The effects of aging treatment and strain rates on damage evolution in AA 6061 aluminum alloy in compression

AA 6061 aluminum alloy in T4, T6 and T8 temper were subjected to quasi-static compressive loading at a strain-rate of 3.2 × 10⁻³ s⁻¹ and dynamic compressive loading at strain-rates between 7.0 × 10³ and 8.5 × 10³ s⁻¹. The effects of strain rates and temper condition on the deformation behavior of the alloy are discussed. Under the quasi-static loading, deformation was relatively homogeneous and controlled by strain hardening, which is more pronounced in the naturally aged than the artificially aged alloys. Thermal softening played a dominant role under impact loading leading to strain localization along narrow bands called adiabatic shear bands (ASBs). Both deformed bands consisting of aligned second phase particles and transformed bands consisting of fine recrystallized grains were observed. The average size of the recrystallized grains in the transformed bands is about 600 nm and varies slightly depending on the temper condition. The fine grains are suggested to form by dynamic recrystallization. The T4 alloy showed the highest propensity for thermal softening, strain localization and cracking under impact loading while the T8 alloy showed the least tendency. The degree of recrystallization in the transformed band is influenced by temper condition with T8 alloy having the highest fraction of unrecrystallized grains inside the transformed bands. This is related to the temperature rise in the transformed bands that was estimated to be highest in the T4 alloy and lowest in the T8 alloy. The combined effects of high temperature and severe strain inside the transformed bands caused dissolution of second phase particles and induced microstructural changes that resulted in less silicon inside the transformed bands than in the adjacent region.     

Effects of particulate reinforcement and strain‐rates on deformation and fracture behavior of Aluminum 6061‐T6 under high velocity impact

Aluminum matrix composites (AMC) exhibit an attractive combination of mechanical and physical properties such as high stiffness and low density, which favors their utilization in many structural applications. Thus, increasing the structural applications of AMC is the driving force for the need to adequately understand their deformation and failure mechanisms under various types of loading conditions. In this study, plastic deformation of alumina particle reinforced Aluminum 6061‐T6 matrix composite is investigated and compared to that of an un‐reinforced Aluminum 6061‐T6 alloy at high strain‐rates under compressive loading. Dynamic stress‐strain curves are obtained using direct impact Split Hopkinson Pressure Bar (SHPB). Particulate reinforcement increases the deformation resistance of the aluminum alloy at high strain‐rates. Strain localization along narrow adiabatic shear bands is observed in both the reinforced and un‐reinforced alloy. Whereas the microstructure of shear bands in un‐reinforced alloy showed finer grain size compared to that of the bulk material, the shear bands observed in the AMCs are darker than the bulk material and the reinforcing particles are observed to be more closely spaced along the shear bands.     

Texture evolution and dynamic mechanical behavior of cast AZ magnesium alloys under high strain rate compressive loading

In this study, texture and compressive mechanical behavior of three cast magnesium alloys, including AZ31, AZ61 and AZ91, were examined over a range of strain rates between 1000 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. Texture measurements showed that after shock loading, initial weak texture of the cast samples transformed to a relatively strong (00.2) basal texture that can be ascribed to deformation by twinning. Furthermore, increasing the aluminum content in the alloys resulted in increase in the volume fraction of β-Mg₁₇Al₁₂ and Al₄Mn phases, strength and strain hardening but ductility decreased at all strain rates. Besides, it was found for each alloy that the tensile strength and total ductility increased with strain rate. By increasing the strain rate, the maximum value of strain hardening rate occurred at higher strains. Also, it is suggested that a combination of twinning and second phase formation would affect the hardening behavior of the cast AZ magnesium alloys studied in this research.     

Effects of heat treatment on the ballistic impact properties of Inconel 718 for jet engine fan containment applications

The effects of heat treating Inconel 718 on the ballistic impact response and failure mechanisms were studied. Two different annealing conditions and an aged condition were considered. Large differences in the static properties were found between the annealed and the aged material, with the annealed condition having lower strength and hardness and greater elongation than the aged. High strain rate tests show similar results. Correspondingly large differences were found in the velocity required to penetrate material in the two conditions in impact tests involving 12.5 mm diameter, 25.4 mm long cylindrical Ti-6-4 projectiles impacting flat plates at velocities in the range of 150–300 m/s. The annealed material was able to absorb over 25 percent more energy than the aged. This is contrary to results observed for ballistic impact response for higher velocity impacts typically encountered in military applications where it has been shown that there exists a correlation between target hardness and ballistic impact strength. Metallographic examination of impacted plates showed strong indication of failure due to adiabatic shear. In both materials localized bands of large shear deformation were apparent, and microhardness measurements indicated an increase in hardness in these bands compared to the surrounding material. These bands were more localized in the aged material than in the annealed material. In addition, the annealed material underwent significantly greater overall deformation before failure. The results indicate that lower elongation and reduced strain hardening behavior lead to a transition from shear to adiabatic shear failure, while high elongation and better strain hardening capabilities reduce the tendency for shear to localize and result in an unstable adiabatic shear failure. This supports empirical containment design methods that relate containment thickness to the static toughness.     

