PM Materials for Joining

Most iron and steel powder compositions can be joined or welded without difficulty. However, some additions or material grades should be avoided if possible. In general, atomized iron grades have lower residual and tramp elements than sponge or other types of reduced iron powders. The cleanliness of these materials does not play a predominant role in weldment integrity, if held within acceptable limits. Nevertheless, the subtle influence of acid insolubles, oxides, and silicates over a period of time influences the service and fatigue performance. For this reason, the atomized grades are preferred for fusion, high-strength, and critical welding applications.

Carbon content has a pronounced influence on the overall weldability of a material. As a general rule, the carbon content should be held to as low a level as possible. However, carbon also greatly enhances the strength characteristics of a material. Joining processes and techniques can be developed to accommodate intermediate to high carbon levels that exhibit acceptable weld soundness and strength characteristics. For example, Fig. 6 illustrates a high-strength GMA weldment between an FN-0205 steering gear and an AISI 1035 steel shaft using an E70S-type filler wire.

Fig. 6 High-strength GMA weldment between an FN-0205 steering gear (6.9 g/cm3) and a AISI 1035 steel shaft using an E70S-type filler wire

Materials containing sulfur additions should be avoided for welding and brazing applications. The sulfur can migrate to the grain boundaries and may cause hot cracking when fusion welded. When brazing sulfur-bearing components, the manganese in the brazing alloy can form manganese sulfide in the presence of free sulfur, reducing the ability of the material to flow. If a machining enhancement is necessary, a more appropriate choice would include a manganese sulfide (MnS) addition.

Premixes with copper additions of 2.0% can be readily joined to other materials using most processes. The exception, however, involves compositions that include both sulfur and copper additions. Too high a copper content (4.0%) was found to lower the weldment strength to levels below the strength of the parent metal (Ref 28).

Phosphorus additions (Fe3P), somewhat like sulfur, are not particularly attractive for fusion welding applications. The low melting Fe3P addition may promote hot cracking in the weld zone. However, GTA and HPW processes have been used for a limited number of Ancorsteel 45P applications. Admixed additions of nickel to iron or steel powders generally enhance the toughness of the material and do not pose any particular difficulties involving weldability.

Stainless steel P/M components have been successfully welded using various joining processes (Ref 29). Gas metal arc welds of 316L P/M parts at various density levels, using 316L filler metal with an argon shield, provided good overall properties. The 303 free-machining grade and those identified as nitrogen strengthened are not good candidates for welding applications. The 410 martensitic grade can be welded, but precautionary measures must be observed with regard to its hardenability.

A growing number of automotive exhaust system flanges and bosses involve welded P/M stainless materials. Extensive testing programs involving static and dynamic characterization (Ref 30, 31) of flanges and welded assemblies indicate P/M stainless materials successfully meet all the appropriate performance criteria. However, during the initial prototype welding development, it was determined that sintering conditions have a pronounced influence on welding performance.

Production trials (Table 1) indicated a change from dissociated ammonia to pure hydrogen atmosphere substantially reduces the level of carbon, nitrogen, and oxygen when processing 409 stainless materials. Relatively high interstitial levels promoted the formation of fine martensite during welding, along with substantial outgassing that resulted in excessive porosity in the weldment. Weld trials using the hydrogen sintered parts proved successful (Fig. 7). The resulting weldments were free of porosity with completely ferritic microstructures.

Table 1 Influence of sintering conditions on properties

Property

Sinter environment

NH3

at H2

at Vacuum

at Vacuum

at Vacuum at

1230 °C (2250 °F) 1260 °C (2350 °F) 1150 °C (2100 °F) 1205 °C (2200 °F) 1260 °C (2350 °F)

Density, g/cm3

6.57

6.70

6.68

6.95

7.02

Dimensions, %

-0.6

-0.8

-2.0

-3.5

Carbon, %

0.189

0.060

0.017

0.007

0.007

Nitrogen, %

0.339

0.042

0.018

0.003

0.001

Oxygen, %

0.376

0.144

0.225

0.209

0.176

Weldability

Poor

Good

Good

Good

Good

Fig. 7 P/M stainless steel weld variation. (a) Unacceptable weld quality associated with high interstitial gas levels. (b) Weld microstructure representing the same weld parameters but a hydrogen-sintered, low-interstitial P/M stainless boss.

Nickel-molybdenum admixed composition with a nominal 0.5% C, 5.0% Ni, 0.5% Mo having a sintered density of 7.0 g/cm3 was successfully welded using the GMA process and an austenitic filler metal without a preheat or postheat treatment (Ref 32). The friction/inertia process also provided a satisfactory, high-integrity weldment with this composition. The diffusion-bonded grades would respond in a similar manner if processed using the same techniques.

Joining Dissimilar Metals. Two considerations must be taken into account to achieve successful weldments. First, it is necessary to define the environmental conditions both metals must satisfy to ensure the best selection for the task. In addition, the physical compatibility of the metals must also be determined.

Compatibility involves several physical characteristics that affect the weldment. The properties to be considered are coefficients of expansion, electrochemical potential differences, melting temperatures, strength levels, and potential intermetallic compounds that can form between the two metals. When the coefficients of expansion are widely different, there will be internal stresses within the weldment that cannot be reduced by a postweld treatment. If these stresses are significant, the weld integrity will be jeopardized and the weldment prone to premature failure. Weldments involving dissimilar metals may also be subject to electrochemical corrosion. The differences in electrochemical potential indicate the susceptibility of the weldment to corrosion. If the alloys are at opposite spectra on the electrochemical scales, the nobility of one metal is far superior to the other and corrosion can be a potential problem. The solubility effects when joining certain metals can form brittle intermetallics when alloying occurs. The combinations most susceptible are steel and aluminum, magnesium or titanium; copper and aluminum; aluminum and magnesium; or similar combinations of titanium. When joining these alloys, caution should be exercised in choosing a welding procedure so as to prevent the formation of any brittle constituents.

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