Introduction

JOINING P/M components increases the possibilities of manufacturing more complex configurations than conventional die-pressed geometries. Most P/M materials can be welded or joined successfully. However, not every potential P/M welding application is cost effective in terms of labor intensity, equipment, and overall manufacturing costs.

Several references detail the related joining processes and weldability of wrought materials (Ref 1, 2). However, very little comprehensive information is available regarding the application of these practices to P/M components. This article attempts to characterize the physical differences between P/M and wrought or cast materials as they apply to joining. In addition, acceptable procedures and techniques are discussed along with the weldability of various P/M materials.

Effects of Porosity on Properties. Perhaps the most important physical characteristic of a P/M component, which makes it uniquely different in regard to joining, is porosity (Ref 3). The pore volume or relative density has a pronounced influence on several important properties that affect the welding characteristics. These include:

• Thermal conductivity is dependent on the amount of porosity, and greater amounts of porosity change the heat transfer mechanism and ultimately influence the welding parameters.

• Hardenability is dependent on the rate of heat dissipation or thermal conductivity. Pores act as thermal insulators that slow the transfer of heat, making P/M components less hardenable than wrought parts of similar composition. However, because of potential densification associated with particle melting in the heat-affected zone (HAZ), the higher stress levels associated with greater hardenability can occur in P/M components and may increase the material's susceptibility to cracking.

• Thermal expansion is not directly associated with porosity, but this characteristic also plays an important role in determining susceptibility to cracking. Likely differences in composition between P/M, wrought, or cast materials result in dissimilar volume changes when metals are heated and subsequently cooled. Extreme differences in expansion or contraction rates increase the potential for cracking at the weld interface.

Porosity can also cause erratic fluctuations in welding performance because of entrapped oxides or impurities. Precautionary measures must be taken to eliminate any of the following residual elements that may remain in the pores as a consequence of related process conditions:

• Lubricants

• Machining coolants

• Plating solutions

• Impregnating materials

• Cleaning or tumbling agents

• Free graphite or residual ash

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