, and 70 A. The base material used was AISI 1045 steel, and Castolin 6825 was used as the welding electrode. The results showed that three wear mechanisms were dominant: abrasion, adhesion, and delamination. The porosity in the coated samples increased with increasing currents for both AC and DC. The welding current and current type influenced the coating thickness and penetration depth. When dipped in warm NaCl solution, the corrosion mechanism experienced by both sets of coated samples was pitting corrosion. In both cases (alternating and direct currents), the hardness values increased towards the coating surface from the substrate.