Abstract:  The phase transition temperature is very important for the processing  technology of titanium alloy. The processing of titanium alloy needs  heat treatment. It is necessary to calculate the specific value of the  phase transition temperature range of titanium alloy. In  this paper, the titanium alloy samples were measured by calculation  method, differential thermal analysis method and continuous heating  metallographic method, and the phase transition temperature range was  obtained. The three test methods were analyzed and the phase transition temperature of TC4 titanium alloy was 998 °C.

Key words: TC4 titanium alloy; phase transition temperature; continuous heating metallographic method

Foreword

TC4  titanium alloy has a wide application range, and its application fields  are in the aerospace and aerospace industries, and its requirements for  the process are high. In  the process of titanium alloy process modification, it needs to be heat  treated. This requires analysis of the phase transition temperature  range of titanium alloy. In this paper, three methods are tested for a  titanium alloy sample to obtain the phase transition temperature value.

1. Properties and phase transition temperature of TC4 titanium alloy

The  composition of TC4 titanium alloy (Ti-6Al-4V) is composed of α and β  two-phase titanium alloy, and its advantages are: (1) strong  processability; (2) strong plasticity; (3) weldability and corrosion  resistance. TC4 titanium alloy is widely used in China, mainly in the aviation industry and aerospace industry.

The  processing of titanium and titanium alloys requires heat treatment.  Therefore, the measurement of the phase transition temperature of TC4  titanium alloy is very important. It is also the application parameter  of TC4 titanium alloy treatment process. It is processed by hot  processing during processing to form titanium alloy to form the desired  shape. It  is necessary to control the applicable temperature of titanium alloy,  which is also an important reference for oxygen and nitrogen pollution  indicators in the heat treatment process of titanium alloy. In  the process of using titanium alloy materials, the phase transition  temperature or phase transition temperature range needs to have accurate  values, and the phase transition temperature of titanium alloy varies  with the composition of titanium alloy and processing history, and the  phase transition temperature of each batch of raw materials. It is also different.

2. Determination and analysis of phase transition temperature of TC4 titanium alloy

2.1 Determination of phase transition temperature by different methods

2.1.1 Calculation method for the determination of phase transition temperature

The  change of the phase transition temperature of titanium alloy is the  change of each element in the process after hot working, and the  temperature change is calculated by calculation method. The calculation  method can provide the selection range of quenching temperature in the  continuous heating metallographic method [1].

The formula for determining the phase transition temperature of titanium alloy using the calculation method is:

In  the formula, 885 ° C is the phase transition temperature of pure  titanium; W is the mass value of each element; q is the influence of  each element on the phase transition temperature.

According  to the influence of the chemical composition and impurity content of  TC4 titanium alloy on the phase transition temperature, the calculation  formula is:

The effect of the content of the components in the titanium alloy on the phase transition temperature is shown in Table 1.

Table 1 Effect of the content of components in titanium alloy on phase transition temperature

Phase Type Element Mass Content (ppm) Impact Value (°C)

α phase Al 6.12±0.21 +14.5±0.2

N 0.01±0.02 +23.2±0.1

O 0.16±0.01 +5.5±0.5

C 0.012±0.11 +2.5±0.2

β H 0.005±0.110 -5.1±0.1

V 4.02±0.05 -15.0±0.2

Fe 122.0±1.21 -15.3±1.2

Si 0.01±0.12 -1.0±0.2

According  to the calculation method, the phase transition temperature of TC4  titanium alloy is 885 ° C + (aluminum) 126.75 ° C - (vanadium) 56.7 ° C -  (iron) 2.01 ° C - (silicon) 0.1 ° C + (carbon) 4.4 ° C + (oxygen) 34.0 °C + (nitrogen) 5.5 ° C - (hydrogen) 1.1 ° C = 995.7 ° C.

2.1.2 Determination of phase transition temperature by differential thermal analysis

The  differential thermal analysis method uses a differential thermal  analyzer to analyze the sample, compares it with the sample under the  same conditions, and establishes a curve comparison chart based on the  temperature difference relationship between the two to determine the  state of the material [2]. The  sample adopts air-cooling cooling method, which is difficult to control  to eliminate deformation stress. Therefore, there is residual stress in  the DSC sample. After the test starts, the temperature will gradually  increase. During the period, the residual stress is released, and the  release of residual stress belongs to Thermal behavior, so the DSC curve in the pre-sampling phase will deviate from the baseline and will radiate upward.

2.1.3 Determination of phase transition temperature by continuous heating metallographic method

In  the thermal process operation, the quenching temperature range is  determined according to the phase change temperature value obtained by  the calculation method and the differential thermal analysis method, and  the temperature is controlled at 980, 985, 990, 995, 1000, 1005 ° C,  respectively, and the quenching temperature interval is determined as 5 ° C. The  average temperature between the quenching temperature of the alpha  phase and the disappearing temperature of the adjacent alpha phase is  the phase transition temperature.

3. Conclusion

In  summary, the application range of TC4 titanium alloy is relatively  wide, and the manufacturing process requires a heat treatment process  for the titanium alloy. Therefore, it is necessary to understand the  phase transition temperature value of the titanium alloy. In  this paper, the phase transition temperature range of the sample  titanium alloy is tested by three test methods, and the measured values  are not much different. Each method can provide accurate data, but the  operation methods are different. Finally, the average phase transition temperature of the TC4 titanium alloy was 998 °C.

 references:

[1]Zhou  Li,Liu Huijie.Effect of temporary alloying element hydrogen on  microstructure and mechanical properties of TC4 titanium alloy friction  stir welded joints[J].Rare Metal Materials and  Engineering,2013,02(03):500-506.

[2]ZHU  Hong, LAO Hong. Discussion on Several Test Methods of α+β/β Phase  Transition Point of Titanium Alloy[J].现代机械,2013,06(03):85-87.

[3]  Huang Jianfeng. Study on microstructure and properties of high-strength  and high-plastic TC4 titanium alloy sheet during hot rolling and heat  treatment [D]. Kunming: Kunming University of Science and Technology,  2013.