Research on adiabatic shear failure character of pure copper and aluminum bronze based on empirical electron theory

High-speed machining experiment of pure copper and aluminum bronze (QAl9-4) is carried out at the same cutting condition to obtain the different chips. The valence electron structure (VES) parameters that affect adiabatic shearing sensitivity were studied in this paper. The results show the bond energy and lattice electron number are related to adiabatic shearing sensitivity. Adiabatic shearing sensitivity increases with the increase of bond energy, and the decrease of lattice electron number. For pure copper, the bond energy is smaller and the lattice electron number is higher, so its adiabatic shearing sensitivity is low. The shape of chip is approximate ribbon. For aluminum bronze (QAl9-4), the bond energy is increased and the lattice electron number is decreased due to aluminum (Al) addition, so its adiabatic shearing sensitivity is higher than that of pure copper. The serrated chip divided uniformly by Adiabatic Shear Band (ASB) was formed. The basis can be provided for optimizing process parameters, improving and selecting materials with different cutting performance by studying the specific alloy elements on the influence of adiabatic shearing sensitivity to predict the chip morphology to some extent in VES level.

In this paper, the influence of valence electron structure parameters on the adiabatic shearing sensitivity has been studied for two grade steels (30CrMnMo and C45E4) as a function of covalent pair number and lattice electron number by empirical electron theory of solids and molecules in high‐speed penetration process. The research shows that the adiabatic shearing sensitivity increases with the increase of the covalent electron pair number nA, decreases with the increase of the lattice electron number nl. The nA of each structure unit in 30CrMnMo steel is larger than that in C45E4steel, the nl of each structure unit in 30CrMnMo steel is smaller than that in C45E4steel. Therefore, the adiabatic shearing sensitivity is higher for 30CrMnMo targets damaged by adiabatic shear failure, and yet C45E4 targets were damaged by ductile fracture without any adiabatic shear band. The basis can be provided for appropriately selecting and designing materials with different adiabatic shearing sensitivity by studying the specific alloy elements on the influence of adiabatic shearing sensitivity in valence electron structure level.

The effect of specific alloy elements on adiabatic shearing sensitivity was studied by cutting experiment for the industrial pure titanium (TA2) and titanium alloy (Ti-6Al-4V) in the level of valence electron structure (VES). The valence electron structure (VES) parameters that affect adiabatic shearing sensitivity were studied in this paper. The results show the electron density of heterophase boundary and the lattice electron density are related to adiabatic shearing sensitivity. The higher the electron density of heterophase boundary, the lower the lattice electron density, the higher adiabatic shearing sensitivity, and the easier the formation of the serrated chips. The electron density of heterophase boundary and the lattice electron density have coupling effect on adiabatic shearing sensitivity. The adding of Al and V elements for Ti-6Al-4V can increase the electron density of heterophase boundary and reduce the lattice electron density, so its adiabatic shearing sensitivity is higher than that of TA2, which would produce easily serrated chips divided evenly by the adiabatic shear band (ASB) and enlarge the degree of serration to exacerbate the tool vibration.

In this paper, the microscopic mechanism of 080A15 and 30CrMnMo steel chip failure modes has been studied at the level of valence electron structure (VES) based on the empirical electron theory (EET) of solids and molecules. Studies show the bonding ability and the lattice electron density are related to the chip failure form. If the bonding ability is weak and the lattice electron density is high, the chip is more prone to ductile fracture. Conversely, the chip is more prone to adiabatic shear failure to form the serrated chip divided uniformly by adiabatic shear bands (ASBs). For 080A15 steel, the bonding force between atoms is weak, and the lattice electron density is very high. It is difficult to produce the thermal-mechanical instability. Thus, the chip was formed by ductile fracture. For 30CrMnMo steel, the total bonding ability of each structural unit of carbon and alloy elements is stronger, and the lattice electron density of each structural unit is weaker, the temperature can rise instantaneously to a very high level to form the serrated chip with the adiabatic shear failure. The results of this research provide useful insights on material design and selection in high-speed cutting by studying the specific alloy elements on the influence of chip failure mechanism at the VES level.

The cutting experiments were conducted for the industrial pure titanium (TA2) and titanium alloy (TC4) under the same cutting condition to obtain the chips of different shapes. The mechanisms of action of different alloy elements for adiabatic shearing sensitivity were studied from the perspective of valence electron structure. The results show that the interface electron density and the lattice electron density are related to adiabatic shearing sensitivity. The bigger the interface electron density is, the higher the interfacial bonding strength, and the higher the adiabatic shearing sensitivity. Also, the lower the lattice electron density is, the lower the thermal conductivity, and the higher the adiabatic shearing sensitivity. For TA2, the electron density is smaller and the lattice electron density is higher, so its adiabatic shearing sensitivity is low. The shape of chip is approximate ribbon. For TC4, the bi-phase interfaces are caused, interface electron density is increased and the lattice electron density is decreased due to aluminum (Al) and titanium (Ti) addition, so its adiabatic shearing sensitivity is higher than that of TA2. The saw-tooth chip divided uniformly by Adiabatic Shear Band (ASB) was formed. The basis can be provided for optimizing process parameters and selecting materials with different cutting performance by studying the specific alloy elements on the influence of adiabatic shearing sensitivity to predict the chip morphology tosomeextent in valence electron structure level.

