Other Alloying Methods

Admixing. Press-ready mixes contain all the necessary alloying additions for powder users. To preserve premix uniformity against the natural tendency to demix during handling and subsequent processing, various proprietary mixing processes use patented binders to bond additives to the base powders. The benefits of premix processing with binders are:

• Improved (or uniform) flow rate and better die filling while retaining similar green strength

• Potential for improved productivity

• Reduced variability in the sintered properties

• Opportunities for new alloy development

• Utilization of fine particle additives

• Potential for achieving increased P/M part densities and weight control without the need to resort to double-pressing/double-sintering techniques

Hoeganaes QMP, and Kawasaki have developed these "segregation-free" premixed powders by bonding graphite and other alloying additives to the surface of iron powder.

Mechanical Alloying (MA). In the MA process, composite metallic (or ceramic) powders are produced by simultaneous and repeated sequences of extensive plastic deformation, cold welding, and fracturing of a mixture of metallic and alloying ingredient particles during a dry, high-energy (attrition, vibratory, or large-diameter tumbler) ball-milling process.

The mechanically alloyed powders are characterized by dense, intimate mixing of constituent metals on a fine scale, homogeneous with a grain-refined (submicron grain size) microstructure, extended solid solubility, and formation of nonequilibrium phases. These particles have irregular shape, suitable for high packing density during compaction, and, in addition, they are free of the interdendritic microsegregation and pores that are occasionally encountered. This results in a unique combination of high strength and corrosion resistance. Another advantage is the production of amorphous alloys where an extended range of compositions can be processed, which is not possible by rapid solidification.

Mechanical alloying is the most successful method for the production of high-temperature creep-resistant fine (submicron) oxide-dispersion-strengthened (ODS) iron-base superalloys, amorphous iron-titanium, and iron-tantalum alloy powders. Further information is contained in the article "Mechanical Alloying" in this Volume.

Rapid Solidification Process (RSP). In RSP, the local solidification time is reduced with increasing cooling rate. Typically, the cooling rates of conduction processes range between 106 and 108 °C/s (1.8 x 106 and 1.8 x 108 °F/s), whereas the convection processes may be limited to the range 104 and 106 °C/s (1.8 x 104 and 1.8 x 106 °F/s).

Important attributes of RSP are: increased homogeneity, highly refined microstructure and second-phase particle refinement, extended solid-solubility limit (i.e., alloying flexibility), and formation of unique nonequilibrium crystalline and noncrystalline (amorphous or glassy) metastable phases that have significant influence on the properties and structural engineering applications of alloys.

Rapidly solidified low-alloy steel powders with a fine homogeneous structure and dispersion of fine, stable sulfide and oxide inclusions such as MnS, VS, SiO2, MgO, and Al2O3 have been produced for hot consolidation processing to high-strength and ultrahigh-strength P/M parts. This enhancement of mechanical properties is attributed to the retention of fine grain size during austenitizing at high temperatures because of effective pinning of grain boundaries by finely dispersed, stable inclusions. Production of stress-corrosion-resistant NiMoLa ultrahigh-strength steel via RSP is another landmark in the development of high-performance P/M parts (Ref 11). The composition of this steel is similar to 4340 NiCrMo steel where chromium is replaced by higher concentration of molybdenum (1.5% instead of 0.25%), and lanthanum (as LaNi5) is introduced into the melt to balance phosphorus and sulfur to promote stable fine LaPo4 and La2O2S inclusions (Ref 12).

Other potentials in RSP technology include the development of amorphous soft magnetic materials, amorphous ferromagnetic Fe-B-Si alloys for transformer applications, crystalline soft magnetic Fe-B-Si-Al alloy (Ref 13), and hard magnetic alloys based on crystalline Fe-Nd-B alloy (Ref 14).

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