Sintered Aluminum Powder (SAP)

Abstrakt:

Sintered aluminum powder alloys have properties quite different from those of material fabricated by conventional techniques. The oxide that forms immediately on the surface of aluminum is not reduced back to metal during sintering and the resulting powder product contains a substantial amount of oxide. This oxide prevents grain growth and movement of dislocations at the boundaries or through them and produces high strength, high creep resistance and insensitivity to high-temperature exposure.

Sintered aluminum powder alloys have properties quite different from those of material fabricated by conventional techniques. The oxide that forms immediately on the surface of aluminum is not reduced back to metal during sintering and the resulting powder product contains a substantial amount of oxide. This oxide prevents grain growth and movement of dislocations at the boundaries or through them and produces high strength, high creep resistance and insensitivity to high-temperature exposure.

The amount of oxide varies:

  • Flake powder may contain as much as 20% Al2O3;
  • Atomized powders a few percent;
  • Electrolytic powders have intermediate contents;
  • Ball milled powders are denser and consist of small particles welded together so that the oxide is present not only at the surface but also within the particles.
The material properties depend on the amount of naturally formed oxide. Heating powder to increase the thickness of the oxide film or addition of Al2O3 powder, however, does not increase strength and only reduces ductility.

Reportedly, SAP-type products can also be made by dispersing Al2O3 into molten aluminum with ultrasonic vibration or by blowing reducible oxides of high melting point metals into an aluminum melt. Powders milled wet with hydrocarbons contain Al4C3 in addition to oxide.

The natural oxide formed on the powder is some 100 x 10-10m thick, is amorphous and contains absorbed water. Upon heating at high temperature the oxide crystallizes first to η(Al2O3), then to χ(Al2O3) or γ`(Al2O3) and finally to the stable γ(Al2O3).

The absorbed water reacts with the metal to form additional oxide and release hydrogen. This hydrogen may produce porosity at the grain boundaries and cracking or blistering. The higher the oxide content, the more hydrogen is released and the more pronounced the embrittling and defect formation, especially with cyclic heating and cooling. Vacuum treatment or high-temperature sintering before complete compacting reduces hydrogen content and eliminates most if not all cracking. Small additions of aluminum fluoride also reduce the effect of hydrogen.

The oxide is present as finely dispersed particles, which interact with vacancies and dislocations and prevent their easy movement. The oxide effect persists even above the melting point of aluminum.

The structure of SAP materials depends on the origin of the powder, but mainly on fabrication technique and oxide content. The normal iron and silicon compounds present in aluminum fragment in powdering and become dispersed in the metal. They act as points of weakness and starting points for fracture, but the improvement of properties obtained by the use of Al 99.99% is limited.

The density of the powders themselves is of the order of 900-1000 kg/m3, but when compacted to 95% or better, the density is 2710-2720 kg/m3, slightly higher than that of aluminum. Substantial additions of heavy metals raise the density further.

The thermal expansion coefficient is lower than for pure aluminum and decreases almost linearly with increasing oxide content, to reach values of the order of 20x10-61/K for the 300-600 K range at 15% oxide. Thermal conductivity decreases by approximately 1% for every 1% of oxide present, but is higher in the direction of extrusion. Repeated pressing with intermediate vacuum annealing gives the maximum conductivity. The Lorentz constant is 23 x 10-6W/Ω/K2.

Electric resistivity strongly depends on structure: if the oxide film is unbroken, as in the case after pressing and sintering only, resistivities as high as 1 Ωm have been measured; if, on the other hand, very-high-temperature sintering (>800K) or extrusion is used to break up the film, the resistivity drops by some 5-6 orders of magnitude to values ranging from 2.9 x 10-8 Ωm for 1-2% oxide to 4-4.5 x 10-8 Ωm at 15% oxide. The increase in resistivity with temperature parallels that of aluminum up to ≈900K, where it becomes steeper. Quenching from high temperature increases the resistivity because the impurities (silicon, iron, manganese, etc.) are retained in solution. Resistivities of alloys are higher, especially if made from atomized powders.

Grain strength is proportional to the area of contact of the particles; strength after sintering is directly and ductility inversely proportional to oxide content. Grain size of the sintered product has little or no effect, but coarse grain produces an increase in strength and ductility at high temperature. The mosaic block size in the matrix, together with distance between oxide particles, controls the properties. Typical properties at various temperatures are shown in the Table 1.

A tendency for intercrystalline fracture is reported due to separation of metal from oxide. High-temperature sintering tends to reduce strength and increase ductility. Up to 20-40% cold working increases strength but then further work breaks up the oxide network and tends to facilitate coalescence of the oxide films. Consequently, strength and creep resistance decrease. Material rolled to foil has properties close to those of pure aluminum.

