Cryogenic Milling of Titanium Powder

Abstract

Ti Grade 2 was prepared by cryogenic attritor milling in liquid nitrogen and liquid argon. Two types of milling balls were used-stainless steel balls and heavy tungsten carbide balls. The effect of processing parameters on particle size and morphology, contamination of powder and its microhardness was investigated. Milling in liquid nitrogen was not feasible due to excessive contamination by nitrogen. Minor reduction of particle size and significant alterations in particle morphology depended on type of milling balls and application of stearic acid as processing control agent. Heavily deformed ultra-fine grained (UFG) internal microstructure of powder particles was observed by the method of "transmission Kikuchi diffraction".

Mechanical Properties of Ti-15Mo Alloy Prepared by Cryogenic Milling and Spark Plasma Sintering

“…It was found that titanium remains ductile even at cryogenic temperatures (unless contaminated by nitrogen [21]) and powder particles are not significantly refined [22]. On the other hand, powder particles are significantly deformed by repetitive plastic deformation [15,22,23]. In our previous study we proved that deformation by ball milling resembles the multi-directional forging and causes grain refinement [24].…”

“…Cryogenic milling employing the Szegvari type attritor has been previously applied to commercially pure titanium [18][19][20][21]. It was found that titanium remains ductile even at cryogenic temperatures (unless contaminated by nitrogen [21]) and powder particles are not significantly refined [22]. On the other hand, powder particles are significantly deformed by repetitive plastic deformation [15,22,23].…”

“…On the other hand, HPT samples are very small and thermal stability of ultra-fine grained Ti alloys prepared by SPD methods is limited [29,30]. Similarly to bulk SPD methods, grain size after cryogenic milling can be reduced to tens of nanometers as shown for commercially pure Ti (CP Ti) in [22]. However, the grain size in CP Ti significantly increases after sintering due to recrystallization [24].…”

“…Note that liquid nitrogen cannot be used, because N atoms diffuse into the powder during milling causing embrittlement [21,31]. In order to prevent cold welding of the powder to the milling tank, shaft and balls, a process control agent such as stearic acid must be added to it [15,22].…”

“…For comparison, initial gas atomized powder was sintered using the same parameters. Due to the fact that titanium is a strong gatherer of nitrogen and oxygen, some contamination is unavoidable during powder metallurgy processing [22,32]. Therefore, contamination by oxygen, nitrogen (and possibly by carbon and hydrogen) must be always monitored in order to assess strengthening mechanisms appropriately.…”