2A12-T4 Aluminum Alloy Research Articles

Crack initiation and early propagation plane orientation of 2A12-T4 aluminum alloy under tension-torsion fatigue loading including mean tensile stress

The specimen fractures appearances are analyzed to investigate the effects of mean tensile stress on the fatigue crack initiation and early propagation plane orientation under axial-torsion fatigue loading for 2A12T4 aluminum alloy. The fatigue crack initiation and early propagation plane orientations are measured by optical microscope, the results of macro-analysis show that both the maximum shear stress amplitude and normal mean stress have effects on the orientations of crack initiation and propagation plane orientation, which are close to the plane of the maximum shear stress amplitude plane. With increasing the mean tensile stress, more cracks are inclined to initial and propagate on or near the maximum shear stress amplitude plane with larger normal mean stress, and the angle of deviation from the plane of maximum shear stress amplitude increases. The predicted plane orientations based on critical plane methods are compared with experimental measured results.

Preparation and Application of Film Sensor for Metal Structure Crack Monitoring

A crack monitoring technique is desired to ensure the safety and reliability of metallic structures. In the present study, a conductive film sensor was presented to monitor structural cracks in metal structures in real-time based on the electrical potential method. First, a Ti/TiN film sensor was prepared on the fatigue critical portion of a 2A12-T4 aluminum alloy specimen by vacuum ion plating technology, which allows firm integration with the metal surface. A finite element model (FEM) of the Ti/TiN film sensor was then constructed and the changes in the output of the sensor along with corresponding changes in crack propagation were discussed. The results indicated that the Ti/TiN film sensor has high sensitivity to cracks and it is feasible to monitor structural surface cracks using the sensor. Finally, crack monitoring experiments were carried out based on the Ti/TiN film sensor. Experimental results showed that the output potential curve of the Ti/TiN film sensor contained several regions, which corresponded to plastic deformation accumulation, crack propagation, and sensor failure, respectively. Therefore, the information on the origination and propagation of structural cracks can be gained through analyzing changes in slope of the output potential values of the Ti/TiN film sensor with respect to time. DOI: http://dx.doi.org/10.5755/j01.ms.21.4.9623

Fatigue crack paths and properties in A356-T6 aluminum alloy microstructurally modified by friction stir processing under different conditions

A356-T6 cast aluminum alloy is a light weight structural material, but fatigue crack initiates and propagates from a casting defect leading to final fracture. Thus it is important to eliminate casting defects. In this study, friction stir processing (FSP) was applied to A356-T6, in which rotating tool with probe and shoulder was plunged into the material and travels along the longitudinal direction to induce severe plastic deformation, resulting in the modification of microstructure. Two different processing conditions with low and high tool rotational speeds were tried and subsequently fully reversed fatigue tests were performed to investigate the effect of processing conditions on the crack initiation and propagation behavior. The fatigue strengths were successfully improved by both conditions due to the elimination of casting defects. But the lower tool rotational speed could further improve fatigue strength than the higher speed. EBSD analyses revealed that the higher tool rotational speed resulted in the severer texture having detrimental effects on fatigue crack initiation and propagation resistances.

Influence of Cyclic Action of Corrosion and Alternate Load on Fatigue Life of 2A12-T4 Aluminum Alloy

Tests of alternate corrosion and fatigue for 2A12-T4 aluminum alloy were performed in order to reveal the influence of factors related with corrosion environment and load on its fatigue life. The effect of the alternate frequency of corrosion/fatigue and the type of corrosion tests such as alternate immersion, salt spray and salt spray+hot and humidity corrosion on the degradation of 2A12-T4 aluminum alloy was analyzed. The results show that for a designated total time of corrosion, the alternate frequency of corrosion/fatigue has important influence on the fatigue life of the alloy. The fatigue life of the alloy increases gradually with the increasing alternate frequency. The total fatigue life of aluminum alloy for 2, 4 and 6 cycles were 16.9%, 30.7% and 50.3% respectively higher than that for the first cycle. Besides, the type of corrosion tests also has important influence on the fatigue life of the alloy, of which the impact intensity may be ranked in a descending order as follows: salt spray corrosionalternate immersion corrosionsalt spray+hot and humidity corrosion.

