On optimization of the powder plasma arc surfacing process
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On Optimization of the Powder Plasma Arc Surfacing Process J.N. DuPONT The plasma arc welding (PAW) process is frequently used with alloy powder additions for surfacing components subjected to wear and corrosion. Although the deposition rate of this process is typically lower than consumable electrode processes such as gas metal arc welding and submerged are welding,[1,2] the ability to independently control arc power and filler metal feed rate generally allows for a higher degree of control over dilution.[3] Minimizing dilution is important for maintaining the composition, and thus the corrosion/wear resistance, of the weld overlay. As discussed in a recent article,[3] dilution is minimized by operating the process at a high filler metal feed rate (Vfm) to melting power (ha hm VI) ratio, where ha and hm are the J.N. DuPONT, Research Scientist and Associate Director, is with the Energy Liaison Program, Lehigh University, Bethlehem, PA 18015. Manuscript submitted November 24, 1997. 932—VOLUME 29B, AUGUST 1998
arc and melting efficiency, respectively, and VI is the arc power. The melting power is simply the fraction of arc power utilized for melting the filler metal and substrate. If the melting power is too low for a given filler metal feed rate, then insufficient power remains to completely fuse the filler alloy to the substrate. In this condition, incomplete fusion can result between the weld overlay and the substrate, which may be susceptible to spallation during service. In addition, the deposition efficiency of the process can be low if insufficient power is available for melting the volumetric feed rate of powder, which is directed to the molten weld pool. (The deposition efficiency is the fraction of total powder that is melted and deposited as part of the weld overlay.) A scheme for controlling dilution has recently been discussed in detail.[3] In this work, it is demonstrated that the Vfm/hahm VI parameter can also be used for estimating deposition efficiency and fraction of overlay/substrate fusion, Ff. Figure 1 shows the change in dilution, fraction of overlay/substrate fusion, and deposition efficiency, which are expected to result as the filler metal feed rate is increased for a given melting power. In Figure 1(a) the filler metal feed rate/melting power ratio is low. Only a small portion of the total melting power is required for melting the filler alloy. The remaining power is used by melting a rather large portion of the substrate. Although the deposition efficiency is likely to be high in this condition and there is complete overlay/substrate fusion, the level of dilution is high due to the large quantity of substrate melted. As the filler metal feed rate is increased. (Figure 1(b)), a larger portion of the total power is required for melting the filler alloy and less power is available for melting the substrate. As a result, the quantity of melted substrate decreases, which, in turn, reduces the dilution. Figure 1(b) represents a good balance of parameters because dilution is low while maintaining
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