Metallurgical Characteristics

Metallurgically, the mechanical properties of spray cast alloys normally meet or exceed those of the counterpart ingot alloy (Ref 9). This fact is illustrated in Table 2 for stainless steel tubing in the spray cast condition and after cold working.

Table 2 Tensile properties of stainless steel

N/mm2

Elongation, %

Spray formed (as-deposited)

260

618

57

Ingot metallurgy (as-extruded)

290

632

58

Spray formed (cold Pilger rolled)

980

1058

15

Ingot metallurgy (cold Pilger rolled)

931

1007

Source: Ref 9

The advantages of spray forming in its application to aluminum alloys are vested in low oxygen and hydrogen levels and a refined uniform microstructure (Ref 9). In the absence of oxide films or particle boundaries, minimal hot working is required to optimize mechanical properties. In the standard 2000 and 7000 series aluminum alloys, spray forming enhances damage tolerance. This is attributed to refinement of the grain structure ("-'50 t'm) and of the primary phase distribution (""1 /,!m). particularly in alloys that are sensitive to the presence of iron-containing intermetallics or to the distribution of transition element phases. This refinement results in attractive increases in ductility, toughness, and fatigue resistance. Fracture toughness data for spray formed 7075, as a function of yield strength, are given in Fig. 3; corresponding data for the direct chill (DC) cast alloy are included for comparison.

Fig. 3 Fracture toughness of 7075 aluminum alloy as a function of strength and mode of fabrication. Source: Ref 9

Alloy modification via spray forming has led to further strengthening of the 7000 series aluminum alloys (Ref 9). For example, strengths from 700 to 800 MPa have been achieved by increasing the levels of zinc and magnesium. In the T6

condition, fracture toughness is 27 to 32 MPav/^, with attendant improvements in fatigue resistance compared to 7075. Significantly, a 30% increase in strength has been achieved with this new alloy composition compared to 7050 at the same toughness level. Thus the new alloy has the potential for replacing 7075 and 7050 extrusions and 7050 forgings.

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