Ultrasonic velocity varia on of Ti6Al4V Ti-alloy bars under conven onal forging combined with triple heat treatment

In this paper, the microstructure evolu on and ultrasonic velocity varia on of Ti6Al4V Ti-alloy bars under different conven onal forging deforma on degrees and triple heat treatment were inves gated, it was found that the the orienta on of microstructure is the main factor influencing the ultrasonic velocity. Meanwhile, taking the ultrasonic velocity as target, the ultrasonic velocity varia on under given conven onal forging process combined with different triple heat treatment condi ons (hea ng temperatures, holding me) were revealed, it was found that the α-β phase ra o, the volume frac on and morphology of equiaxed αp and lamellar αs are also influencing the ultrasonic velocity.


Introduc on
Ti6Al4V alloy is a typical (α+β) tanium alloy, possessing excellent comprehensive proper es such as high strength, high specific strength and good creep performance at high temperature, is extensively used to manufacture structural components in aerospace field, such as joints and frames [1] .As a conven onal nondestruc ve tes ng method, ultrasonic tes ng applies a variety of physical and chemical phenomena to test without damaging the materials, so as to evaluate the physical proper es, state and internal structure of the materials, and to determine whether or not they are qualified. By measuring the ultrasonic velocity of the material, the microstructure and proper es of the material can be detected and evaluated [2][3] .
Ultrasonic velocity is a basic physical quan ty describing the propaga on characteris cs of ultrasonic wave in the medium, which is closely related to the material and the bonding force between material atoms and the atomic spacing. Meanwhile, to some extent, the proper es of the material are determined by the microstructure of the alloy, so there is a certain rela onship between the ultrasonic velocity value and the microstructure [4][5][6][7] The purpose of the present paper is to inves gate the microstructure evolu on and ultrasonic velocity varia on of Ti6Al4V Ti-alloy bars under different conven onal forging deforma on degrees, and to reveal the effect of triple heat treatment condi ons on the ultrasonic velocity varia on.

Star ng materials
The Ti6Al4V Ti-alloy used in the experiments was from Western Superconduc ng Technologies Co., Ltd with a transus β temperature of 985-990℃, its chemical composi on is listed in Table 1. The microstructure of as-received material consists of above 60% equiaxed α p and transforma on β matrix, as shown in Fig. 1.

Experimental procedure
The bars with a diameter of 95.0mm were cut from the as-received bars. The conven onal forging process was conducted on a GFM precision forging machine. The ultrasonic velocity was measured on a CL400 ultrasonic pulse reflector, as shown in  In conven onal forging process, forging deforma on degree of 75% and 90% was considered. In subsequent triple heat treatment two heat treatment condi ons (hea ng temperature, holding me) were considered, in detail, solu on temperature of 950-970℃, ageing temperature of 500-600℃, ageing holding me of 1-3h. The ultrasonic velocity value was taken as evalua on index to inves gate the effect of conven onal forging deforma on degree and heat treatment condi ons, so as to reveal which factor affec ng the ultrasonic velocity most.
The bars were heated to the conven onal forging temperature at a hea ng rate of 10℃/min and held for 70 min to achieve thermal equilibrium and then forged. A sec on of 85.0mm specimen was cut off from the conven onal forging bar, and then turned two end faces. The ultrasonic velocity was measured on the end faces with three mes and the average values are taken, the measurement uncertainty on velocity value is 0.2%. The specimens were axially sec oned and prepared for metallographic observa on by an op cal microscope.