EBSD study on microstructure and texture of friction stir welded AA5052-O and AA6061-T6 dissimilar joint

Friction stir welded AA5052-O and AA6061-T6 dissimilar joint has a more obvious impact on microstructure and texture evolution compared to single material welding due to differences in physical and chemical parameters between two aluminum alloys. Microstructure, texture evolution and grain structure of AA5052-O and AA6061-T6 dissimilar joint were investigated by means of OM,EBSD and TEM measurements. Experimental results showed that FS weld was generalized in four regions–nugget zone (NZ),thermomechanically affected zone (TMAZ),heat affected zone (HAZ) and base metals (BM), using standard nomenclatures. NZ exhibited the complex structure of the two materials with flowing shape and mainly composed of the advancing side material Subgrain boundaries in weld nugget zone gradually transformed into high angle grain boundaries by absorbing dislocation and accumulating misorientations. Grain refinement of weld nugget zone was achieved by dynamic recrystallization. In the friction stir welding process, the presence of the shear deformation in weld made {001} < 100 > C cube texture, {123} < 634 > S texture in BM gradually transformed into {111} < 1(−)12(−) > A1¹ shear texture. HABs distribution were most significant in nugget followed by RS and then by AS. In TMAZ and NZ, numerous precipitates and lots of dislocations were observed.     

Relationship between mechanical properties and microstructural response of 6061-T6 aluminum alloy impacted at elevated temperatures

The high temperature impact properties and microstructural evolution of 6061-T6 aluminum alloy are investigated at temperatures ranging from 100 to 350 °C and strain rates ranging from 1 × 10³ to 5 × 10³ s⁻¹ using a compressive split-Hopkinson pressure bar (SHPB) system. It is found that the flow response and microstructural characteristics of 6061-T6 aluminum alloy are significantly dependent on the strain rate and temperature. The flow stress and strain rate sensitivity increase with increasing strain rate or decreasing temperature. Moreover, the temperature sensitivity increases with both increasing strain rate and increasing temperature. The flow stress–strain response of the present 6061-T6 alloy specimens can be adequately described by the Zerilli–Armstrong fcc model. The grain size and dislocation cell size increase significantly with a decreasing strain rate or an increasing temperature. The higher flow stress is the result of a smaller grain size and smaller dislocation cell size. The stacking fault energy of the deformed specimens has a value of 145.78 mJ/m².     

The stamping behavior of an early-aged 6061 aluminum alloy

The stamping behavior of 6061 aluminum alloy with various conditions of early-aging is investigated in the present study. The relationship between the stamping performance, microstructure and mechanical property for this alloy is also discussed. Experimental results show that the 6061 aluminum alloy with a 10–30 min. early-aging at 160 °C will exhibit excellent stamping performance. The burnished surface of these treated alloys can reach a quite high value of 47%. Meanwhile, the mechanical strength and impact toughness have important effects on the stamping behavior of 6061 aluminum alloy. The moderate values of mechanical strength and toughness will exhibit an optimal stamping performance.     

Temperature dependent work hardening in Ti–6Al–4V alloy over large temperature and strain rate ranges: Experiments and constitutive modeling

The plastic deformation behaviors of Ti–6Al–4V alloy over wide ranges of strain rate (from 10⁻⁴ to 10⁴ s⁻¹) and temperature (from 20 to 900 °C) are investigated by the quasi-static and dynamic uniaxial compression tests. The microstructure evolution of Ti–6Al–4V alloy at different temperatures is discussed. Material generates higher ductility and formability when temperature is higher than 500 °C, which leads to the decrease of work hardening rate. The true stress–strain responses are modeled with the JC, modified JC, KHL and modified KHL models. In detail, a temperature dependent work hardening function is introduced into the original JC and KHL models. The parameters of the four models for Ti–6Al–4V alloy are calculated by GA optimization method. The average standard deviations between the experimental and calculated flow stresses range from 4% to 13%, which validates the accuracy of the models. In addition, comparison of flow stresses at dynamic (10,000 s⁻¹), the work hardening rates at dynamic (7500 s⁻¹), as well as the quasi-static jump experiments were proposed to further validate the models. The modified JC and modified KHL models could characterize the temperature dependent work hardening effect for Ti–6Al–4V alloy over large strain rate and temperature ranges.     

