Yuepeng Song

Finite element analysis of the effect of friction in high pressure torsion

High pressure torsion (HPT) is one of the most important techniques among various methods that create severe plastic deformation in the production of bulk materials with nano/ultrafine grained microstructures. Since the driving force in deforming the workpiece in HPT is surface friction, understanding of the friction effect is critical for successful application of HPT. In this study, the friction effect in HPT was analyzed using the finite element method. The distribution of effective strain on the contact surface of the HPT samples under different friction conditions was investigated. The friction force influenced the effective strain more in the middle and edge regions than in the central region. The condition for the minimum friction factor that could achieve a sticking condition between the surfaces of the dies, and the samples in the middle and edge regions, was investigated. There was a critical friction coefficient in which the effective strain varies sharply with an increasing friction coefficient.

Analysis of stress states in compression stage of high pressure torsion using slab analysis method and finite element method

High pressure torsion (HPT) is useful for achieving substantial grain refinement to ultrafine grained/nanocrystalline states in bulk metallic solids. Most publications that analyzed the HPT process used experimental and numerical simulation approaches, whereas theoretical stress analyses for the HPT process are rare. Because of the key role of compression stage for the deformation of HPT, this paper aims to conduct a theoretical analysis and to establish a practical formula for stress and forming parameters of HPT process using the slab analysis method. Three equations were obtained via equations derivation to describe the normal stress states corresponding to the three zones of plastic deformation for HPT process as stick zone, drag zone and slip zone. As to the compression stage of HPT, the stress distribution results using the finite element method agree well with those using the slab analysis method. There are drag and stick zones on the contact surface of the HPT sample, as verified by the finite element method (FEM) and slab analysis method.

Thickness inhomogeneity in hardness and microstructure of copper after the compressive stage in high-pressure torsion

Distinction between plastic deformation occuring in compression and compression-torsion stages is important for understanding the properties and microstructures of materials processed by high-pressure torsion (HPT). In the present study, remarkable through-thickness inhomogeneities of hardness and microstructure were found in the samples processed by compression stage of HPT. Three regions on the radial direction plane of compressed disks were defined to display the inhomogeneity: edge zone (high hardness), radial medium zone (uniform hardness) and center zone (low hardness near the surface and high hardness in the thickness central plane). A low hardness region in the center near the surface was detected and found to stretch along the upper and bottom surfaces of the disks compressed by low pressure. This low hardness region was also found to decrease with increasing the pressure. Not only the hardness but also the microstructure through-thickness inhomogeneity is attributed to stress and strain distribution in the disk as well as to a huge friction between the anvil and the disk during processing.

Microstructural evolution during superplastic bulge forming of Ti-6Al-4V alloy

Abstract An analytical model reflecting the microstructural evolution was made on the basis of Dutta and Mukherjees work, and applied to the superplastic bulge forming of Ti–6Al–4V alloy. Bulge forming of fine grain (2.5 μm) Ti–6Al–4V alloy sheets was conducted at 900°C, at three strain rates of 2.5×10 −4 , 5×10 −4 and 1×10 −3 s −1 . It was found that the grain growth rate of biaxially bulge formed samples was quite different from that of uniaxially deformed specimens and that the grain growth rate was independent of the strain rate imposed at a fixed temperature. After incorporating the grain growth rate of biaxially deformed material into the analytical model, the bulge forming process was successfully controlled at different strain rates. Prediction of the thickness distribution by the modified model was in agreement with the experimental results.

Hardness and microstructure of interstitial free steels in the early stage of high-pressure torsion

The hardness and microstructure distributions in interstitial free (IF) steel disks processed via the high-pressure torsion (HPT) process with an early stage (up to 1 turn) are investigated using experimental and simulation approaches. The results indicate that the deformation in the HPT-processed IF steel disk is inhomogeneous, providing almost linearly increasing hardness from the center to the edge regions. In particular, near the surface of the disk is a soft region that shrinks with increasing numbers of revolutions. Compared with the compression-only disks by the HPT die, there is a hardness hill in the center region of the HPT-processed disk. The hardness distributions in the HPT disks indicate that the deformation proceeds gradually from the edge to the center with the degree of revolutions. In addition, as the degree of revolutions increases, the strain in the center region increases and the plastic deformation becomes uniform along the radial direction. The finite element analyses strongly support the conclusions of the experimental results.

Fabrication of W-Cu alloy via combustion synthesis infiltration under an ultra-gravity field

Tungsten copper alloy with a tungsten concentrate of 70 vol% was prepared by self-propagating high-temperature synthesis in an ultra-gravity field. The phase structures and components of the W-Cu alloy fabricated via this approach were the same as those via traditional sintering methods. The temperature and stress distributions during this process were simulated using a new scheme of the finite element method. The results indicated that nonequilibrium crystallization conditions can be created for combustion synthesis infiltration in an ultra-gravity field by the rapid infiltration of the liquid copper product into the tungsten compact at high temperature and low viscosity. The cooling rate can be above 100,000 K/s and high stresses in tungsten (~5 GPa) and copper (~2.6 GPa) were developed, which passivates the tungsten particle surface, resulting in easy sintering and densifying the W-Cu alloy. The reliability of the simulation was verified through temperature measurement and investigation of the microstructure. The W-Cu composite-formation mechanism was also analyzed and discussed with the simulation results.

Computer Simulation and Verification of Adiabatic Temperature and Apparent Activity Energy of the NiO/Al Aluminothermic System

Recently, self-propagating high-temperature synthesis (SHS), related to metallic and ceramic powder inter- actions, has attracted huge interest from more and more researchers, because it can provide an attractive, energy-efficient approach to the synthesis of simple and complex materials. The adiabatic temperature Tad and apparent activation energy analysis of different thermit systems plays an important role in thermodynamic studies on combustion synthesis. After establishing and verifying a mathematic calculation program for predicting adiabatic temperatures, based on the thermo- dynamic theory of combustion synthesis systems, the adiabatic temperatures of the NiO/Al aluminothermic system dur- ing self-propagating high-temperature synthesis were investigated. The effect of a diluting agent additive fraction on combustion velocity was studied. According to the simulation and experimental results, the apparent activation energy was estimated using the Arrhenius diagram of ln(v/Tad)~1/Tad based on the combustion equation given by Merzhanov et al. When the temperature exceeds the boiling point of aluminum (2,790 K), the apparent activation energy of the NiO/ Al aluminothermic system is 64 ± 14 kJ/mol. In contrast, below 2,790 K, the apparent activation energy is 189 ± 15 kJ/ mol. The process of combustion contributed to the mass-transference of aluminum reactant of the burning compacts. The reliability of the simulation results was experimentally verified.