The microstructure evolu on and ultrasonic velocity varia on under different forging degrees.
Fig . 3 shows the microstructure of original Ti6Al4V Ti-alloy bar with a diameter of 95.0mm. It can be found that the transverse equiaxed α p was small and fine, while the longitudinal microstructure has a certain orienta on but was not obvious. Fig. 4 shows the microstructure in different direc ons of dia. 45.0mm and dia. 30.0mm bars a�er forging at 940 ℃ , the corresponding ultrasonic velocity was shown in Table 2. It was obvious that longitudinal microstructure of two specifica ons has processing orienta on, which was growing stronger as the forging deforma on degree increased from 75% of dia. 45.0mm to 90% of dia. 30.mm, the primary equiaxed α p was elongated and even broken. Meanwhile, the longitudinal ultrasonic velocity decreased from 6120m/s of original dia. 95.0mm to 6080m/s of dia. 30.0mm, as shown in Fig. 4(a, c). On the contrary, the transverse ultrasonic velocity increased from 6230m/s of original dia. 95.0mm to 6305m/s of dia. 30.0mm. In addi on, the difference of ultrasonic velocity between transverse and longitudinal direc ons increases as forging deforma on degree increasing. The original difference of dia. 95.0mm was 110m/s, however, the difference increased to 155m/s and 225m/s a�er the forging deforma on degree increased to 75% and 90% respec vely.  In an infinite solid medium, the longitudinal ultrasonic velocity can be described by the following formula: In the above formula, E is the elas c modulus (N/m 2 ), ρ is the medium density (kg/m 3 ), and σ is the poisson's ra o. For solids, the σ is usually in the range of 0~0.5. Therefore, for Ti6Al4V Ti-alloy, when the composi on was determined, the main cause for the ultrasonic velocity varia on of forging bar was the change of elas c modulus caused by microstructure under different processes, while the elas c modulus was mainly affected by the plas c deforma on, crystal structure and phase transforma on. A�er large plas c deforma on, obvious processing texture will emerge in metal materials. Due to the existence of processing texture, the proper es of bars in different direc ons will be different. Meanwhile, there is a big difference in the elas c modulus between the longitudinal and transverse direc on [8] , thus leading to a big difference in the ultrasonic velocity. Fig. 5 shows the microstructure of dia. 45.0mm forging bar a�er 730℃/1h, AC, 950℃/1h, AC+730℃/1h, AC and 950℃/1h, AC+730℃/1h, AC+550℃/2h, AC heat treatment, the corresponding ultrasonic velocity was shown in Table 3. It can be found that a�er 730 ℃ /1h, AC common annealing, the longitudinal microstructure did not change obviously compared to the original thermal-forging state (R state) , the orienta on of microstructure and α-β ra o changed li�le as shown in Fig. 5(a, b). The ultrasonic velocity was consistent with the R state. However, a�er 950℃/1h, AC+730℃/1h, AC two-step heat treatment, the ultrasonic velocity increased significantly, from 6110m/s in R state to 6165m/s. This was because the volume frac on of equiaxed α p decreased significantly as well as the orienta on due to the α→β phase transforma on and recrystalliza on at 950℃ high temperature, as shown in Fig. 5(c). A�er 950 ℃ /1h, AC+730 ℃ /1h, AC+550 ℃ /2h, AC triple heat treatment, the ultrasonic velocity increase further to 6175m/s. The orienta on of microstructure was barely changed, but the volume frac on and thickness of secondary lamellar α s increased, as shown in Fig. 5(d). This was because in two-phase Ti-alloy, the atomic density of α-Ti (HCP) was higher than that of β-Ti (BCC, the atomic density was 0.68). Therefore, the elas c modulus of α phase was higher than that of β phase, the ultrasonic wave propagated faster in α phase.  Apparently, given the conven onal forging process, the ultrasonic velocity of Ti6Al4V forging bars varied significantly under different triple heat treatments. Therefore, it was necessary to study the microstructure evolu on and ultrasonic velocity varia on under different triple heat treatment condi ons, so as to provide a reference for adjus ng the ultrasonic velocity in different direc ons of forging bars.

The microstructure evolu on and ultrasonic velocity varia on under different solu on temperatures.
Fig . 6 shows the microstructure of Ti6Al4V dia. 45.0mm forging bar heated at different solu on temperatures of 950℃,960℃and 970℃and subsequent 730℃/1h, AC+550℃/2h, AC heat treatment, the corresponding ultrasonic velocity was 6175m/s, 6190m/s and 6208m/s respec vely. As the solu on temperature increasing from 950℃ to 960 ℃ , equiaxed α p will transform into high-temperature β phase and its volume frac on will reduce. Meanwhile, the orienta on of microstructure decreased because of recrystalliza on at high temperature, the ultrasonic increased from 6175m/s to 6190m/s. As the solu on temperature increasing further , equiaxed α p decreased faster, and there is basically no obvious orienta on or processing texture, as shown in Fig. 6(c). However, for a given Ti-alloy the content of α stable element is certain, when the volume frac on of equiaxed α p decreased, that of lamellar α s will increase [9] . Thus the ultrasonic velocity increased further, from 6190m/s to 6208m/s.   7 shows the microstructure of dia. 45.0mm forging bar heated at 960℃/1h, AC+730℃/1h, AC and different ageing temperatures of 500℃,550℃and 600℃ heat treatment, the corresponding ultrasonic velocity was 6200m/s, 6190m/s and 6178m/s respec vely. It can be found that as ageing temperature increasing, the orienta on of microstructure changed li�le, but the volume frac on and thickness of secondary lamellar α s decreased significantly. As men oned above, the ultrasonic wave propagated faster in α phase compared to β phase, the ultrasonic velocity decreased as the volume frac on of α phase decreased.   8 shows the microstructure of dia. 45mm forging bar heated at 960℃/1h, AC+730℃/1h, AC and different ageing holding me of 1h, 2h and 3h heat treatment, the corresponding ultrasonic velocity was 6186m/s, 6190m/s and 6195m/s respec vely. As the ageing holding me increasing, the lamellar α s precipitated from β transformed matrix gradually, leading to an increase in the volume frac on and thickness of α s , the ultrasonic velocity increased from 6186m/s to 6195m/s, but the extent of increase was modest.

Conclusion
(1) Due to the existence of processing orienta on in conven onal forging of Ti6Al4V Ti-alloy bar, there is a big difference in the ultrasonic velocity in the longitudinal and transverse direc on, the longitudinal ultrasonic velocity decreased with the increase of forging deforma on degree.
(2) The ultrasonic wave propagated faster in α phase compared to β phase, thus the ultrasonic velocity increased as the volume frac on and thickness of α phase increased.
(3) The processing orienta on of microstructure is the main factor influencing the ultrasonic velocity. As the solu on temperature increasing, the volume frac on of equiaxed α p decreased while that of lamellar α s increased, the processing orienta on decreased but the ultrasonic velocity increased significantly. With the decrease of ageing temperature and the increase of ageing holding me, the longitudinal ultrasonic velocity increased. but the effect of ageing temperature on ultrasonic velocity is more effec ve than that of ageing holding me.
(4) The results would provide a guide to obtain a Ti-alloy bar with high-ultrasonic velocity through conven onal forging combined with triple heat treatment process.