The effect of retrogression and re‐aging treatment on mechanical and tribological features of AA7075 aluminium alloy

In this study, T651‐applied AA7075 alloy was subjected to retrogression and re‐aging (RRA) process. Various retrogression temperatures (180 °C, 280 °C, 370 °C) and times (15 min, 30 min, 90 min) were used to determine the effects of temperature and time on the mechanical and tribological properties of the AA7075 alloy. All re‐aging stages were performed at 120 °C for 24 hours. Retrogression and re‐aging‐applied specimens were characterized by scanning electron microscope, transmission electron microscope, x‐ray diffraction, Charpy V‐notch impact and tensile tests. Brinell hardness measurements and ball‐on‐disc type tribometer measurements by using AISI 316 ball as a counterpart have also been conducted. Grain boundary precipitates in the T651‐applied specimen was transformed from continuous to the discontinuous structure after retrogression and re‐aging process. Continuous MgZn₂ precipitates at grain boundaries were disintegrated and re‐precipitated along the grain boundaries. The sizes of intragranular precipitates have become coarsened by comparison with the T651 condition. Hardness, tensile strength and wear resistance were decreased whereas impact toughness values were increased with increasing retrogression temperature and time. The best wear resistance was obtained in the sample treated at 180 °C for 15 minutes.     

Rapidly modifying microstructure and mechanical properties of AA7150 Al alloy processed with electropulsing treatment

Studies show that the proper solid solution treatment (SST) is a key step in the precipitation strength-ening of AA7150 Al alloy.Despite the superior characteristics of the fully dissolved phase,it has major drawbacks,including high consumption of energy and low efficiency.Recently,electropulsing treatment(EPT) has been proposed to study dissolved precipitations and modify microstructures of AA7150 Al alloy faster than conventional SST.Experiments have been conducted in the present article,and the obtained results show that EPT can promote the rapid dissolution of the rl'phase at relatively low temperatures in only 20 s.Meanwhile,the strength and ductility of electropulsed samples decrease drastically.Compared with conventional SST,EPT accelerates recrystallization and obtains relatively fine grains after 20 and 50 s electric pulses.Moreover,as the EPT time increases,the corresponding non-uniform local heating and the electron force promote dislocation generation and annihilation.     

Improving the properties of aluminium alloys by retrogression and reageing

Retrogression and reageing heat treatments offer the potential of improved tensile properties in combination with greatly increased resistance to stress corrosion cracking. The potential of this technique is reviewed with respect to the current application in the European and North American Aerospace Industry. To illustrate the performance increase associated with RRA treatments, the stress corrosion cracking performance of the established aerospace plate and forging alloy 7010 has been evaluated using an alternate immersion constant load tensile type test (ECSS‐Q‐70‐37A) Specimens were cut from a large aerospace rectilinear forging and tested in three different tempers, T652, T7452 and a retrogressed and reaged condition (RRA). In the T652 condition the material has been shown to be highly susceptible to intergranular corrosion and stress corrosion cracking. In the T7452 and RRA conditions 7010 showed much improved resistance to SCC but pitting corrosion resulted in failure of some specimens within the 30day requirement of the test standard.     

Effects of heat treatment on the microstructure and mechanical properties of AA2618 DC cast alloy

Direct chill (DC) cast ingot plates of AA2618 alloy have been increasingly used for large-mold applications in the plastics and automotive industries. The effects of different heat treatments on the microstructure and mechanical properties of AA2618 DC cast alloy were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and hardness and tensile testing. The as-cast microstructure contained a considerable amount of coarse intermetallic phases, including Al₂CuMg, Al₂Cu, Al₇Cu₄Ni, Al₇Cu₂(Fe,Ni) and Al₉FeNi, resulting in poor mechanical properties. Solution treatment at 530 °C for 5 h dissolved the first three phases into the solid solution and consequently improved the mechanical properties of the alloy. By utilizing the appropriate aging temperature and time, different combinations of strength and ductility could be obtained to fulfill the design requirements of large-mold applications. The strengthening of AA2618 DC cast alloy under the aging conditions studied was caused by GPB zones and S′ precipitates. The evolution of both precipitates in terms of their size and density was observed to have a significant effect on the mechanical properties of the alloy.     