The cutting experiments were conducted for the industrial pure titanium (TA2) and titanium alloy (Ti-6Al-4V) under the same cutting condition to obtain different chip shapes. The energy barrier formed adiabatic shear band (ASB) was calculated. It shows that the smaller the energy barrier, the stronger the adiabatic shearing sensitivity and the easier the occurrence of serrated chip. The mechanisms of action of different alloy elements for adiabatic shearing sensitivity and surface roughness were studied. For TA2, its adiabatic shearing sensitivity is low because of low strength and high thermal conductivity. The shape of the chip is approximate ribbon. For Ti-6Al-4V, the bi-phase interfaces are caused, strength is increased, and thermal conductivity is decreased due to Al and Ti addition, so its adiabatic shearing sensitivity is higher than that of TA2. The serrated chip divided uniformly by ASB was formed. By and large, there is a positive correlation between adiabatic shear sensitivity and surface roughness in the other same conditions. The basis can be provided for optimizing process parameters, improving surface quality, and selecting materials by studying the specific alloy elements on the influence of adiabatic shearing sensitivity to some extent.

As a model material, commercial pure titanium was rolled to plates with different dislocation boundaries. The dynamic mechanical response of Ti specimen was analyzed during impacted with Split Hopkinson Pressure Bar (SHPB) at different strain rates, and microstructure evolution was investigated using optical microscopy and transmission electron microscopy. It was found that adiabatic shear sensitivity was decreased with increasing strain rates for all as-annealed, 25% and 50% cold rolled states. To the contrary, for 70% cold rolled state the adiabatic shear sensitivity was increased with increasing strain rates. The microstructure of adiabatic shear bands (ASBs) were developed from elongation morphology to fine equiaxed grains in the specimens of 25% cold rolled state, and ASBs became broader with increasing strain rate.

Microstructure evolution of adiabatic shear bands and mechanisms of saw-tooth chip formation in machining Ti6Al4V

Adiabatic shear sensitivity of ductile metal based on gradient-dependent JOHNSON-COOK model

Influence of pre-twinning on adiabatic shear sensitivity of AM30 magnesium alloy

Quasi-static compression/tension, dynamic compression/shear mechanical property tests, fracture morphology characterization, dynamic response experiment, numerical simulation, and calculation of the ultimate penetration velocity of the high-strength steel under the impact of large mass low-velocity fragments and small mass high-velocity fragments were carried out to study the ballistic performance of high-strength steel used for armor vehicle protection and clarify its dynamic response under Ballistic impact. The fracture mode of the high-strength steel under dynamic impact compression and shear was shear failure. A large number of fine parabolic shear dimples were formed on the fracture surface. The high-strength steel had an obvious strain rate strengthening effect and adiabatic shear sensitivity. The plug formed under the low-velocity impact of a large mass fragment was a cone shape, and the impact area of the target plate underwent slight bending plastic deformation. The damage of the fragment to the target plate includes both adiabatic shear failure and ordinary shear failure. Under the high-velocity impact of small mass fragments, the fragments were seriously deformed and destroyed, and the damage model of the target plate is adiabatic shear failure.

New conception of adiabatic shear bands (ASB) and adiabatic shear failure mechanisms are proposed as special type of critical phenomen, structural-scaling transition, in the ensembles of microshears, governed by the characteristic non-linearity (metastability) of stored (free) energy of solid with mesodefects. Corresponding free energy release kinetics provides experimentally observed ASB induced staging of plastic strain localization and transition to adiabatic shear failure. ASB staging follows to collective properties of microshears ensemble given by the self-similar solutions of evolution equation providing spatial-temporal microshears localization, momentum transfer and damage localization. The criticality of ASB induced plastic strain localization and failure allows us to avoid the discrepancy in the interpretation of ASB effects as thermo-plastic instability in the balance of the stored energy and structural DRX transformation. The microshear ensemble is considered as the second phase and initiation of collective modes provide different staging according to the metastability decomposition and ASB scaling properties following to the self-similar solutions. Self-similar nature of microshears collective modes providing the ASB dynamics is analyzed as the mechanism of steady plastic wave front unversality in shocked materials. The dynamic split Hopkinson pressure bar tests were conducted with AlMg6 alloy combined with “in-situ” imaging of temperature kinetics by CEDIP Silver 450M high-speed infrared camera with conclusion of the secondary role of thermoplastic instability at the ASB staging. The microstructural study performed by an electron microscopy revealed the correlated behavior of the ensemble of defects, which can be classified as a structural transition and precursor of ASB induced strain localization and failure. The modeling reflecting the links of self-similar solutions in microshear ensembles with relaxation properies and damage localization was applied for the comparative analysis of ASB staging and temperature dynamics given be the infrared imaging.