Crack monitoring method based on Cu coating sensor and electrical potential technique for metal structure

Advanced crack monitoring technique is the cornerstone of aircraft structural health monitoring. To achieve real-time crack monitoring of aircraft metal structures in the course of service, a new crack monitoring method is proposed based on Cu coating sensor and electrical potential difference principle. Firstly, insulation treatment process was used to prepare a dielectric layer on structural substrate, such as an anodizing layer on 2A12-T4 aluminum alloy substrate, and then a Cu coating crack monitoring sensor was deposited on the structure fatigue critical parts by pulsed bias arc ion plating technology. Secondly, the damage consistency of the Cu coating sensor and 2A12-T4 aluminum alloy substrate was investigated by static tensile experiment and fatigue test. The results show that strain values of the coating sensor and the 2A12-T4 aluminum alloy substrate measured by strain gauges are highly coincident in static tensile experiment and the sensor has excellent fatigue damage consistency with the substrate. Thirdly, the fatigue performance discrepancy between samples with the coating sensor and original samples was investigated. The result shows that there is no obvious negative influence on the fatigue performance of the 2A12-T4 aluminum alloy after preparing the Cu coating sensor on its surface. Finally, crack monitoring experiment was carried out with the Cu coating sensor. The experimental results indicate that the sensor is sensitive to crack, and crack origination and propagation can be monitored effectively through analyzing the change of electrical potential values of the coating sensor.

A6061 合金FSW継手材の疲労特性におよぼすジルコニアピーニングの影響

Fatigue tests of friction stir welded (FSWed) joints of A6061-T6 aluminum alloy have been conducted with fully reversed plane bending conditions (R= -1) to investigate the effect of shot peening on the high cycle fatigue property. Shot peening was performed with two types of fine zirconia shot grid. The results showed that the fatigue strength at 107 cycles of unwelded specimens (base material) was 130 MPa and fine zirconia shot peening enhanced the strength by 10 MPa with B120 shot grid type. The fatigue lives were dramatically improved in all stress levels by the peening. On the other hand, the fatigue strength at 107 cycles of the SP treated FSWed specimens was nearly the same as that of the base material, and the fatigue lives were improved at only lower stress levels. To discuss these fatigue properties, fracture surfaces were observed and peening effects such as surface roughness, hardness and residual stress distributions were examined.

Effect of mean shear stress on torsion fatigue failure behavior of 2A12-T4 aluminum alloy

Fatigue experiments with solid cylindrical bar specimens were carried out to investigate the effects of mean shear stress on the torsion fatigue failure behavior of 2A12-T4 aluminum alloy. The torsion S–N curves under different values of mean shear stress were obtained. Experiment results showed that fatigue life under the same shear stress amplitude gradually reduces with increasing the mean shear stress, when mean shear stress is lower than a threshold value. Macro-analysis and micro-analysis of specimen fracture appearance were conducted in order to obtain the fracture characteristics for different mean shear stress values under torsion fatigue loading.

Rosette Eddy Current Sensor for Structural Health Monitoring

To monitoring of the crack damage located at the bolt hole of metallic bolted-joint structure in real-time, a fatigue crack monitoring scheme based on rosette eddy current sensor is put forward. The characteristics of sensor are analyzed via semi-analytic model, and feasibility of proposed scheme is verified through monitoring experiment using constructed fatigue crack monitoring system. The result of semi-analytic model shows that the magnitude of transimpedance is more sensitive to changing of conductivity and liftoff than phase of transimpedance, and the sensitivity of magnitude increases with the frequency. The result of fatigue crack monitoring experiment of 2A12-T4 aluminum alloy under constant load indicates that viewing the break point as indication of crack position, the rosette eddy current sensor achieves quantitative-monitoring capability of fatigue crack with precision of 1mm and the sensing-channels near fatigue source are capable of qualitative-monitoring of accumulated fatigue damage.

