Metals densification

Activating additives (mainly transition metals) ean be used as in conventional sintering allowing maximal densification at much lower T, thus preventing only chemical interaction between the sample and the die ". 

An example where one metal melts before the densification process, is the formation of bronze from a 90 10 weight percentage mixture of copper and tin. The tin melts at a temperature of 505 K, and the liquid immediately wets the copper particles, leaving voids in the compact. The tin then diffuses into the copper particles, leaving further voids due to the Kirkendall effect. The compact is therefore seen to swell before the final sintering temperature of 1080 K is reached. After a period of homogenization dictated by the criterion above, the alloy shrinks on cooling to leave a net dilatation on alloy formation of about 1%. 

When the powder is isostatically compacted at elevated temperatnres, the process is called hot isostatic pressing (HIP). In this case, the flexible dies are often made of thin metals, and high-pressnre gases snch as argon are nsed to heat the part rapidly and rednce thermal losses. Pressnre np to 100 MPa and temperatnres in excess of 2000°C are possible nsing HIP, and parts up to 600 kg can be fabricated. A schematic diagram of a typical HIP apparatus is shown in Figure 7.18. Metals that are processed commercially by HIP include various specialty steels, superalloys, hard metals, refractory alloys, and beryllium. We will see in Section 7.2 that HIP is also particularly useful for the densification of ceramic components. 

The thus obtained high-density Mn-Zn ferrite was investigated in detail from the view of physical and mechanical properties, that is, the relationships between the composition of metals (a,) ) and <5 the magnetic properties such as temperature and frequency dependence of initial permeability, magnetic hysteresis loss and disaccommodation and the mechanical properties such as modulus of elasticity, hardness, strength, and workability. Figures 3.13(a) and (b) show the optical micrographs of the samples prepared by the processes depicted in Fig. 3.12(a) and (b), respectively. The density of the sample shown in Fig. 3.13(a) reached up to 99.8 per cent of the theoretical value, whereas the sample shown in Fig. 3.13(b) which was prepared without a densification process, has many voids. 

Peigney, A., Rul, S., Lefevre-Schlick, F. and Laurent, Ch., Densification during hot-pressing of carbon nanotube-metal-magnesium aluminate spinel composites , to be submitted, 2006. 

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