The decrease in density after sintering of magnesium alloy AZ91 can be attributed to several factors:
Pore Formation: During the sintering process, the powder particles are compacted and heated to fuse together. However, the process may also lead to the formation of pores or voids within the material. These pores can result from incomplete particle bonding, gas evolution, or other factors. The presence of pores reduces the overall density of the material.
Gas Evolution: Magnesium and its alloys are known to release gas, often hydrogen, during sintering. This gas evolution can contribute to the formation of pores within the material and, consequently, reduce the density. The gas may come from the decomposition of residual lubricants or other organic materials present in the powder.
Oxide Formation: Magnesium readily forms oxides on its surface when exposed to air. During sintering, oxides may form at the particle interfaces, hindering proper bonding between particles. This oxide layer can act as a barrier, preventing effective particle fusion and contributing to the formation of pores.
Incomplete Particle Bonding: Sintering involves the partial melting and bonding of powder particles. However, achieving perfect particle bonding is challenging, and incomplete bonding can result in voids or gaps within the material, leading to a decrease in density.
Pore Coalescence: During sintering, smaller pores may coalesce to form larger ones. This coalescence process can result in a decrease in density as the material becomes less compact.
To mitigate these issues and improve the density of sintered magnesium alloys, process parameters such as temperature, pressure, and sintering atmosphere need to be carefully controlled. Additionally, the use of proper sintering aids, alloy modifications, and powder processing techniques can help enhance the overall densification of the material
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