Development of Al356–Al2O3 Nanocomposite Coatings by High Velocity Oxy-fuel Technique

In this research, development of Al356–Al2O3 nanocomposite coatings has been investigated. Al356–Al2O3 composite powders were prepared by mechanical milling of Al356 powder and 5 vol.% micro and nanoscaled alumina particles. The milled powders were used as feedstock to deposit composite coatings on A356–T6 aluminum alloy substrate using high velocity oxy-fuel (HVOF) process. X-ray diffractometry, optical and scanning electron microscopy, microhardness and wear tests were used to characterize the composite powders and coatings. The hardness of composite coatings containing micro and nanosized Al2O3 were 114.1 ± 5.9 HV and 138.4 ± 6.9 HV, respectively which were higher than those for substrate (79.2 ± 1.1 HV). Nano and microcomposite coatings revealed low friction coefficients and wear rates, which were significantly lower than those obtained for Al356–T6 substrate. Addition of 5 vol.% micro and nanoscaled alumina particles improved the wear resistance by an average of 85% and 91%, respectively. This is mainly caused by the presence of Al2O3 in matrix and nanocrystalline structure of matrix. Scanning electron microscopy tests revealed different wear mechanisms on the surface of the wear test specimens.

Effect of anodizing treatment on the very high cycle fatigue behavior of 2A12-T4 aluminum alloy

The effects of chromic acid anodizing (CAA) and sulfuric acid anodizing (SAA) treatments on the very high cycle fatigue behavior of 2A12-T4 aluminum alloy were investigated through ultrasonic fatigue tests. The results showed that CAA and SAA treatments resulted in an appreciable reduction in the fatigue strength and crack initiation from the interface between the anodizing film and substrate, whereas the fatigue crack of substrate specimens initiated from the specimen subsurface over 5×108cycles. The decrease in fatigue strength of the CAA specimens was associated with the residual tensile stress arising from the elastic mismatch between the anodizing film and the substrate. The defects in SAA film, such as segmentation cracks and overgrowth of the anodizing film into the substrate, contributed to the reduction of fatigue strength and multi-crack initiations. The predicted fatigue limit for SAA specimens well agreed with the experimental results.

Reducing residual distortion of thin-plate weldments by rotating extrusion

A new method named rotating extrusion was developed to mitigate residual distortion of thin-plate weldments. The basic principle and characteristic of rotating extrusion as well as an efficient rotating extrusion device were introduced. Systematic trials were conducted to investigate the influence of several technological parameters including the distance between the extrusion tool and welding torch, the pressure acting on weldment, the dimension of the extrusion tool and its rotational speed on distortion control effect. Experimental results show that, as for 2A12T4 aluminum alloy weldment with 2 mm in thickness, 150 mm in width and 350 mm in length, when appropriate technological parameters are adopted, rotating extrusion can reduce its buckling deflection to below 3% of the original value. Implementing rotating extrusion during welding with an extrusion tool more than 100 mm away from the welding torch may achieve better distortion mitigation effect.

Microstructural Characteristics and Mechanical Properties of Friction Stir Spot Welded 2A12-T4 Aluminum Alloy

2A12-T4 aluminum alloy was friction stir spot welded, and the microstructural characteristics and mechanical properties of the joints were investigated. A softened microstructural region existed in the joint, and it consisted of stir zone (SZ), thermal mechanically affected zone (TMAZ), and heat affected zone (HAZ). The minimum hardness was located in TMAZ, and the average hardness value in SZ can be improved by appropriately increasing welding heat input. The area of complete bonding region at the interface increased with increasing welding heat input because more interface metals were mixed. In a certain range of FSSW parameters, the tensile shear failure load of the joint increased with increasing rotation speed, but it decreased with increasing plunge rate or decreasing shoulder plunging depth. Two kinds of failure modes, that is, shear fracture mode and tensile-shear mixed fracture mode, can be observed in the tensile shear tests, and the joint that failed in the tensile-shear mixed fracture mode possessed a high carrying capability.

