Microstructure and mechanical properties of friction stir welded AA7075-T651 aluminum alloy thick plates

Friction stir butt welding of AA7075-T651 plates with thicknesses of 10 and 16 mm was investigated. Defect-free, full- penetration welds were obtained after careful process parameter selection. While the nuggets in both welds exhibited very fine reerystallized grains, and finer grains were observed in welds made on 10 mm thick plates. Microhardness surveys revealed that significant loss in hardness occurs in the heat-affected zone. The reduction in hardness due to the welding process is higher in the case of welds made on 16 mm thick plates. Welds made on 10 mm thick plates exhibited superior tensile properties compared with those made on 16 mm thick plates. Fracture during tensile test occurred in the heat-affected zone in both cases. TEM images of specimens revealed that the heat-affected zone consisted of widened precipitate-free zones along grain boundaries and partial dissolution of precipitates in the grain interiors. It is concluded that defect-free single pass welds can be made on AA7075-T651 thick plates using friction stir welding and the welds made on 10 mm thick plates exhibit high joint efficiency.

Investigation on the plastic work-heat conversion coefficient of 7075-T651 aluminum alloy during an impact process based on infrared temperature measurement technology

The plastic work-heat conversion coefficient is one key parameter for studying the work-heat conversion under dynamic deformation of materials. To explore this coefficient of 7075-T651 aluminum alloy under dynamic compression, dynamic compression experiments using the Hopkinson bar under four groups of strain rates were conducted, and the temperature signals were measured by constructing a transient infrared temperature measurement system. According to stress versus strain data as well as the corresponding temperature data obtained through the experiments, the influences of the strain and the strain rate on the coefficient of plastic work converted to heat were analyzed.The experimental results show that the coefficient of plastic work converted to heat of 7075-T651 aluminum alloy is not a constant at the range of 0.85–1 and is closely related to the strain and the strain rate. The change of internal structure of material under high strain rate reduces its energy storage capacity, and makes almost all plastic work convert into heat.

Residual Stress Relaxation of Thin‑walled Long Stringer Made of Aluminum Alloy 7050‑T7451 under Transportation Vibration

Thin-walled long stringer made of aluminum alloy 7050-T7451 is prone to deformation during transportation,so a research of residual stress relaxation was launched in this paper.The transport resonance stress of long stringer was analyzed based on the power spectral density of road transport acceleration.The residual stress relaxation experiment of aluminum alloy 7050-T7451 under different equivalent stress levels was designed and carried out.According to the amount of residual stress relaxation in the experiment,an analytical model was established with the equivalent stress level coefficient.The deflection range of long stringer was evaluated under different damping ratios.The results show that when the equivalent stress exceeds 0.8σ0.2,the residual stress relaxation of the thin-walled samples occurs.The residual stress relaxation increases linearly with the equivalent stress,which is logarithmically related to the loading cycle.The deformation caused by residual stress relaxation of the long stringer is proportional to the square of the length and the bending moment caused by stress rebalance,and inversely proportional to the moment of inertia of the structure.As the damping ratio decreases from 0.03 to 0.01,the total deflection of the long stringer increases from 0 to above 1.55 mm.

Effect of Temperature on Material Transfer Behavior at Different Stages of Friction Stir Welded 7075-T6 Aluminum Alloy

In this work, the morphologies of weld of 7075-T6 aluminum alloy via friction stir welding （FSW） were analyzed by optical microscopy, the temperature field was attained by numerical simulation, and the effect of temperature on material transfer behavior in the thermal-mechanical affected zone （TMAZ） at different stages was mainly investigated. The FSW process consists of three stages. It is very interesting to find that the maximum transfer displacement of material appears at the final stage of welding process, then at the stable stage and at the initial stage, which results from the difference of peak temperatures at different stages. At any stage, the material in TMAZ near the surface of weld transfers downwards, the material in the middle of weld moves upwards and the material near the bottom of weld hardly moves. In any cross section of weld, the largest transfer displacement of material appears in the middle of weld. The increase of rotational velocity and the decrease of welding speed are both beneficial to the transfer displacement of material in the middle of weld.

Effects of dry/wet ratio and pre-immersion on stress corrosion cracking of 7050-T7451 aluminum alloy under wet-dry cyclic conditions

The stress corrosion cracking(SCC) behavior and mechanism of 7050-T7451 aluminum alloy under wet-dry cyclic conditions were investigated. Slow strain rate tests(SSRTs) and electrochemical tests were used to study the effects of dry/wet ratio(DWR) and pre-immersion on SCC.Fracture and side surface characterizations were observed by scanning electron microscopy(SEM).The results demonstrate that SCC susceptibility decreases with an increase of the DWR. With an increase of the pre-immersion time, both continuous pre-immersion(CP) and wet-dry cyclic preimmersion(WDP) samples are more sensitive to SCC, and the cracking mode in the SCC fracture region is intergranular. Furthermore, the effect of WDP on SCC is greater than that of CP when the total time immersed in solution before an SSRT is the same with each other. In fact, each single wetdry cycle can be divided into three processes with respect to the change of solution on samples’ surface. Volatilization of water on the surface results in an increase in solute concentration, thus accelerating corrosion.

Study on the reaming process of aluminum alloy 7050-T7451 under different cooling conditions

Aluminum alloy 7050 is widely used in the aeronautical industries.However,owing to their highly ductile property,chips created during high-speed machining cannot be naturally broken,and long continuous chips are unavoidably formed,impacting the machining stability and quality of the parts.Because a smaller cutting allowance is required compared with conventional machining operations,the behavior of the chips during reaming operation may be more complex and different from those determined in previous investigations.Therefore,studying the characteristics of chip formation and hole quality during the reaming process is essential to improve the machinability of aluminum alloy 7050.In this study,three different cooling conditions were applied to reaming aluminum alloy 7050-T7451 with polycrystalline diamond(PCD)reamers.The finite element models(FEMs)were established to simulate the chip formation.The macro-and micro-morphologies of chips under the three cooling conditions were compared to analyze the chip behaviors.The diameter,surface roughness,and micro-morphologies of the reamed holes were also analyzed to evaluate the hole quality.The results showed that the chip morphology was strongly influenced by the cutting parameters and cooling strategies.It was found that the desired chip morphologies satisfactory geometrical accuracy and surface quality during the reaming of aluminum alloy 7050-T7451 could be achieved using internal cooling at a spindle speed of 8000 r/min and a feed rate of 0.0l mm/z.This study also demonstrates the feasibility of an internal cooling strategy for breaking chips when reaming aluminum alloy 7050-T7451,which opens new possibilities for improving the chip-snarling that occurs during hole machining.