Analytical and experimental studies on mechanical behavior of composites under high strain rate compressive loading

An analytical method is presented for the prediction of compressive strength at high strain rate loading for composites. The method is based on variable rate power law. Using this analytical method, high strain rate compressive stress–strain behavior is presented up to strain rate of 5000 s⁻¹ starting with the experimentally determined compressive strength values at relatively lower strain rates. Experimental results were generated in the strain rate range of 472–1957 s⁻¹ for a typical woven fabric E-glass/epoxy laminated composite along all the three principal directions. The laminated composite was made using resin film infusion technique. The experimental studies were carried out using compressive split Hopkinson pressure bar apparatus. It was generally observed that the compressive strength is enhanced at high strain rate loading compared with that at quasi-static loading. Also, compressive strength increased with increasing strain rate in the range of parameters considered. Analytically predicted results are compared with the experimental results up to strain rate of 1957 s⁻¹.     

Impact and fracture response of sintered 316L stainless steel subjected to high strain rate loading

This paper describes the use of a material testing system (MTS) and a compressive split-Hopkinson bar to investigate the impact behaviour of sintered 316L stainless steel at strain rates ranging from 10^− 3 s^− 1 to 7.5 × 10³ s^− 1. It is found that the flow stress–strain response of the sintered 316L stainless steel depends strongly on the applied strain rate. The rate of work hardening and the strain rate sensitivity change significantly as the strain rate increases. The flow behaviour of the sintered 316L stainless steel can be accurately predicted using a constitutive law based on Gurson's yield criterion and the flow rule of Khan, Huang and Liang (KHL). Microstructural observations reveal that the degree of localized grain deformation increases at higher strain rates. However, the pore density and the grain size vary as a reversible function of the strain rate. Impacts at strain rates higher than 5.6 × 10³ s^− 1 are found to induce adiabatic shear bands in the specimens. These specimens subsequently fail as a result of crack propagation along the dominant band. The fracture surfaces of the failed specimens are characterized by dimple-like structures, which are indicative of ductile failure. The depth and the density of these dimples are found to decrease with increasing strain rate. This observation indicates a reduction in the fracture resistance and is consistent with the observed macroscopic flow stress–strain response.     

Influence of spectrum loading sequences on fatigue life in a high-temperature environment

This paper discusses the fatigue life behaviour of aluminium alloy AA6061-T6 under spectrum loadings. Load sequences in spectrum loadings can have significant effects on fatigue life at room temperature and within the elevated temperature range. The main objective of this paper is to investigate the influences of load sequences effect on fatigue life at elevated temperature. Fatigue strain signal was obtained from the engine mount bracket of an automobile under normal driving conditions. Constant amplitude loading, high-to-low, and low-to-high loading sequences were then derived from the original fatigue strain signal to observe the fatigue behaviour at both room and elevated temperatures. The fatigue test was performed on AA6061-T6 specimen according to the ASTM E466 standard using a 100 kN servo-hydraulic fatigue testing machine within the temperature range of 27–250 °C. The elevated temperature range was chosen based on the maximum temperature of the engine mount bracket and the extreme temperature of the cylinder head that can be reached in service. After the test, fatigue fracture surfaces were sectioned and inspected using a high-magnification microscope. Results show that fatigue life behaviour at room temperature was significantly influenced by the load sequences in spectrum loadings. On the other hand, the effect of load sequences at a higher temperature was reduced.     

Control of load/displacement responses of AA6061-T6 and T4 circular extrusions under axial compressive loads

Axial crushing of AA6061-T6 and T4 circular extrusions with variable wall thicknesses was completed under both dynamic and quasi-static loading conditions to investigate the capability of controlling the load/displacement responses of the extrusions. Circular specimens with a nominal original wall thickness of 3.175 mm, an outer diameter of 50.8 mm, and a length of 300 mm were considered. Variations of the wall thickness in the axial direction were completed by material removal from the extrusion sidewall prior to crushing tests. Cutters used in this research had a height of 20 mm and blade tip widths of 1.0 mm. A curved deflector was used to flare the cut petalled sidewalls and facilitate the cutting system. Results from the impact tests illustrated that an initial peak cutting force with a magnitude of 1.08-1.74 times higher than that for the quasi-static loading was needed to initiate the cutting deformation mode. After this transient cutting stage, the load/displacement responses were observed to be similar to that from the quasi-static tests except for some slight fluctuations resulting from a minor amount of material fracture which occurred on the petalled sidewalls. A lesser extend of material fracture was observed on the T4 temper specimens due to the work hardening material property of the T4 temper condition. The mean cutting force from the dynamic tests were determined to be in the range of 0.92-1.09 times the mean force from the corresponding quasi-static test. Control of load versus displacement responses of the extrusions under both impact and quasi-static compressive loading conditions was accomplished through the variation of the wall thickness along the axial direction of extrusions.     