Correlation of microstructure with mechanical properties and fracture behavior of A356-T6 aluminum alloy fabricated by expendable pattern shell casting with vacuum and low-pressure, gravity casting and lost foam casting

In the present work, a novel casting process named expendable pattern shell casting process with vacuum and low-pressure (EPSC-VL) was introduced to produce A356-T6 aluminum alloy. The correlation of microstructure with mechanical properties and fracture behavior of A356-T6 aluminum alloy was investigated. The results were then compared with lost foam casting (LFC) and gravity casting (GC) processes. Obtained results showed that the sizes of the α-Al primary phase and eutectic silicon particles as well as SDAS value of A356-T6 aluminum alloy obtained from the EPSC-VL process were smaller than that from the LFC and GC processes, and the porosity defects significantly decreased compared to the other processes, especially the LFC process. The significant improvements in microstructure and porosity level of the EPSC-VL process resulted in superior mechanical properties compared to that obtained from the other two processes. The fractographs of samples obtained with the EPSC-VL process clearly exhibited a dimple morphology, and the fracture mode was intergranular. The fracture surfaces of the GC samples exhibited a transgranular fracture mode, while, the fractographs of samples obtained with the LFC process displayed a brittle fracture mode, and the fracture path preferentially initiated at porosity defects due to its high porosity level in these samples.

Investigations on the material property changes of ultrasonic-vibration assisted aluminum alloy upsetting

Numerous studies have shown the benefit of ultrasonic-vibration assisted metal forming. This benefit include a reduction in forming forces, which might be attributed to the superposition of stress, increased temperatures, the effects of interface friction, and energy absorption of dislocation. This study conducts a series of experiments and analyses to investigate the main mechanisms of a reduction in forming forces during ultrasonic-vibration assisted A6061-T6 aluminum alloy upsetting.The findings of this research confirm that, under frictionless conditions, ultrasonic vibration still reduced forming forces, and ultrasonic vibration can increase the temperature of specimens and soften specimen surface during upsetting. From metallographic analyses and micro-hardness tests, the results reveal that energy absorption of dislocation was occurred during upsetting, which also contribute to the reduction of forming force.This research concludes that the mechanisms of increased temperatures and energy absorption of dislocation can affect the material property and make a reduction in forming forces; however, the interface friction effect has nothing to do with a reduction in forming forces.

Effect of Root Flaw on Tensile Behavior of Friction Stir Welded A6061-T6 Aluminum Alloy

The tensile behavior of friction stir joints containing root flaw in A6061-T6 aluminum alloys was investigated. First, plates of this material were butt-jointed by friction stir welding, and the metallographic observation and micro-hardness analysis were made on the cross-section of the weld. Then, tensile tests were performed in MTS 810 testing machine, and a digital CCD camera was used to monitor the process during testing.. Finally, the tensile fracture surface analysis was examined by scanning electron microscopy (SEM). The results show that the root flaw has no apparent effect on the tensile properties. However, it was observed that a crack about 0.3 mm in length occurred in the root part of the weld center in the early stage of tensile test, and the crack did not further propagate during the sequent tensile experiment. The tensile specimens were all fractured in the heat-affected zone (HAZ) of the retreating side, which is the area coincident with the region of maximum local strain and lowest hardness.

A corrected damage law for high cycle fatigue

It is an effective method to use continuum damagemechanics (CDM) to solve fatigue problems. A corrected damage law for highcycle fatigue is postulated to accord with experimental data based on thelatest Lemaitre damage law. Stress amplitude and mean stress can be takeninto account in this corrected damage law. Take 2A12-T4 aluminum alloy as anexample, the material parameters were gotten for this correction damage model.Let this model be an UMAT subroutine of ABAQUS. Fatigue lives of 14 loadingsituations for two components were computed. The result shows that the meanerror between experimental and computational result is about 15%, whichshows this corrected fatigue damage law can preferably forecast high cyclefatigue life for metallic components.

