Experimental Study on Effect of Cryogenic Treatment on Cutting Force of Titanium Alloy

Three titanium alloy materials TA7, TB6 and TC4 were selected, and three titanium alloys with different phases were placed in liquid nitrogen at -196°C for cryogenic treatment, and the change of the cutting performance of titanium alloy samples before and after cryogenic treatment was studied.Six speed combinations of 30m/min, 40m/min, 50m/min, 60m/min, 70m/min and 80m/min were selected to carry out cutting tests.The cutting experiments show that the cutting performance of TA7, TB6 and TC4 titanium alloys has been significantly improved by cryogenic treatment.At different cutting speeds, the cutting force was compared. After cryogenic treatment of three titanium alloy materials, the cutting force was reduced in machining.After cryotreatment, the cutting performance of TB6 was significantly improved, while that of TA7 was relatively low. Among them, TB6 showed the greatest improvement, followed by TA7 and TC4.The three titanium alloys without cryogenic treatment have a cutting speed of 50m/min at maximum cutting force.The maximum cutting force speed of cryogenic titanium alloy is 40m/min.


Introduction
After the 1980s, with the emergence of liquid nitrogen cryogenic equipment at home and abroad, the research on improving the structure and properties of materials through cryogenic treatment technology has become more extensive and in-depth.The method of cryogenic treatment is to stand the material in liquid nitrogen at -196℃ [1] , and the low temperature treatment for a certain time can improve the performance of the material to some extent.For more than 30 years, Chinese researchers have done some research on cryogenic treatment technology, equipment and mechanism, the early research object focused on tool steel, bearing steel and high-speed steel, in recent years, gradually transition to non-ferrous metals and their composite materials such as copper alloy, aluminum alloy, magnesium alloy, titanium alloy and non-metallic materials.Xue Luwei [2] treated pure titanium at low temperature and analyzed its surface residual stress, and found that low temperature treatment did not affect the microstructure of α-Ti, but it would produce residual compressive stress on the surface of the sample.Li Yonggang, Radar etal [3][4] .conductedcryogenic treatment of titanium alloy TC4 at different low temperatures, and analyzed its microstructure and tribological characteristics.They found that at a certain low temperature, low temperature treatment can effectively improve the microhardness of TC4 titanium alloy [5] Titanium alloy is a kind of representative refractory metal.Experimental research on mechanical processing of titanium alloy is an unavoidable link in the industrial production of titanium alloy.Cryogenic treatment can change the microstructure of titanium alloy to some extent [6] , so as to change its mechanical properties.Therefore, the change of cutting force caused by the change of material properties in cutting titanium alloy is a topic that can be studied in depth.The purpose of this paper is to conduct cryogenic treatment on three titanium alloy materials [7][8] , TC4, TB6 and TA7, and carry out cutting force experiments at different cutting speeds to study the changes in cutting performance of titanium alloy materials before and after cryogenic treatment.

Experimental workpiece and tool
The titanium alloy materials used in the experiment, is directly from domestic factories, procurement to before leaving the factory, according to the different heat treatment process of titanium alloy, its cold extraction or annealing processing, such as its specifications are 100 mm, TC4, TB6, two TA7.It was not annealed in the test, but cryogenic treatment was carried out in its factory condition.The cutting samples used in the test are pre-processed and the cutting samples are cut on a CNC machine with a width of 1 mm.To this end, we processed the cryogenic titanium alloy bar material into a 100 mm outer diameter and 98 mm inner diameter cylinder, and reserved a certain processing allowance in the feeding direction to ensure the stability of the test.The actual machining and installation effect is shown in Figure 1.In this paper, the change of machinability of three titanium alloy materials TC4, TB6 and TA7 under the same treatment time and the same cryogenic treatment temperature was compared and analyzed.Therefore, you can choose the low cost, the treatment effect is better quenching method.The pre-prepared rod material is directly put into the liquid nitrogen tank.The cryogenic treatment equipment is shown in Figure 2. The volume of the liquid nitrogen tank is 10L, and the temperature of cryogenic treatment is -196 .After soaking, remove it immediately, air cool to room temperature, rinse with alcohol, wash with ultrasonic cleaning equipment, dry and reserve.Figure 3. Titanium alloy material after cryogenic treatment.

Experimental design and system acquisition
The titanium alloy sample was cut at right Angle.In the process of right-angle cutting, the basic plane of the tool is the cutting force concentration site, the reduction of freedom of cutting force, in the calculation of cutting force, can greatly reduce the calculation error.Based on the stress of cylindrical samples cut at right angles in cutting, the decomposition of resultant force in cylindrical samples is mainly concentrated on the tangential direction on the axis, as shown in Figure .The main decomposition of resultant force is the main cutting force and feed cutting force on cylindrical samples, and the conditions of resultant force and two component forces are shown in Formula (1).In the experiment, only the cutting speed as the only variable of the experiment, to discuss the cutting force affected by cryogenic treatment .Select integer cutting speed, select 30, 40, 50, 60, 70, 80.Cutting force data were collected at each speed.

Experimental results and analysis
The average cutting force under different cutting speeds is obtained by analyzing and processing the original data collected by the test with a computer, and the cutting force is calculated from formula (1) Based on the processing of cutting force, the three kinds of TC4 titanium alloy material, TB6, TA7 cryogenic cutting speed and cutting force before and after processing‫ܨ‬ change curves were drawn.See Figure 5.The cutting force changes of titanium alloy before and after cryogenic treatment were compared and analyzed at cutting speeds of 30, 40, 50, 60, 70 and 80 respectively.The maximum cutting force on titanium alloy material without cryotreatment (UDCT) is 50m/min.When the cutting speed was increased, the cutting force of the three titanium alloys began to decline, TC4 titanium alloy and TB6 titanium alloy decreased slightly, and the waveform showed a stable performance, while TA7 titanium alloy decreased significantly.Titanium alloy material after cryotreatment (DCT), by the maximum cutting force, at the cutting speed is 40m/min, because in high-speed cutting conditions, the material under high-speed cutting conditions will appear work hardening phenomenon, when high-speed cutting, cutting heat will reduce the cutting force, and in high-speed cutting conditions,The critical cutting speed can be reduced from 50m/min to 40m/min.

Summary of this chapter
In this paper, through the design of cutting test, titanium alloy was first placed in -196℃ liquid nitrogen, cryogenic treatment for 48 hours, and then machining, the cutting force data collected during the cutting process was analyzed, compared with the data before and after cryogenic treatment at the same speed, the conclusions are drawn as follows: (1) After cryogenic treatment, the cutting force of three groups of titanium alloy materials has been reduced to varying degrees.The reduction of cutting force is particularly obvious when cutting titanium alloy TB6.At the same cutting speed, the effect of cryocooling treatment on cutting stress is TC4 < TA7 < TB6.
(2) Under the cutting speed in this test, the maximum cutting force generated by three titanium alloys is reduced from 50m/min to 40m/min after cryogenic treatment, and the cutting force of titanium alloys gradually decreases with the increase of the speed, which indicates that cryogenic treatment has a good effect on the improvement of titanium alloy materials in machining.

Figure 1
Figure 1 Actual installation effect of sample.

Figure 4
Figure 4 Schematic diagram of cutting force decomposition.

Figure 5
Figure 5 Cutting force waveforms of titanium alloy at different cutting speeds before and after cryogenic treatment.