Hot stamping forming of large thickness titanium alloy spherical head

Titanium and titanium alloy are excellent engineering structural materials. With the increase of their consumption year by year, they are more and more used in the processing and manufacturing of various engineering structures and equipment. Due to the inherent characteristics of titanium and titanium alloy, it is difficult to form and process. If the method is improper or the process is unreasonable, the workpiece can not meet the technical requirements and even be scrapped. Therefore, the research on the forming methods and processes of titanium and titanium alloys has attracted great attention. Titanium alloy has high strength, relatively poor plasticity, large processing deformation resistance, narrow deformation range, small elastic modulus and large rebound. For most of the workpiece forming with high alloy strength, large plate thickness, complex shape or large deformation, hot forming process is often used. In this paper, combined with the large thickness of titanium heat exchanger（ δ= The design of forming die, determination of process parameters, selection of forming equipment and change of head size after forming are discussed.

Materials, specifications and dimensions

According to the design requirements, the titanium alloy spherical head material is Ti31 alloy, belonging to Ti Al Mo Ni Zr alloy system, and its conventional mechanical properties are as follows: σ b＝590MPa， σ 0.2＝490MPa， δ 5＝16%， ψ＝ 35%.
Blank plate thickness: 60mm; Blank type: forging.
Titanium alloy spherical head size: SD380 / 60mm.

Mold design

Reasonable design of forming die is the key to ensure the qualified product and the smooth forming. During the die design, the main factors that have a great impact on the forming process and product size are: the upper die indenter diameter D, the lower die pull ring outlet diameter D, the lower die pull ring fillet R, the gap Z between the upper and lower dies, and the selection of blank holder ring.

Upper die: during the cooling process of the stamped head, the metal shrinks, reducing the titanium alloy spherical head diameter. Therefore, when designing the upper die, the diameter of the upper die indenter should be appropriately increased. The shrinkage of heads with different metal materials, different plate thicknesses and different diameters is different. Therefore, the diameter of upper molding head:

• Dup = D0 (1+ ε%)

In the formula: d0 is the theoretical diameter of the titanium alloy spherical head, ε Is the average shrinkage of the titanium alloy spherical head diameter. In addition, in order to facilitate the demoulding of the titanium alloy spherical head after forming, the straight edge of the upper part of the die is designed with a certain taper.

Upper mold material: cast steel. Figure 1 (a) shows the structural dimensions of the upper mold.

Figure. 1 die structure dimensions
Lower die: in the design, the lower die ring outlet diameter D and the lower die ring fillet R are mainly considered, which are the key dimensions to ensure the smooth forming. Generally, the lower die ring outlet diameter Dup = Ddown + 2Z, and Z is the gap between the upper and lower dies. The determination of Z value depends on the nominal thickness of the blank, the positive tolerance of the blank thickness and the thickening of the upper part of the titanium alloy spherical head after forming Size has a great influence on forming quality.
Z. Too small R will increase the radial tensile force during forming, make it difficult to stretch and form, and increase the thinning of the workpiece. In serious cases, it will cause the workpiece to tear. Too large Z and R are easy to cause wrinkling. In design, z = 1.08 δ， R=（2～3） δ. Lower mold material: inner mold cast iron and outer mold cast steel. Figure 1 (b) shows the structural dimensions of the lower mold.
Blank holder ring: generally, blank holder conditions are judged by blank diameter D and thickness δ Because the stamping blank is thick, the flange will not wrinkle due to instability during stretching, so it is not necessary to use flattening ring in the stamping process.

Selection of forming equipment

Like other metal materials, the hot forming of titanium alloy is generally carried out on hydraulic press or hydraulic press. The selection of equipment tonnage can be determined by the minimum tensile force required for workpiece forming. The tensile force can be calculated according to the following empirical formula [1]:

• P pull = (1.6 ~ 2.0) LND / D1 · π · D1 δ·σ bt

In the formula: P is the tensile force, kg; D is the blank diameter, mm; D1 is the diameter of the neutral surface of the titanium alloy spherical head, mm; δ Is the blank thickness, mm; σ BT is the tensile strength of metal at stamping temperature, kgf / mm-2.
According to the alloy process plasticity diagram, the alloy is between 700 ~ 900 ℃, σ bt＜150MPa.
It is worth noting that when selecting the equipment, on the premise of meeting the tensile force, the stamping equipment shall be reasonably selected according to the workpiece size and die structure size, comprehensively considering the size of the worktable, working stroke, die mounting height and other parameters.

Determination of forming process parameters

Reasonable selection of hot forming process parameters is very important. The main process parameters for hot forming of titanium alloy include heating temperature, holding time, deformation temperature, deformation amount, etc. the important basis for the selection of process parameters is the process plasticity diagram of the alloy. Generally, with the increase of deformation temperature, the deformation resistance of titanium alloy decreases, and the degree of reduction varies with the alloy. According to ti-3 1. The high temperature deformation resistance and plasticity diagram of the alloy (Fig. 2) shows that when the heating temperature exceeds 700 ℃, the deformation resistance of the alloy is small (below 150MPa), the plasticity is greatly improved, and the elongation exceeds 100%, which is very conducive to forming processing.

