PM Welds

Successful P/M welding requires proper design and the proper choice of material(s) and joining process. This requires a thorough understanding of the performance characteristics for the application. These include, but are not limited to, the following:

• Strength requirements: applied load in tension, shear, torsion, and so forth

• Dimensional restrictions: potential problems with distortion and/or shrinkage

• Environmental factors: for example, galvanic corrosion from dissimilar metal combinations or effects of elevated temperature on the joint strength

• Appearance: surface condition

• Economics: cost effectiveness compared with other manufacturing methods

Once the application requirements have been established, consideration can be given to the major factors involved in the joining process.

Type(s) of Material. If strength requirements necessitate greater hardenability, control of hardness and related stresses in the weldment become more critical in welding. Also, the potential for any metallurgically incompatible elements must be eliminated. Compositions containing sulfur or high phosphorus levels are particularly suspect.

Joint design should ensure that the joint interface is not subjected to excessive loads or stress concentrations. In addition, the component geometry must lend itself to the type of joining process to be used, for example, the ability to press and control density in projections for RPW applications.

Process selection must be considered when evaluating the minimum strength requirement of the application. Typically, the fusion processes GTA, GMA, EBW, and LBW provide the highest joint strengths, while FRW and RPW are often near parity with these techniques. Brazing, diffusion, and adhesive bonding provide somewhat lower levels with respect to potential joint strength.

Fusion Weld Cracking. The most common problem with fusion welding involves cracking in or near the weld interface (Ref 23). Successful welding can be accomplished if proper consideration is given to why P/M weldments crack.

The formation of cracks in welded P/M components is most often associated with the stresses generated during solidification and cooling of weld metal (Ref 24, 25). The mass of material surrounding the weld puddle and HAZ resists the contraction forces as the metal cools, resulting in tensile stresses that initiate cracks. These stresses can be minimized by the use of several techniques:

• Preheating can drive off moisture (hydrogen) and lessen the thermal gradient across the weld zone.

• Postheating immediately after completing the weldment reduces stresses, particularly for high hardenability materials that form appreciable amounts of martensite in the weld metal and HAZ.

• Austenitic filler metals on steel or low-alloy components can be beneficial because of their superior toughness, strength, and lack of martensitic transformation.

• Processes that allow the operator to manipulate the amount of heat input can also assist in minimizing stresses (GTA, GMA).

Joint configuration plays a major role in stress formation. Mismatched joints, poor gap spacing, or narrow (low width-to-depth ratio) joints along with insufficient amount of filler metal to counteract densification can all have deleterious affects on the weldment. For example, Section A in Fig. 5 shows a throat crack resulting from increased volume of affected metal and related stress concentrations by using a large-diameter filler wire. In contrast, section B incorporates a smaller-diameter filler wire and lower energy input, which provides a successful weldment.

Fig. 5 P/M Alloy FL-4405 (6.95 g/cm3) joined to low-carbon steel rod without different filler wire diameter. See text for details.

In addition, fusion welding is not recommended for steam-treated or quenched-and-tempered P/M components. The oxides resulting from steam treatment act as contaminants in the weld zone, resulting in erratic performance and potential for cracking. Quenched-and-tempered parts, even with a sufficient temper to drive off entrapped quenchant, are not good candidates for welding (Ref 26). However, this is not to suggest that it cannot be accomplished. The most substantial drawback centers on the relatively high heat input associated with fusion processes that can change the structural constituents. This results in lower strength in the weld zone compared with the adjoining base material resulting in a stress gradient and potential fracture initiation site. It should also be noted that when welding infiltrated parts, free copper in the pores is likely to melt and subsequently migrate to the former austenitic grain boundaries.

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