Experimental investigations on the behaviour of short to long square aluminium tubes subjected to axial loading

The transition between progressive and global buckling of axially loaded aluminium extrusions in alloy AA6060 temper T6 was studied by quasi-static and dynamic tests. The primary variables in the tests were the local (b/h=17.78–40) and global (L/b=5–24) slenderness of the extruded members and the impact velocity. The critical global slenderness is defined as the slenderness where direct global buckling or a transition from progressive to global buckling occurs. In the quasi-static tests and for an impact velocity of 13 m/s, the critical global slenderness was found to be an increasing function of the local slenderness. In contrast, the critical global slenderness was a decreasing function of the local slenderness when the impact velocity was 20 m/s. The energy absorption was found to be very dependent on the collapse mode. Significantly more energy is absorbed in the progressive buckling mode than in the global bending mode. In the case of transition from progressive to global buckling, the energy absorption depends on the time of transition. The difference in energy absorption between the different modes decreases for increasing impact velocity due to inertia forces preventing the direct global buckling mode and the early transition from progressive to global buckling.     

On the role of matrix grain size and particulate reinforcement on the hardness of powder metallurgy Al–Mg–Si/MoSi2 composites

Six Al–Mg–Si composites reinforced with 15 vol.% of MoSi₂ intermetallic particles, together with three unreinforced monolith Al–Mg–Si (AA6061) alloys have been processed by powder metallurgy to quantify the roles of alloy matrix grain size and reinforcement particle on their solutionized hardness and ageing response. In the range studied, hardness of solutionized composites follows a Hall–Petch mechanism. Moreover, it can be rationalised as the sum of the hardness of the alloy matrix with the same matrix grain size (d) and a term H_R, that accounts for 17–27% of total hardness, is roughly constant and independent of reinforcing size and distribution. Matrix grain size is responsible for 50–65% of hardness, whereas the contributions of solid solution and Orowan strengthenings account for 17–26%. Upon heat treatment at 170 °C, hardening ability decreases linearly with d^?1/2, fitting all data points to a single equation independently of whether they correspond to the composites or to the monolith alloys.     

Combined effect of strain-rate and mode-ratio on the fracture of lead-free solder joints

The critical strain energy release rate for the solder joint fracture was measured as a function of the strain rate and the mode ratio of loading. These data are useful in predicting the fracture of solder joints loaded under arbitrary combinations of tension and shear during the impact conditions typical of falling portable electronic devices. In this study, strain rates from quasi-static (close to 0 s^− 1) to 61 s^− 1 were investigated at phase angles from 0 to 60°, typical of the range found in microelectronic devices. Copper–solder–copper double cantilever beam (DCB) model specimens were prepared using SAC305 solder at cooling rates and times above liquidus typical of actual ball grid arrays (BGAs). A drop tester was designed and built to achieve different strain rates at various mode ratios. The critical initiation strain energy release rate, J_ci, increased about 70% from quasi-static to intermediate strain rates, before decreasing by more than 67% from intermediate strain rates to 42 s^− 1.     

Effects of nano-sized α<Superscript>2</Superscript> (Ti<Superscript>3</Superscript>Al) particles on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy with bimodal microstructure

A bimodal microstructure containing very fine α²(Ti³Al) particles was produced by over-aging a Ti-6Al-4V alloy. The effects of α² precipitation on quasi-static and dynamic deformation behavior were investigated in comparison with an unaged bimodal microstructure. Quasi-static and dynamic torsional tests were conducted on them using a torsional Kolsky bar. The quasi-static torsional test data indicated that the over-aged bimodal microstructure showed higher fracture shear strain than the unaged bimodal microstructure, while their maximum shear stresses were similar. Under dynamic torsional loading, both maximum shear stress and maximum shear strain of the over-aged microstructure were higher than those of the unaged microstructure. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed by introducing concepts of critical shear strain, absorbed deformation energy, and void initiation. In the over-aged microstructure, the energy required for forming adiabatic shear bands was higher than that in the unaged microstructure, thereby lowering the possibility of the adiabatic shear band formation. The α² precipitation in the over-aged microstructure was effective in both the improvement of quasi-static and dynamic torsional properties and the reduction in the adiabatic shear banding, which suggested a new approach to improve ballistic performance of Ti alloy armor plates.     