High Strength AA7050 Al alloy processed by ECAP: Microstructure and mechanical properties

Commercial AA7050 aluminium alloy in the solution heat-treated condition was processed by ECAP through routes A and B_C. Samples were processed in both room temperature and 150 °C, with 1, 3, and 6 passes. The resulting microstructure was evaluated by optical microscopy (OM) and transmission electron microscopy (TEM). Only one pass was possible at room temperature due to the low ductility of the alloy under this condition. In all cases, the microstructure was refined by the formation of deformation bands, with dislocation cells and subgrains inside these bands. The increase of the ECAP temperature led to the formation of more defined subgrain boundaries and intense precipitation of spherical-like particles, identified as η′ and η phases. After the first pass, an increase in the hardness was observed, when compared with the initial condition. After 3 passes the hardness reached a maximum value, higher than the values typically observed for this alloy in the overaged condition. The samples processed by route B_C evolved to a more refined microstructure. ECAP also resulted in significant strength improvement, compared to the alloy in the commercial overaged condition.     

The effect of pre-straining on the mechanical behaviour of self-piercing riveted aluminium alloy sheets

Self-piercing riveting as an alternative joining method to spot-welding has attracted considerable interest from the automotive industry and has been widely used in aluminium intensive vehicles. Pressing and stamping are important processes in automotive production and result in additional straining on the vehicle body sheet material. It is therefore important to have knowledge of the effect of sheet pre-straining on the quality of the self-piercing riveted joints and on the mechanical behaviour of the riveted aluminium alloy sheets. This paper reports the influence of sheet pre-straining on the static and fatigue behaviour of self-piercing riveted aluminium alloy sheet. Wrought aluminium alloy sheet, NG5754 with a nominal thickness value of 2 mm was used to obtain pre-strained NG5754 sheets with pre-straining levels of 3%, 5% and 10%. Pairs of pre-strained NG5754 sheets were joined to create single-riveted lap joints which subsequently underwent lap-shear and fatigue testing. Microscopic inspection showed that the joint quality was satisfactory despite the increasing sheet straining levels. The results showed that by increasing the pre-straining level up to 10%, the shear and fatigue strength also increased. The rate of increase of the static and fatigue strength differed as the pre-straining levels varied.     

Effect of recrystallization and texture on mechanical properties of extruded AA7003 alloy

The microstructure, texture and mechanical properties of AA7003 extrusion sheets processed with different parameters were investigated. Furthermore, the effects of the microstructure and texture on the mechanical properties were discussed. The grain morphology and the texture were analyzed by using a combination of scanning electron microscopy and electron backscatter diffraction. Moreover, the mechanical properties were measured by tensile tests and the tensile fracture morphology was also analyzed by scanning electron microscopy. The results showed that the maximum recrystallization extent occurred at the extrusion process conditions of T = 450 °C and v = 1 mm/s, and the increase in recrystallization extent had a negative effect on the tensile strength. Moreover, a relatively strong recrystallization cube orientation <100> existed when the extrusion temperature was 470 °C, showing a high tensile strength and elongation. Thus, it can be concluded that the cube texture is beneficial to the tensile strength and elongation in extruded AA7003 alloy.     

Effect of deposition time on morphology,corrosion resistance and mechanical properties of Ti-containing conversion coatings on zinc

Ti-containing conversion coatings were obtained on zinc electroplated steel. The influence of deposition time on the coatings morphology, chemical composition, corrosion resistance and mechanical properties were studied. Their chemical composition strongly depends on deposition time. As the time increases from 5 s to 300 s, the Ti content decreases and the Si content increases. For the deposition time of 5 s the Si:Ti atomic ratio in the conversion coating is 0.32 and after 300 s it increases to 9.1, which is accompanied by reduction in corrosion resistance. As deposition time increases, the microhardness of the coatings and their wear resistance deteriorate. Corrosion test results show that the coatings can be applied to protect zinc against corrosion.     

