Free forging process of astm b381 titanium forgings

 Free forging is mainly used for the primary processing of astm b381 titanium forgings ingots, that is, to manufacture semi-finished bar products with round wearing surface, square section or rectangular section. When free forging is more economical than die forging in single piece or small batch production, free forging is usually used to produce large-size blanks.

From ingot to finished bar, the forging process is usually divided into three stages.

1. Billet

When the initial forging (blooming) temperature is 150-250 ℃ above the β transition point, the plasticity of the casting structure is the best. At the beginning, the ingot should be deformed by light and fast striking until the primary coarse grain structure is broken. The deformation degree must be kept in the range of 20% - 30%. The ingot is forged into the required section, and then cut into a fixed size blank.

The plasticity increases after the casting structure is broken. The aggregation recrystallization is intensified with the increase of temperature, holding time and grain refinement. In order to prevent aggregation recrystallization, the forging temperature must be gradually reduced with grain refinement, and the heating holding time must be strictly controlled.

2. Multi-directional repeated upsetting

It starts forging at 80-120°C above the β-transition temperature, and alternately performs 2 to 3 times of upsetting and drawing, while alternately changing the axis and edge. In this way, the entire cross-section of the blank obtains a very uniform recrystallized fine-grained structure with β zone deformation characteristics. If the blank is rolled on a rolling mill, such multi-directional upsetting is not necessary.

3. The second multi-directional repeated upsetting

It is the same as the first multi-directional repeated upsetting and drawing, but the starting forging temperature depends on whether the semi-finished product after forging is the blank of the next process or the delivered product. The initial forging temperature can be 30-50 ℃ higher than the β - transition temperature for the blank in the next process, and 20-40 ℃ lower than the β - transition temperature for the product delivered

Because of the low thermal conductivity of astm b381 titanium forgings, if the preheating temperature of the tool is too low, the impact speed of the equipment is low, and the deformation degree is large, X-shaped shear band is often formed on the longitudinal section or cross section when upsetting or drawing long billets on the free forging equipment. This is especially true for non isothermal upsetting on hydraulic press. This is because the temperature of the tool is low, and the contact between the blank and the tool causes the surface layer of the metal to chill. In the process of deformation, the deformation heat generated by the metal has no time to conduct heat to the surrounding, forming a large temperature gradient from the surface layer to the center, resulting in the formation of a strong flow strain band of the metal. The larger the deformation degree is, the more obvious the shear band is, and finally cracks are formed under the action of tensile stress with opposite sign. Therefore, in the free forging of titanium alloy, the striking speed should be faster, the contact time between the blank and the tool should be shortened as far as possible, the tool should be preheated to a higher temperature as far as possible, and the deformation degree in one stroke should be properly controlled.

When forging, the corner cooling is the fastest. Therefore, it is necessary to turn over the blank several times and adjust the hammering force to avoid sharp angle. When forging on hammer, the deformation should be less than 5% ～ 8% at the beginning, and then the deformation can be increased gradually.