Experimental study on tensile property of AZ31B magnesium alloy at different high strain rates and temperatures

As the lightest metal material, magnesium alloy is widely used in the automobile and aviation industries. Due to the crashing of the automobile is a process of complicated and highly nonlinear deformation. The material deformation behavior has changed significantly compared with quasi-static, so the deformation characteristic of magnesium alloy material under the high strain rate has great significance in the automobile industry. In this paper, the tensile deformation behavior of AZ31B magnesium alloy is studied over a large range of the strain rates, from 700 s⁻¹ to 3 × 10³ s⁻¹ and at different temperatures from 20 to 250 °C through a Split-Hopkinson Tensile Bar (SHTB) with heating equipment. Compared with the quasi-static tension, the tensile strength and fracture elongation under high strain rates is larger at room temperature, but when at the high strain rates, fracture elongation reduces with the increasing of the strain rate at room temperature, the adiabatic temperature rising can enhance the material plasticity. The morphology of fracture surfaces over wide range of strain rates and temperatures are observed by Scanning Electron Microscopy (SEM). The fracture appearance analysis indicates that the fracture pattern of AZ31B in the quasi-static tensile tests at room temperature is mainly quasi-cleavage pattern. However, the fracture morphology of AZ31B under high strain rates and high temperatures is mainly composed of the dimple pattern, which indicates ductile fracture pattern. The fracture mode is a transition from quasi-cleavage fracture to ductile fracture with the increasing of temperature, the reason for this phenomenon might be the softening effect under the high strain rates.     

Fatigue behavior and dislocation substructures for 6063 aluminum alloy under nonproportional loadings

In this paper, the fatigue behavior and dislocation substructures of 6063 aluminum alloy were studied under several nonproportional path loadings, which were circle, ellipse, rectangle and square paths. After fatigue test the micro-structure especially the dislocation substructures of the failure materials was carefully observed with the transmission electron microscope (TEM) method. Under the same 93 MPa equivalent stress amplitude loading, the alloy has the shortest life and the most severe cyclic additional hardening with circle path loading among all the loading paths. This attributes to the complicated dislocation substructures and severe stress concentration of the alloy during the cycling process. While under the ellipse path loading, the alloy has a comparably long life and light cyclic additional hardening. The deformation of the alloy and the morphology of the dislocation substructures determine the fatigue behavior of 6063 alloy under the same equivalent stress amplitude loading. Under the circle path loading, the fatigue life decreases while the cyclic strain increases as the loading stress amplitude increases from 47 MPa to 163 MPa. The dislocation evolution of 6063 alloy during the cycling process under circle path loading was examined with TEM. It was found that the dislocation merges with each other and changes from single lines to crossed bands. The movability of dislocation reduces and the stress concentration degree rises during the cycling process.     

Fatigue behavior of friction stir weld joints of Al–Zn–Mg alloy AA7039 developed using base metal in different temper condition

This paper presents the influence of base metal temper conditions (O, W and T6) on fatigue behavior of friction stir weld joints of Al–Zn–Mg alloy AA7039. Fatigue tests were performed at stress ratio of 0.1 and tensile to tensile stress in the range of 85–215 MPa to develop S–N curves. Fractured surfaces were investigated by SEM to determine the mode of fracture. On the basis of this study, friction stir welding of Al–Zn–Mg alloy AA7039 is recommended to be performed in W temper condition because of better tensile properties and superior fatigue strength.     

Effect of heat treatment on ballistic impact behavior of Ti–6Al–4V against 7.62 mm deformable projectile

The present study describes the effect of heat treatment on mechanical properties and ballistic impact resistance of Ti–6Al–4V alloy against 7.62 mm deformable lead projectiles. As-received plates were solution treated above and below the β-transus temperature followed by aging. The plates that were solution treated below the β-transus temperature followed by aging exhibited good combination of strength and ductility, and better ballistic impact resistance. Post ballistic microstructural examination showed formation of adiabatic shear bands (ASBs) and adiabatic shear band induced cracks. Both as-received plates and the plates that were solution treated above β-transus temperature followed by aging showed higher number of ASBs and ASB induced cracks compared to plates that were solution treated below β-transus temperature and then aged. Plug formation was observed through the ASB induced shear localization in planes parallel to the direction of projectile impact in all the conditions.