Microstructural aspects and mechanical properties of friction stir welded AA2024-T3 aluminium alloy sheet

In the present work, aluminium alloy AA2024-T3 thin sheets were joined by the Friction Stir Welding – FSW – process. Butt joints were obtained in 1.6 mm sheets, using an advancing speed of 700 mm/min. These joints were characterised by optical, scanning electron microscopy, tensile and fatigue mechanical tests. The results showed that the resulting microstructure is free of defects and the tensile strength of the welded joints is up to 98% of the base-metal strength. Fatigue tests result indicates an equivalent stress intensity factor (kt) of approximately 2.0 for the welded samples. Consequently, the FSW process can be advantageous compared to conventional riveting for airframe applications.     

Microstructural evolution,mechanical properties and corrosion behavior of as-cast Mg-5Li-3Al-2Zn alloy with different Sn and Y addition

Influences of Sn and Y on the microstructure,mechanical properties,and corrosion behavior of as-cast Mg-5Li-3Al-2Zn (LAZ532) alloy were investigated.The addition of Sn and Y refines grains and results in the formation of Mg2Sn and Al2Y phases,thus improving the mechanical properties of alloy by second phase strengthening and grain refinement strengthening.As-cast LAZ532 alloy shows typical filiform corrosion morphology,and the addition of Sn and Y does not change the corrosion mode of alloy.Ascast LAZ532-0.8Sn-1.2Y alloy shows excellent mechanical properties with yield strength of 166.2 MPa,ultimate tensile strength of 228.6 MPa and elongation of 14.8 ％,and exhibits the best corrosion resistance with the smallest corrosion current density and the lowest anodic dissolution rate.     

Nd对AM50力学性能及高温性能的影响

对加Nd的AM50镁合金铸态试样进行固溶处理(420℃/12 h),研究了Nd对其显微组织、力学性能和抗高温蠕变性能的影响.结果表明:Nd的加入细化了晶粒,导致AM50合金室温力学性能的提高.Nd在AM50合金中形成了Al11Nd3高温稳定相,Al11Nd3的存在使加Nd的AM50合金在200℃的稳态蠕变率及高温(150℃)力学性能大幅度提高.     

The microstructure,mechanical properties and corrosion behaviour of Ce-containing Mg-3Sn-1Ca alloy

The effects of minor Ce on the microstructure, mechanical properties and corrosion behaviour of Mg-3Sn-1Ca alloy are investigated systematically. The minor Ce can refine the eutectic structure between α-Mg and CaMgSn and make the microstructure more uniform. The maximum ultimate tensile strength and elongation of the alloy with the Ce content of 0.3?wt-% were 152.5?MPa and 4.1%, respectively. Meanwhile, the corrosion resistance of the Ce-containing alloys is improved by Tafel curves and electrochemical impedance spectra, especially when the Ce is up to 0.3?wt-%. The minor Ce can promote the formation of the surface film and makes the corrosion product film to become more compact, which effectively prevents further the occurrence of corrosion of the Mg matrix.     

