Ultrasonic Impact Strengthening of Titanium Alloys: State-of-the-art and Prospectives

This review started with the introduction to the principles and research progresses of the ultrasonic impact strengthening technology for titanium alloys. The influences of the properties of titanium alloys were investigated, which was associated with the different parameters of ultrasonic impact strengthening processes(static pressure, ultrasonic amplitude and numbers of rolling). Results show that the optimization of different processing parameters has a significant improvement on the performance strengthening of the titanium alloys. However, there is a critical value of the different processing parameters. Once the critical values arc exceeded, continuing to increase the parameter values will reduce the service performance of the titanium alloys. Finally, the difficulties of ultrasonic impact strengthening technology which used in the engineering applications were summarized. Combined with the development of intelligent manufacturing, the future development of ultrasonic impact strengthening technology was prospected

Research on the oxidation resistance and ultra-high frequency thermal fatigue shock failure mechanisms of the bilayer and multilayer nano-coatings on cemented carbide tools

The multilayer nano-coating shows excellent thermal stability and oxidation resistance which could be attributed to the multielement composition and multilayer structure. It is widely used in the machining of hard-to-cut materials, such as titanium alloy. The formation of serrated chip during cutting process of Ti-6Al-4V would result in the ultra-high frequency thermal fatigue shock on the coating surface, and this could induce the thermal fatigue failure of coating. In this study, the ultra-high frequency (20 kHz) thermal fatigue shock testing was developed to investigate the thermal fatigue failure mechanisms of the bilayer TiSiN/TiAlN coating and multilayer TiSiN/TiAlN nano-coating. It was achieved by using the pulsed laser system to irradiate the coating surface under different shock cycles, which could produce alternating thermal stress on coating surface. This creates a close simulation of the actual working conditions in high-speed machining of Ti-6Al-4V with serrated chip produced. Moreover, the high temperature oxidation experiments were performed to study the oxidation resistance of coating. The failure mechanisms of coating/substrate system under the constant thermal loads and dynamic thermal loads were compared and analyzed. The cutting performance of related coated tools was evaluated after turning Ti-6Al-4V. The correlation between the thermal fatigue shock resistance of coatings at ultra-high frequency and the wear behavior of coated tools was built. It can be concluded that the multilayer TiSiN/TiAlN nano-coating showed better properties of the resistance to thermal fatigue shock in machining Ti-6Al-4V

Coupling mechanisms of static and dynamic loads during the ultrasonic impact strengthening of Ti-6Al-4V

The ultrasonic impact-strengthening technology was used to effectively improve the fatigue resistance of the key components, which could be attributed to the association of static and dynamic impact loads. It is widely used in the high-performance manufacturing of critical aerospace components, such as titanium alloys. During the ultrasonic impact-strengthening process, the static and dynamic loading can provide two different deformation mechanisms, which may cause differences in the strengthening effect of the titanium alloys. This study developed an ultrasonic impact-strengthening test platform to investigate the influence mechanisms of static loads and cyclic dynamic impact loads in the ultrasonic impact-strengthening process. Meanwhile, the experiment platform was based on displacement control and could apply either static loads or cyclic dynamic impact loads individually on the surface of the Ti-6Al-4V. The force values in the static load experiments, cyclic dynamic impact experiments, and ultrasonic impact strengthening experiments were analyzed. The results show that the force value in the ultrasonic impact strengthening process is not only the superposition of the static load and the cyclic dynamic impact load, but indicating a coupling effect. The force of ultrasonic impact strengthening process increased by more than 55% compared to the sum of the static load and the maximum dynamic impact load. Moreover, the deformation strain rate of Ti-6Al-4V under separate cyclic dynamic impact loading was simulated. During the ultrasonic impact strengthening process, the deformation strain rate of Ti-6Al-4V could reach 960 s−1, 1587 s−1, and 2043 s−1 when the cyclic impact depths of 5 μm, 10 μm, and 15 μm, respectively. At the same time, the material surface hardening mechanism under the high strain rate cyclic impact loading was analyzed. The hardness of Ti-6Al-4V after the ultrasonic impact-strengthening process increased by more than 11% compared to the original hardness. At last, the strengthening performance of Ti-6Al-4V after the ultrasonic impact strengthening was evaluated. The strengthening mechanisms of static and dynamic loads during the ultrasonic impact strengthening of Ti-6Al-4V was investigated

