Previous investigations of the plasma transferred arc welding (PTAW) surfacing technique have primarily focused on the wear and corrosion resistance of the coating in aggressive applications, such as oil sands abrasion [i]. The metallurgical effects of the carbide type, carbide fraction, carbide dissolution and matrix alloy composition are commonly investigated [ii,iii,iv]. There are few studies where the main objective is investigating the influence of welding parameters [v]. Figure 1 illustrates how PTAW parameters do, in-fact, influence the final overlay service performance. Thus, this study investigated the influence of welding parameters during PTAW overlaying of a metal matrix composite (MMC) [vi]. An AISI 410 stainless steel matrix with 65wt% tungsten carbide MMC was welded utilizing industrial capacity equipment (Figure 2). Comparative experiments were first completed with the heat input equation; however, the equation was found to be inadequate. The effective heat input was better characterized by balancing the combined interrelationships of torch motion, arc energy, and powder delivery rate. An optimization methodology was developed to negotiate this balance and develop "operating windows" (Figure 3). This methodology provides a repeatable method for determining optimal welding parameters (Table 1). Very little work has been done on the effects of plasma, carrier and shielding gas compositions during PTAW. Previous work typically concentrated only on the effect of gas flow rates [vii,viii]. In this study, the typical argon gas was varied with additions of helium, hydrogen, nitrogen and carbon dioxide. The changes in the effective heat input, oxidation/reduction behaviour and weld bead formation were recorded with video photography (Figures 4 through 7). Metallurgical and productivity improvements were found with additions of hydrogen and helium, while nitrogen and carbon dioxide produced detrimental fusion flaws (Figure 8). Future collaborative work will further optimize the gas combinations for new metallurgies to maximize MMC performance.

Figure 1 - Optimization Flow Chart for PTAW Surfacing Parameters and Coating Behaviour

Figure 2 -PTAW Welding Setup & Equipment

Figure 3 - Dependence of Total Welding Time on Torch Velocity and Dwell Time

Table 1 - Standard Welding Parameters, Cross-section, and Bead Profile for 410SS-WC MMC

Figure 4 - Macrographs of Cleaned 410SS Overlays with Plasma and Carrier Gas Variations (Argon

Shielding Gas)

Figure 8 - Excessive Oxidation and Flaw Formation with Ar-CO2 Shielding Gas (1.5 seconds)

References

i Llewellyn, R., Tuite, C., "Hardfacing Fights Wear in Oil Sands Operation", Welding Journal, Vol. 74, No. 3, 1995, pp. 55-60

ii Babiak, Z., Dudzinski, W., "Tungsten Carbide Stability in Plasma Weld Surfacing", Surfacing Journal International, Vol. 1, No. 3, 1986, pp. 87 " 90

iii Yan, M. et al "The Microstructure and Wear Resistance of Plasma-arc Remelted Ni-base Overlay", Journal of Materials Science Letters, Vol. 15, 1996, pp 2038-2041

iv Sandy, K., "High Temperature Behaviour of WC and B4C in Ni-Based Metal Matrix Composites", M.Sc. Thesis, The University of Alberta, Edmonton, Canada, 2006

v Dören, H., Wernicke, K., "Influence of Welding Parameters in Plasma-Arc Overlay Welding With Powder", Deutscher Verband fur Schweisstechnik (DVS) - Berichte, Vol. 100, 1985, pp. 72 " 77

vi Yarmuch, M.A.R., "Effect of Welding Parameters on the Plasma Transferred Arc Welding (PTAW) Process for Autogenous Beads and 410SS-WC Overlays", M.Sc. Thesis, The University of Alberta, Edmonton, Canada, 2005

vii Deuis, R.L., Yellup, J.M., Subramanian, C., "Metal-Matrix Composite Coatings by PTA Surfacing", Composites Science and Technology, Vol. 58, No.2, 1998, pp. 299-309 viii Harris, P., Smith, B.L., "Factorial Techniques for Weld Quality Prediction", Metal Construction, Vol. 15, No. 11, 1983, pp. 661 " 666