Table 1: Mechanical properties of SAP as a function of temperature
Property %
oxide
Temperature, K
170 300 500 600 700 800 900
UTS
MPa
1 - 80-140 - 60-80 40-70 - -
3 - 150-220 - - 60-80 - -
7 350 230-280 120-150 100-140 70-100 50-70 -
12 400-500 320-380 180-220 150-190 100-140 60-90 -
15 - 400-500 200-250 150-200 100-150 70-100 40
YS
MPa
1 - 40-70 - 40-60 - - -
3 - 100-140 - - - - -
7 - 120-160 100-140 80-120 60-90 40-80 -
12 250-300 180-240 140-180 110-150 80-120 60-90 -
15 - 200-260 170-220 140-200 100-150 80-100 30
%A 1 - 25-30 - - 27-34 - -
3 - 18-24 - - 14-18 - -
7 - 14-18 14-18 12-15 4-8 2-4 -
12 - 8-12 4-9 3-6 3-6 2-5 -
15 - 5-9 5-8 5-8 3-5 2-4 2
HV 1 - 300 - 200-250 - - -
3 - 500 - - - - -
7 - 550-700 - - - - -
12 - 900-1000 - - - - -
15 - 1000-1100 600 500 400 300 100

Fatigue strength is of the order of 60-70 MPa at 107 cycles and the decrease with temperature parallels that of the strength. Fatigue resistance is increased by some 10-20% by high (12-14 ppm) hydrogen in solution but decreased sharply by notches and slow strain rates. It is higher in vacuum.

Creep resistance is extremely high and exceeds that of all aluminum alloys. Activation energy for creep of 6.5 eV has been reported. Impact strength rises with increasing temperature up to 800-850 K and then declines; shear strength behaves similarly.

Another important characteristic of sintered aluminum powders is their insensitivity to high temperature: exposure for several years at temperatures up to 800 K produces practically no change in structure or properties, especially in the higher-oxide-content alloys.

The modulus of elasticity increases with oxide content to reach values of the order of 77-80 GPa at 15-16% Al2O3, declining with temperature as does the strength. The damping capacity of SAP is some 20 times higher than that of aluminum. Abrasion resistance does not differ substantially from that of pure aluminum. Neutron or ion irradiation hardens the material but not enough to prevent use in nuclear reactors.

The potential of sintered products is practically the same as that of commercial aluminum; the oxide has little effect on the pitting potential. Aluminum 99.99% is anodic to SAP but aluminum 99.3% is not. Corrosion resistance is slightly worse than that of the corresponding wrought product. It is good for products containing only oxide and manganese, chromium and magnesium; somewhat lower with additions of silicon, iron, nickel, etc.; poor if copper and tin are added. Oxide distribution and content affect somewhat the corrosion resistance of SAP to water at elevated temperatures. However, products with iron, nickel and tungsten show the same (or slightly better) resistance of the corresponding alloys fabricated by conventional methods. Cladding with aluminum, preferably 99.99% pure or aluminum-magnesium alloys improves corrosion resistance to seawater.

Alloys containing large amounts (up to 20%) of chromium, nickel, cobalt, iron, manganese, titanium and titanium carbide have moduli of elasticity of up to 100-120 GPa and high creep resistance, but if prepared by mixing aluminum powder, and metal, they are brittle and the compounds that form in sintering tend to fragment when deformed. On the other hand, if they are prepared from alloy powders, their properties and corrosion resistance are better, especially if the rapid cooling or atomizing is exploited to supersaturate the aluminum, and no segregation is present.

Additions of SiO2, SiC, B4C and AlPO4 embrittle the material without a corresponding increase in strength. Chromium, iron, tungsten, etc. oxides or carbonates, which react with aluminum to increase the amount of A12O3 and liberate the corresponding metal, on the other hand, increase strength. Alloys containing up to 50% Si have low density and low expansion coefficients. Mixtures containing boron or boron carbide can be used to extrude moderator rods for atomic reactors.

High-diffusivity, elements, whose compounds tend to coalesce on high-temperature exposure, lose strength rapidly. Thus, dural, aluminum-copper and aluminum-zinc-magnesium alloys made by powder metallurgy have a strength at room temperature some 30-50% higher than the corresponding aluminum, but after 6 months exposure at 500-550 K have lost their extra strength at room temperature and at higher temperatures are weaker and less creep resistant. Age hardening of SAP alloys is not different from that of normal alloys.

Texture in extrusion tends to be of aluminum, but less pronounced, especially with fine size powders. In sheet a variety of textures has been reported: (111) [112]; (110) [447]; (315) [513]. The use of backpressure on extrusion produces materials with higher high-temperature strength. High-speed extrusion (2-4 m/sec) produces a coarser structure but no substantial difference in properties.

Both deformation and creep mechanisms change with temperature. Slip is on the (111) and (211) planes, with [01-1] and [206] directions.

Recovery of SAP is very similar to that of pure aluminum, but the mosaic structure is smaller, because dislocations tend to be pinned at oxide particles. Low-oxide materials (<3% Al2O3) recrystallise easily, but the recrystallisation temperature rises steeply with oxide content, so that above 5-7% Al2O3 recrystallisation, especially of extrusions, is very rare. Supersaturation with Fe, Mn and Ni further increases the recrystallisation temperature. Activation energy for grain growth is of the order of 2 eV. Diffusion of elements is faster in SAP than in aluminum.

The data on compatibility of SAP and atomic fuels are somewhat contrasting: some scientist report no reaction up to 900 K when SAP is used for canning uranium oxide or carbide; reaction above 700 K, on the other hand, is reported by others.

The data on compatibility of SAP and atomic fuels are somewhat contrasting: some scientist report no reaction up to 900 K when SAP is used for canning uranium oxide or carbide; reaction above 700 K, on the other hand, is reported by others.

Extrusion, forging or hot rolling of compacted borings, fine chips or granules of aluminum and alloys have been tried, either as a means of recovering machining residues or as a short cut in the production of thin sheet, foil, or complex shapes, but the properties obtained are very close to those of the conventional materials rather than those made from powders.

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