Element Ti and Zr Influence TZM Alloy (2)

After adding elements Ti and Zr TZM alloy grain is fine, and there are a lot of second-phase grains and pore between Mo grains. The study found that TZM second phase grains and pore for TZM alloy has some inhibiting effect. Grain boundaries provide resistance to deformation of the material, with the grain refinement, polycrystalline strength improved. Despite the added TiH2 and ZrH2 of TZM alloy pores more, the relative density is low, and there is plenty of space particles, but due to finer grain, so the strength of TZM is higher.

However, the second phase particles and pores with grains Mo interface is easy to become a source of micro-cracks to reduce the fracture toughness of the material, thereby causing material happen intergranular brittle fracture. Wherein when the size of the pores and the second phase particles become smaller, more dispersed distribution, strengthening the effect is more obvious. Conversely, when a large particle size when the second phase, it will affect the properties of the alloy.

Element Ti and Zr Influence TZM Alloy (1)

Study found that the addition elements Ti and Zr will make TZM alloy density slightly less than molybdenum sintered body’s. This is due to the elements Ti and Zr are added in the form TiH2 and ZrH2, when sintered at 400 ~ 800 ℃ condition, the TiH2 and ZrH2 after dehydrogenation to generate hydrogen to increase the porosity of the material slightly. Although Ti and Zr slightly reduced the density of the TZM alloy’s sintered body, but refines the grain size of the sintered body. Mo grain size is generally between 10~20um and there are no second phase particles between grains, fewer voids. TZM alloy which added TiH2 and ZrH2 the grain size generally between 5~15um, comparing to molybdenum is small. Meanwhile, Mo grains which exist a large number of second-phase particles and pores and there are a large number of spherical particles and spherical pits. What’s more there is distribution uniformly.

By analyzing the TZM alloy spherical particles found that these particles are (Mo, Ti, Zr) xOy compound composite by Mo, Ti, Zr and O elements. (Mo, Ti, Zr) xOy alloy particles in favor of oxygen purification, generation of dislocation pinning effect, but, to refine the grain, thus contributing to improve the alloy strength.

TZM Alloy Cogging-down

Firstly, TZM alloy billet temperature should not be too high or too low and a reasonable amount of cogging-down process not only reduces the deformation resistance, and will reduce the chance of breakage of the alloy.

In addition, the cogging reduction can reflect the deformation parameters of TZM alloy, which determines the microstructure and properties of the alloy. Cogging-down reduction of TZM alloy is similar to molybdenum which is usually used 25%~ 30% deformations amount. This is because a large amount of deformation, not only it can help the TZM alloy billet has uniform deformation, so that in subsequent processing, there will not produce delamination, burst and other defects. But also it can make the TZM alloy obturator, porosity, and other crushing closure.

On the other hand, due to the quality of alloy billet will affect the subsequent processing steps, so often before the billet into the rolls, which fall under for two in the instruments on stage to shake off most of the scale, and then quickly push into a roll. Meanwhile, TZM alloy billet in the heating stage to have hydrogen protection system to prevent oxidation of the alloy. 

Because the alloy is easily oxidized in the air, and the resulting oxide not only be pressed into the molybdenum rolling, forming pits, affecting the surface quality and smooth TZM alloy blanks for subsequent rolling process, and will significantly reduce the friction, thereby undermining the bite condition and stable rolling conditions, slippage phenomenon.

TZM Alloy Cogging-down Temperature

Sintered molybdenum and its alloys due to the melting point is too high, large deformation resistance, narrow plastic range, so the heating temperature is cannot too low, otherwise the rolling process can not proceed. When TZM alloy billet heating temperature is higher than 1100 ℃, the alloy is only a little plastic, but when the temperature exceeds 1250 ℃, the alloy plasticity improved significantly. Reasonable cogging-down temperature TZM alloy can not only reduce the deformation resistance in cogging process. Besides, at the large cogging-down pressure, alloy billet fracture does not occur.

The experiment found that the cogging-down temperature selected too high, not only easy to make billet has coarse grains, waste of energy, but also have a negative impact on organizational performance of the finished TZM alloy. The cogging-down temperature is too low, TZM alloy billet easy to crack. Therefore, in selecting the appropriate cogging-down temperature, not only to refer to past experiences, but also consider process control, general TZM alloy billet reasonable cogging-down temperature is at 1400 ℃. At this temperature the alloy none cracking scrapped, and has good performance.