Plastic Working Of Molybdenum Alloys

Plastic forming is not only a method for shaping molybdenum alloys, but it can also improve their strength and low-temperature plasticity. Molybdenum and its alloys can be produced into plates, strips, foils, tubes, bars, wires, and profiles using conventional plastic forming methods. A characteristic of processing molybdenum alloy materials is that each hot deformation process has a significant impact on the final product's properties. Molybdenum oxidizes rapidly above 600℃, and at around 725℃, the oxidation products volatilize and a liquid phase appears. Therefore, hydrogen or other reducing atmospheres are typically used for protection when heating molybdenum and its alloys.

Molybdenum and its alloy ingots are mainly produced using powder metallurgy, but smelting can also be used. Small-sized ingots are generally produced using powder metallurgy, while large-sized ingots can be produced using either process. The choice of process depends on the performance requirements of the final product.

Extrusion is used to break up coarse cast grains, improving the machinability of ingots. It can also be used to produce tubes, bars, and profiles. To ensure sufficient grain breakage, the extrusion ratio should be no less than 4, and the extrusion temperature is typically between 1100 and 1315℃.

Forging

Includes rotary forging and conventional forging. Rotary forging is mainly used for producing billets for thin bars and drawn wires with diameters of 2.5 mm or more, using 10-30 mm square sintered bars. The initial forging temperature for pure molybdenum rotary forging is typically around 1400℃, with a per-pass deformation of 10-20%, sometimes reaching around 30%. As the diameter decreases, the forging temperature gradually decreases, dropping to around 800℃ for 3 mm diameter billets. For conventional forging, hammer forging is more suitable than pressure forging.

Rolling

Used for the production of plates, strips, foils, and bars. The initial rolling temperature for billets supplied by smelting-extrusion is generally between 1200-1250℃; the initial rolling temperature for billets supplied by powder metallurgy is generally around 1400℃. To reduce uneven deformation, the per-pass deformation during initial rolling should be between 20-40%, with a total rolling deformation of approximately 75% after each heating.

Heat Treatment

Recrystallization annealing and stress-relief annealing are commonly used. Recrystallization annealing is used in extrusion, forging, and hot rolling processes. Stress-relief annealing is used to eliminate work hardening. Because recrystallization annealing raises the plastic-brittle transition temperature of the material, which is detrimental to subsequent processing, finished products are generally delivered and used in the stress-relief annealed state.