Structure and mechanical properties of the annealed TZAV-30 alloy

The Ti–30Zr–5Al–3V (wt.%, TZAV-30) alloy having good mechanical properties is a potential structural material to apply in the aerospace industry. The microstructure and mechanical properties of ZTAV-30 alloy underwent various annealing heat treatments were investigated. The specimens annealed from 500 to 800 °C are composed of α and β two phases. No compound is detected in specimens annealed in that temperature range. The microstructure of annealed specimens is characterized as a typical basketweave microstructure. Three microstructural parameters, thickness of plate α phase, relative fraction of β phase and aspect ratio of α grains, were measured in those annealed specimens. As the alloy annealed in the range from 500 to 800 °C, the average thickness of plate α grains increases with the increasing annealing temperature from 500 to 700 °C but decreases while annealed at 800 °C. The fraction of retained β phase increases with annealing temperature. And the aspect ratio of plate α grains decreases firstly but increases while the annealing temperature is higher than 700 °C. As the variation of those three microstructural parameters, the strength of examined alloy varies from 1269 to 1355 MPa for tensile strength and from 1101 to 1190 MPa for yield strength, inversely, the elongation changes in the range from 12.7% to 8.4%. The strengthening and toughening mechanism of the TZAV-30 alloy with basketweave microstructure is also discussed in this paper.     

Densification behavior,mechanical properties and thermal shock resistance of tungsten alloys fabricated at low temperature

The low-temperature shrinkage of tungsten was greatly accelerated by the addition of trace Nb and Ni, and the addition of trace Nb and Ni also significantly promoted the final sintering density. The 99.1% of theory density for W–0.1 wt.%Nb–0.1 wt.%Ni material sintered at 1600 °C was obviously greater than 93.7% of theory density for W material sintered at 2000 °C. Ball milling treatment played an important role in promoting the sintering densification of W–0.1 wt.%Nb–0.1 wt.%Ni powder, and the powder milled for 10 h (W10) could be sintered to near full density (99.4% of theory density) at 1600 °C. The ball milling for 15 h has no effect in improving the sintering density, but it induced rapid growth of tungsten grains. The microhardness and tensile strength of the sintered tungsten alloys were highly dependent on its sintering density and grain size. Improving the sintering density while controlling the grain growth could effectively promote the microhardness and tensile strength. Furthermore, the improvement of thermal shock resistance of the W10 alloy was due to good microstructure and the increase in the tensile strength.     

Influence of residual fatigue damage on the fracture toughness parameters of an AA6082-T6 aluminium alloy

The present investigation has been carried out in order to study the influence of the previous accumulated fatigue damage induced during high cycle fatigue (HCF), on the fracture toughness parameters of an AA6082-T6 aluminium alloy. The results show that previous fatigue damage accumulated in HCF does not affect the tensile static mechanical properties of the material, but gives rise to a significant debit of the toughness properties on this aluminium alloy. The fracture toughness results have shown that the crack opening displacement at a crack extension of 0.2 mm (COD_0.2) decreases in the range of ∼18 to 36% whereas the value of the non-linear fracture mechanics parameter  J _0.2, decreases in the range of ∼11 to 25% at applied maximum stresses of 200 and 275 MPa, respectively. Optical microscopy observations conducted on the surface of the specimens subjected to HCF damage indicate the existence of microcracks ∼15 to 25 μm long nucleated along the grain boundaries of the material. Also, the scanning electron microscopy (SEM) observations of the fracture surfaces after the tearing tests show the predominance of a ductile fracture mechanism for the material prior to residual fatigue damage, whereas a mixed ductile–brittle fracture mechanism and the presence of flat facets were observed on the fracture surfaces of the specimens with a fatigue damage of 0.70.     

Study on mechanical properties and constitutive model of 5052 aluminium alloy

The stress–strain relationship of 5052 aluminium alloy was investigated via quasi-static tensile tests and split Hopkinson pressure bar tests. The specimens were exposed to various temperatures (25–500°C) and strain rates (10^?4–0.7?×?10^4?s^?1). At strain rates ranging from 0.001 to 3000?s^?1, the material underwent significant work hardening. When the strain rate exceeded 5000?s^?1, the work hardening effect decreased and the flow stress was relatively constant. The Johnson–Cook constitutive model was modified to describe the deformation behaviour of the material subjected to high temperatures and strain rates. The accuracy of the modified model was verified through ballistic impact testing.     