Fig. 2 high temperature deformation resistance and plasticity of Ti-31 alloy
The deformation temperature and deformation amount of titanium alloy during hot working have an effect on the room temperature properties of the alloy. For Ti-31 alloy, after processing at 700 ~ 900 ℃ according to different deformation, its room temperature σ b、 σ The change of 0.2 is not obvious, but after processing at 900 ℃ and 15% deformation, the δ 5 and ψ drops sharply and reaches the lowest value. Therefore, during actual forming, the heating temperature, initial and final punching temperature are controlled between 700 ~ 900 ℃, and the deformation is controlled below 10%. The holding time is generally selected according to the thickness of the blank.

Anti oxidation protection of workpiece

Generally, the hot working and forming of metals are carried out in the atmospheric environment, and so are titanium alloys. However, titanium alloys are different from other metals in that titanium alloys have strong high-temperature chemical activity. At temperatures above 400 ℃, they are very vulnerable to atmospheric pollution, inhale harmful gases such as oxygen, hydrogen and nitrogen, cause alloy embrittlement, and thus cause cracking in the forming process. Therefore, in order to prevent If the workpiece is polluted by air, the surface of the workpiece must be painted with special high-temperature anti-oxidation coating for anti-oxidation protection. At the same time, the workpiece heating is best carried out in the electric furnace or gas furnace; when the gas furnace is heated, the atmosphere in the furnace shall be controlled to be neutral or slightly oxidizing, and the flame shall be prevented from spraying directly onto the workpiece [2].

Forming process of titanium alloy spherical head

(1) Prepare

• ① Calculate the blank size according to formula , in which: h = straight edge height of head + head radius; cut the blank with oxygen acetylene flame;
• ② Apply high-temperature anti-oxidation coating on the blank.

(2) Heat

• ① Heating equipment: coal fired reverberatory furnace. When the temperature is 600 ℃, the blank is fed into the furnace;
• ② Heating temperature: 820 ℃, holding time: 1.5h.

(3) Shaping

• ① Stamping equipment: 50t oil press;
• ② Initial punching temperature 820 ℃, final punching temperature ≥ 750 ℃;
• ③ Forming times: 2 times;
• ④ Lubrication method: graphite lubricant;
• ⑤ Cooling mode: air cooling after demoulding the workpiece.

Inspection results and discussion

The overall dimension and wall thickness of the formed SD380 / 60mm spherical head are measured, and the results are shown in Table 1.

Table. 1 dimensional inspection and wall thickness measurement results

Project	Inner diameter / mm		Diameter difference / mm	Surface roughness / mm	Thickness / mm		Maximum thinning rate (%)	Maximum thickening rate (%)
					Bottom	upper part
Workpiece	384 385	513.5 515	1～1.5	＜2	51.47 51.79	64.75 65.84	13.7～14.2	7.9～9.7
Mould	384	514	0	–	–	–	–	–

The results in Table 1 and the observation of forming process show that:

• ① The hot stamping process is stable and the deformation is uniform and smooth; the determined hot working system is feasible and the process parameters are reasonable; the workpiece size control is good.
• ② In Table 1, the inner diameter of the workpiece is consistent with the diameter of the die indenter, indicating that the fitting between the workpiece and the die is good during stamping; the shrinkage of the workpiece after stamping is small. Therefore, the shrinkage is controlled within 1% during die design.
• ③ The wall thickness of the upper part of the workpiece increases greatly, and the maximum thickening rate is about 10%. It is basically consistent with the clearance between the upper and lower dies in the die design; it is proved that z = 1.08 δ It is reasonable to determine the outlet diameter of the lower die ring.
• ④ The maximum thinning part of the workpiece after forming appears at the bottom of the workpiece and gradually decreases along the direction from the bottom to the upper end; it begins to thicken at about 100mm away from the upper edge of the workpiece, which is the same as that of spherical heads of other metal materials. The maximum thinning rate is 14.2%, meeting the requirement of thinning rate ≤ 15% in CD130-87 technical conditions for titanium equipment [3]. It is greater than that of steel.

Conclusion

Titanium and titanium alloys have the characteristics of low deformation resistance at high temperature and excellent plasticity, which is very favorable for their hot working forming. However, due to the difference between titanium and titanium alloys and other metal materials, they are different from other metal materials in hot stamping forming. It is very important to reasonably design the forming die and correctly select the forming process parameters for the smooth progress of forming The process plasticity diagram is an important basis for selecting the hot forming process parameters of titanium and titanium alloys. Titanium and titanium alloys are easy to oxidize at high temperature, so the anti-oxidation protection in the hot forming process is very necessary.
Author: Wang Ting

Source: China Titanium Alloy Spherical Head Manufacturer: www.titaniuminfogroup.com

Reference:

• [1] Liu Yousheng. Sheet metal stamping process of head (II) [J]. Forging machinery, 1981, (5): 33-38
• [2] B / t4745-2002, titanium welded vessels [S]
• [3] Cd130a9-1987, technical conditions of titanium equipment [S]