subject: The Rivet Nut Aluminum Metallurgy Process [print this page] The rivet nut aluminum P/M process consists of three basic steps.
1. Rivet nut aluminum powders of controlled purity and particle size are mixed with alloying metal powders in precisely controlled quantities. Generally a powdered lubricant is added to permit the consistent production of high-density parts without seizing of the punches or cold welding to the die walls. This lubricant is carefully chosen to ensure that there is no residual ash to interfere with bonding during sintering.
2. The premix is compacted using precision metal dies in specially designed P/M presses to yield a green compact. Rivet nut aluminum premixes exhibit excellent compressibility and yield high-density parts at low compaction and ejection pressures. Premixes can be compacted to 90% density at only 12 tsi and 95% at 25 tsi. Typical green strengths range from 450 to 1500 psi which is sufficiently strong to withstand normal handling without chipping or breaking.
3. The green compacts are sintered in a controlled atmosphere furnace at closely regulated temperatures. This process elementally bonds the powder particles together and develops the desired physical and mechanical properties. The rivet nut aluminum powder sintering is difficult to achieve because common furnace atmospheres do not reduce the aluminum oxide. However, successful sintering is accomplished in environments containing hydrogen, nitrogen and dissociated ammonia as long as the following conditions are observed:
The lubricant is essentially free of moisture and low in ash contact.
Atmospheres contain low levels of moisture and oxidizing gases.
Alloying elements having a high solubility in aluminum are added to generate low melting phases.
Aluminum P/M mechanical properties are very good and typically dominates in the materials selection process. For certain applications, the dynamic properties as well as other performance properties are more critical and become the determinant criteria. These properties typically include, but are not limited to, fatigue, density, electrical and thermal conductivity, corrosion resistance and machine usability.
Fatigue is an important decision consideration for P/M parts subject to dynamic stresses. The most commonly used fatigue property is the fatigue limit, which is the maximum reversed stress the material will withstand if cycled indefinitely. This limit is defined when the fatigue curve becomes horizontal. For ferrous materials this limit is at 10 million cycles while for aluminum alloys it is at 500 million cycles. This difference is due to the lower density of P/M parts. Hot forging the rivet nut aluminum P/M part results in fatigue limits comparable to the wrought alloys.