Background: Ultrafine-grained (UFG) titanium, which is with high yield strength, biocompatibility and corrosion resistance, is widely used in biomedical and industrial applications. However, adiabatic shear localization (ASL) is often observed in UFG materials due to their worse deformation stability under impact loading. This instability will easily result in formation of adiabatic shear bands (ASB), a narrow band located in the ASL zoom, and finally cause the fracture of material. The main objective of this work is to study the adiabatic shear behavior of UFG titanium under impact loading, including macro- and micro-properties, temperature rise, ASB failure, etc. Methods: A synchronization apparatus, which consisted of Kolsky bar system, high-speed camera system and high-speed infrared temperature measuring system, was set up to carry out the in-situ study of the mechanical properties, temperature rise, and adiabatic shear failure process of UFG pure titanium. Microstructure of the material was also analyzed in this work. Results: The critical strain of UFG pure titanium for adiabatic shear localization is about 0.37 and 0.69 for UFG Ti and CG Ti, respectively. The peak shear stress of UFG Ti is 500MPa. The propagation velocity of ASB in UFG titanium is 533~800m/s, and 160~320m/s for CG Ti. The temperature rsie within ASB of UFG titanium is 307~732℃, and 212-556℃ for CG Ti. The intense temperature rise is after the peak stress and the initiation of ASB most of the time. Conclusion: UFG Ti has good mechanical properties, however, it is easier to form ASB and cause adiabatic shear failure under impact loading when compared with CG Ti. Temperature rise may not play a major role in the formation of ASB in UFG Ti, but may be the consequence of ASB. Results of this work will help researchers better understand the failure of UFG metals under impact loading.

0.1wt% Boron addition effect on dynamic compression properties of Ti-6Al-4V (Ti-64A) alloy are investigated by Split Hopkinson Pressure Bar (SHPB). In the study, and relative damage mechanism is also analyzed. The results show that, as-cast microstructure is refined due to 0.1% Boron addition and also to lower the non-uniform distribution of strain, stress or local concentration due to inharmonic deformation. As well as both dynamic strain and average dynamic flow stress is improved with a reduction of the sensitivity of adiabatic shear behavior. As deformation microstructure loaded at high strain rate with 0.1wt% boron addition, Dynamic strain and maximum absorbed energy is decrease 10%~30% compare with Ti-64A alloy. Both Ti-64A and Ti-6Al-4V-0.1B (Ti-64B), average dynamic flow stress is close. At high speed impact load, it exhibits a damage of adiabatic shear and TiB phases bear loading during fracture. Adiabatic shear band ismain reason of Ti-64A and Ti-64B alloys fracture failure through the deformed specimens’ microstructure observation. Adiabatic shear band formation and expansion is a precursor of material shear fracture failure. Deformation cavity can be formation between TiB phase and matrix during the deformation process, but not the main reason of material fracture failure.

In this paper, the shock phase transformation of β phase in Ti-5Al-5Mo-5V-3Cr-0.5Fe (Ti-5553) was investigated. Split Hopkinson Pressure Bar (SHPB) and light gas gun were employed to investigate the dynamic properties under high strain rates from 1000s-1to 3500s-1. Microstructure characterization was carried out by optical microscopy (OM), scanning electronic microscopy (SEM) and transmission electron microscope (TEM). The experimental results demonstrate that the Ti-5553 alloy with β phase exhibit no obvious strain rate hardening effect with the high strain rate from 1000s-1to 3000s-1. However, compared with the quasi-static compression test results (10-3s-1), this alloy shows an evident strain rate hardening effect, with the yield strength significantly improved. Second time loading indicates light gas gun dynamic tensile loading and then SHPB dynamic compression loading in Ti-5553 alloy with β phase. The results show that the shock-induced β to αʺ martensite phase transformation dramatically influences the postshock mechanical properties of these alloys. The yield strength of this alloy decreased after the shock wave effect of light gas gun, its ductility increasing. Higher shock pressures yielded an increased dislocation density and a gradual increase in the yield strength. Adiabatic shear band (ASB) exists in second time loading Ti-5553 alloy under 103s-1strain rate. SHPB loaded the alloy: The results show that the Ti5553 alloy with β phase is adiabatic shear failure in high strain rate (3000s-1).