Microstructural evolution and mechanical properties of Mg-9.8Gd-2.7Y-0.4Zr alloy produced by repetitive upsetting

A newly developed severe plastic deformation(SPD) technique, i.e. repetitive upsetting(RU), is employed to improve the strength and ductility of a Mg-Gd-Y-Zr alloy. During the RU processing, dynamic recrystallization occurs in the Mg alloy, which leads to a significant grain refinement from 11.2 μm to 2.8 μm.The yield strength(YS), ultimate tensile strength(UTS) and elongation increase simultaneously with increasing RU passes. The microstructural evolution is affected by processing temperatures. Dynamic recrystallization prevails at low temperatures, while dynamic recovery is the main effect factor at high temperatures. Texture characteristics gradually become random during multiple passes of RU processing,which reduces the tension-compression asymmetry of the Mg-Gd-Y-Zr alloy.     

Effect of heat treatment on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy

In this study, effect of various aging tempers (T6, T73 and RRA treatment) on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy was studied using high-resolution electron microscopy, selected area diffraction, and tensile tests. The results indicate that the two types of GP zones, GPI and GPII, are major precipitates for the alloy under T6 condition. No clear precipitation free zone was observed, and the grain boundary precipitates were continuous. Under two-step aging condition, the GP zones and η′ are major precipitates for the alloy, the discontinuous grain boundary precipitates are favorable to SCC resistance in over-aged condition, which reduces its strength 58 MPa (about 7%) compared to the peak-aged condition. After retrogression and re-aging treatment, the grain boundary precipitates are discontinuous, which is closed to that resulting from T73 temper. RRA treatment decreased ultimate tensile strength 25 MPa (about 3%) in values compared with the alloy at T6 condition.     

Investigation of mechanical properties and fatigue life of ECARed AA5083 aluminium alloy

Equal‐channel angular rolling (ECAR) is a continuous severe plastic deformation process. In this process, severe shear strains apply to the sheet. This strain increases the yield or ultimate strength of sheet without significant change in sheet dimension. In this paper, the effect of ECAR process on mechanical properties and fatigue life of manufactured sheets will be studied. Four AA5083 samples have been prepared and annealed for obtaining stress‐free samples. Three samples have been rolled by the ECAR process with one, two and three passes of rolling, respectively. Mechanical tests including tensile test, hardness and axial fatigue tests have been carried out on prepared samples. Fatigue tests have been implemented according to a strain‐based approach with a constant strain ratio equal to 0.75 and 0.5 Hz frequency of loading. All of the tests have been carried out in controlled laboratory conditions. Results show that the ultimate tensile strength of samples increases with increasing the pass of rolling. Also, the maximum elongation of samples decreases. Maximum elongation was 17% in annealed samples, while it decreases to 10% in samples with three passes of rolling. The hardness of samples has been measured, and the results show an increase in hardness for a higher pass of the ECAR process. Fatigue test results show that fatigue life of AA5083 samples decreases in manufactured sheets of the ECAR process. Also, cyclic softening has been observed in the ECARed sample. The fracture surfaces of samples after fatigue test have been observed with a scanning electron microscope. A comparison of fracture surfaces confirms that the crack growth was intergranular in annealed samples while it changes in ECARed samples to transgranular.     

Significant improvement of semi-solid microstructure and mechanical properties of A356 alloy by ARB process

Accumulative roll bonding (ARB) process was used in this study as an effective method for manufacturing high-strength, finely-dispersed and highly-uniform A356 alloy. It was found that when the number of ARB cycles was increased, the uniformity of silicon particles in the aluminum matrix improved, the particles became finer and spheroider and therefore, the tensile strength (TS) and ductility of the samples improved. The microstructure of the manufactured A356 alloy after five ARB cycles indicated a totally modified structure such that it's TS and elongation values reached 269 MPa and 5.3% which were 2.6 and 2.5 times greater than those of the as-cast material, respectively. Also, the hardness value increased from 55.4 (for as-cast sample) to 100.2 HV (after the fifth cycle of ARB), and registered 81% increase.