Experimental Study on Surface Pickling Process of Titanium Alloy Forgings

Titanium and titanium alloys have become an emerging light metal material due to their high strength and corrosion resistance, and have been widely used in the defense and military industry, petrochemical industry, pharmaceutical field, and medical devices, seawater desalination devices, and so on. Moreover, because of its high specific strength, high temperature resistance, low thermal conductivity, non-toxicity, good biocompatibility, and a series of other advantages are used in aviation, aerospace, construction, sporting goods, and other military and civilian fields.
After the Ti-6Al-4V forging is sandblasted, the hardened layer on the surface of the forging is thicker, which makes subsequent machining difficult. Therefore, after the sandblasting process is completed, the hardened layer needs to be removed by pickling and the wall thickness needs to be removed according to the process requirements in order to meet the required process requirements. Titanium alloy pickling generally adopts HF+HNO₃, but due to its larger proportion in the acid ratio and higher concentration, it is easy to cause more serious secondary problems. In view of the above problems, we study the corrosion and hydrogen embrittlement phenomenon under different pickling conditions, analyze the influencing factors and mechanisms, and develop an optimized heat treatment process to reduce the oxide layer and hydrogen embrittlement.

1. Test program

After the forging is sandblasted, it is processed into the required test piece by wire cutting and marked 0#-7#, and the specimen is ground and polished respectively to remove the oxidized layer caused by cutting. The surface of the initial samples were subjected to XRD tests and microhardness tests to determine the surface phase structure and depth of the hardened layer. Then they were corroded in acidic solutions of different formulations, and the specimens were taken out of the water bath at constant temperature for 15 minutes respectively, and they were executed according to the same process to measure the hydrogen content, weight loss, surface phase structure, and hardness changes of the samples after pickling. The formulations of the pickling solution are shown in Table 1.
Table.1 Formulation of pickling solution for forging surface

Note: 0# is the original specimen of forging, without pickling

2. Test results

2.1 Weight loss after pickling
After pickling of Ti-6Al-4V forging specimens, the surface morphology of each specimen is shown in Fig. 1.
As can be seen from Figure 1, the initial specimen surface is dark brown, after pickling, the surface is returned to the metal luster. 7 # sample surface shows nitrided color morphology. The results of the weight loss rate test are shown in Table 2.
2.2 Changes of surface phase structure
The surface phase compositions of Ti-6Al-4V forgings before and after pickling were analyzed by XRD, and the results are shown in Figure 2. It can be seen that the XRD patterns of 1#-7# specimens are more uniform, and the surface α-phase has been better removed. Among them, the main peak of α-phase of 4# and 5# specimens at about 40° is reduced obviously, and the surface α-phase removal is more thorough, while increasing the concentration of the pickling solution has not qualitatively improved the further removal of α-phase.

Figure.1 Surface morphology of Ti-6Al-4V forging specimen after pickling
Table.2 Weight loss statistics of samples after pickling

Fig.2 XRD pattern of the surface of Ti-6Al-4V forging sample after pickling
2.3 Change of hydrogen content of Ti-6Al-4V after pickling
The results of hydrogen content determination and the corresponding composition of the etching solution are shown in Table 3.
From the table, it can be seen that except for 3# and 7# samples, the residual H content of other samples in general does not have a big difference, and 4# and 6# samples have the lowest H content. The high content of HF significantly improves the adsorption of hydrogen in Ti-6Al-4V, but the increase in the content of HNO₃ has a de-adsorption effect on the hydrogen again. Therefore, 4# and 6# can be used as optimal pickling alternative formulations.
2.4 Change of surface hardness after pickling
The hardness changes of the samples after pickling are summarized in Table 4, in which all the sample surfaces were in discrete locations, and five points were selected for hardness testing, with the distance between each two points being much larger than 2.5 times the diagonal length of the indentation.
From the results of the hardness test after pickling, it can be seen that the surface hardness of the 0# initial specimen is extremely high, but after pickling, the surface hardness is significantly reduced. The relatively high surface hardness of specimen 1# was due to the low HF content of the pickling solution, so the hardened surface layer might not be removed completely. The surface hardness of the other specimens is relatively small, taking into account the error of the hardness test itself, it can be assumed that the other specimens have been completely removed from the hardened surface layer.
The above test can be known as the optimal pickling solution formula: 1%-3% HF + 25%-30% HNO₃, forgings after the formula pickling, the surface as shown in Figure 3.
Determined after pickling forging surface hardening layer 0.058mm, hydrogen content of 0.002%, to meet customer requirements.
Table.3 Statistics of H content test results after pickling

Table.4 Sample surface hardness value before and after pickling (HV0.5)

Figure.3 Schematic diagram of surface hardened layer after pickling

3. Conclusion

This article verifies through experiments the changes in the removal of hardened layer on the surface of Ti-6Al-4V forgings under different ratios of acid-washing solution. The optimal acid washing solution formula was found to be 1% -3% HF+25% -30% HNO₃ (acid washing condition is 25 ℃ room temperature, acid washing time is 15 minutes), providing an effective solution for subsequent acid washing treatment of such forgings.

Author: Lv Xiaogan