Fatigue Crack Growth of Aluminum Alloy A6061-T6 in High Pressure Hydrogen Gas and Failure Analysis on 35 MPa Compressed Hydrogen Tanks VH3 for Fuel Cell Vehicles

In order to ensure the safety of carbon fiber reinforced tanks for 35 and 70 MPa fuel cell vehicles, it is necessary to clarify the fatigue crack growth behavior of A6061-T6 aluminum alloy for the tank liner. In this study, the fatigue crack growth tests were carried out under the conditions of test frequencies f = 0.001-10 Hz in 90 MPa (≒70 MPa×125 %) hydrogen gas, 90 MPa nitrogen gas, air and deionized water at room temperature. The fatigue crack growth rates in hydrogen and nitrogen gas at f = 1-10 Hz were lower than those in air and deionized water. However, the fatigue crack growth rates in hydrogen and nitrogen gas at f = 0.001-0.1 Hz were nearly the same as those in air and deionized water. It was found from those results that the fatigue crack growth rate was not accelerated in 90 MPa hydrogen gas. The results of fatigue crack growth tests in this study were applied to the failure analysis of 35 MPa hydrogen tanks. In JARI (Japan Automotive Research Institute), using 35 MPa hydrogen tanks, the pressure cycling tests were conducted at -40, 25 and 85°C by water or PEPF (Perfluoroalkylpolyether) under 125 % filling pressure. The tanks fractured by LBB (Leak Before Break). Increasing in the test temperature increased in the number of cycles to LBB failure. Such a behavior was predicted by the fatigue crack growth properties of A6061-T6 obtained in this study.

High cycle fatigue and fracture mode analysis of 2A12–T4 aluminum alloy under out-of-phase axial–torsion constant amplitude loading

Fatigue experiments with solid cylindrical bar specimens are carried out to investigate the effects of stress amplitude ratios and phase angles on the axial–torsion high-cycle fatigue failure of 2A12–T4 aluminum alloy. Macro- and micro-analysis of the specimen fracture appearance is conducted in order to obtain the fracture characteristics under combined axial–torsion loading with different stress amplitude ratios and different phase angles. The soft factor of fatigue loading is introduced to estimate the failure mode under the combined axial–torsion loading. The result indicates that the estimation about the soft factor of fatigue loading is consistent with the macro- and micro-analysis.

Improvement in fatigue performance of friction stir welded A6061-T6 aluminum alloy by laser peening without coating

The authors have applied laser peening without coating (LPwC) to fatigue specimens cut out from friction stir welded (FSWed) A6061-T6 aluminum alloy plates with a thickness of 3mm. Both crown and root sides of the specimens were peened by laser pulses with an energy of 60mJ and a peak power density of 2GW/cm2 from a frequency-doubled Nd:YAG laser. The effects on the fatigue properties were studied through plane bending fatigue tests with a stress ratio of R=−1. The results showed that the fatigue strength of unwelded specimens (base material; BM) was 110MPa at 107 cycles and LPwC enhanced the strength by 60MPa in spite of increase in surface roughness due to the direct irradiation of the laser pulses to the bare surface of the specimens. Meanwhile the fatigue strength of FSWed specimens was 90MPa and LPwC enhanced it by 30MPa to 120MPa. This increment is a half compared to that in the BM, however the fatigue strength of the FSWed specimens after LPwC was higher than that of the BM. The surface roughness, hardness and residual stress were assessed and characterized as well.

Loading Frequency Effect on Fatigue Crack Growth Rate in Low Pressure Hydrogen Gas in the Case of A6061-T6 Aluminum Alloy

Loading Frequency Effect on Fatigue Crack Growth Rate in Low Pressure Hydrogen Gas in the Case of A6061-T6 Aluminum Alloy