Titanium alloy is widely used because of its excellent performance, but the friction coefficient is high, the titanium alloy is sensitive to adhesive wear and fretting wear, wear resistance, high temperature and high speed friction easy ignition, relatively poor high temperature oxidation resistance, these defects seriously affect the safety and reliability of its structure, greatly limits its application. Therefore, it is urgent to improve the surface properties of titanium alloy such as wear resistance, high temperature oxidation resistance and corrosion resistance. Surface modification of titanium alloy is the most effective method besides improving the composition and preparation process of the alloy.
In recent years, the rapid development of electron beam surface treatment technology, high energy density electron beam action on the material surface can make the material surface with physical and chemical or mechanical properties difficult to achieve by conventional methods, significantly improve the wear resistance, corrosion resistance and high temperature oxidation resistance of the material surface. The surface treatment of titanium alloy by pulsed high current and low energy electron beam has achieved good results, whose method is adopted by MCC East Engineering & Technology Co.
The material used in the experiment was TA15 titanium alloy (Ti-6.5Al-2Zr-1Mo-1V). After the sample surface was polished, the surface was modified by high-current pulsed electron beam. The acceleration voltage of the electron beam was 27kV, the distance of the target electrode was 80mm, the pulse times were 10, and the interval time was 45s.
The hardness test of the samples shows that with the increase of depth, the hardness value decreases first and then increases, and finally tends to a constant value. This special oscillation curve distribution can be explained as: under the pulse high energy rapid irradiation, thermal shock wave will sprout in the material energy absorption layer and will be reflected back when it meets the interface. Multiple irradiation results in the interference and superposition of stress waves among each other, presenting a complex stress distribution state and a special distribution form of microhardness in the section.
After dealing with the electron beam of the sample, the wear volume is 3 times higher than the original sample, shows that the electron beam processing after the TA15 titanium alloy wear resistance is improved, the reason may be that the following three aspects: (1) high energy electron beam moment deposition in material surface of a small area, make the material to the rapid increase in temperature above the phase transition temperature and melting temperature, and then to ultra-fast cooling of thermal conductivity by matrix (about 109 k/s), which makes the material surface quenching effect and has the effect of solid solution strengthening, therefore the surface wear resistance increase; (2) The rapid solidification process of electron beam can refine the surface grain of the material, thus improving the wear resistance of the material; (3) When the electron beam pulse acts on the surface of the material, the temperature begins to rise rapidly, and the compressed thermal stress wave propagates inwards due to the constraint of the rapid thermal expansion of the surface of the material. Residual stress is distributed as compressive stress, which is beneficial to improve wear resistance.
The corrosion performance test shows that the corrosion potential of the original sample increases from -258.3mV to -107.5mV, the polarization resistance increases from 0.796 K /cm2 to 2.424 K /cm2, and the self-corrosion current decreases obviously compared with the original sample. This indicates that the corrosion resistance of the sample is obviously improved. The main reasons for the improvement of corrosion property are as follows :(1) the high temperature caused by the irradiation of the high current pulsed electron beam on the surface of the sample can make the impurities adsorbed or adhered to the surface of the material vaporize or desolve, which plays a cleaning role; (2) The rapid melting of the material surface, followed by solidification at the same high speed, inhibits the equilibrium crystallization, resulting in dense non-equilibrium microstructure and uniform composition, which also inhibits the occurrence of self-corrosion to a certain extent; (3) The rapid cooling of the surface of the material results in the refinement of the surface grain, which leads to the decrease of the ratio of the anode and cathode area and the decrease of the corrosion rate.