Grain size dependence of modified material constitutive model for OFHC copper

Grain refinement occurs during manufacturing process, and grain size has a significant impact on mechanical properties. It is essential to establish the relationship between grain evolution and plastic behavior. The quasi-static and dynamic compression experiments for three kinds of oxygen-free high-conductivity (OFHC) copper with different grain sizes have been carried out. The true stress-strain curves were corrected to the isothermal curves, in which the temperature dependence of the specific heat capacity was used. It was found that the grain size has a significant influence on the yield stress, strain rate sensitivity, and thermal softening. A new phenomenon-based constitutive model considering the grain size and the coupling between strain hardening and strain rate was developed, and the results of the quasi-static and dynamic compression experiments were used to fit the parameters. The experimental data and the predicted results of the newly developed constitutive model agree well, which proves the constitutive model has a high prediction accuracy. The fitted constitutive model depicted that the strain rate sensitivity increases and thermal softening degree declines as the rise of the grain size of OFHC copper. The developed constitutive model could be used to the numerical simulation for the manufacturing process of OFHC copper considering the grain refinement

Asymmetrical cutting-edge design of broaching tool based on FEM simulation

Finite element method (FEM) based model of broaching Inconel 718 was established to optimize an asymmetrical cutting-edge by the commercial software AdvantEdge in this paper. The asymmetrical cutting-edge form by two circles was proposed. The power-law (P-L) material constitutive model, including strain hardening, strain rate hardening and thermal softening was developed by split Hopkinson pressure bar (SHPB) test and high-temperature Vickers hardness test. The parameters of thermal properties were measured by laser flash diffusivity apparatus. The friction model between TiN (The outer layer coating material of the tool) and Inconel 718 was obtained by ball on disc friction test. The simulated and experimental results have been compared comprehensively in the items of chip formation and tool wear width. The results showed that the simulated results were consistent with the experimental results. The average errors of chip curl radius, chip thickness, wear width of rake face and flank face were 7.68%, 3.56%, 13.64% and 12.24%, respectively, which proved the accuracy of the simulation model. Then the optimal asymmetrical cutting-edge of broach were obtained through orthogonal test based on the accurate FEM model. The rise per tooth of the optimal broach tool is 0.04 mm/tooth, the rake angle is 13°, the circle radius R1 is 0.13 mm, and the circle radius R2 is 0.02 mm

FE modeling and simulation of the turning process considering the cutting induced hardening of workpiece materials

The accuracy of the cutting simulation model greatly depends on the constitutive models, thermophysical models, and friction models. However, accurate modeling of physical and mechanical relationships is not enough. The physical and mechanical behavior of the machined surface from the last cut should be modelled in the FE model. In this study, the cutting simulation model of S316L stainless steel was established. The above model consists of two subsequent simulated cuts. The first simulated cut was used to obtain the machined surface with the residual stress, and the second simulated cut was subsequent with the first cut to obtain the actual simulated results. The constitutive model was obtained by the split Hopkinson pressure bar (SHPB) and high-temperature hardness experiments. The specific heat capacity and thermal conductivity models were developed by laser thermal conductivity experiments with various temperatures. The friction model between the workpiece and the tool was established by orthogonal cutting experiments. The simulated cutting forces of the first and second cut were extracted and compared with the experimental results to verify the accuracy of the simulation models. The results showed that the average error of cutting forces for the first cut is 28.33 %, but that for the second cut is 8.02 %, which verifies the accuracy of the two-subsequent cutting simulation model. Additionally, the significant differences in the simulated cutting forces between the first and second cutting depict that the residual stress cannot be ignored for the accuracy verification of cutting simulation models