AA5754 Sheets Research Articles

Formability study and metallurgical properties analysis of FSWed AA 6061 blank by the SPIF process

In this study, in order to obtain the maximum possible formability in tailor-welded blank AA6061 sheets connected by the friction stir welding (FSW) procedure, the incremental sheet forming process has been utilized. The results are presented both numerically and experimentally. To obtain the forming limit angle, the base and FSWed sheets were formed in different angles with conical geometry, and ultimately, the forming limit angle for the base metal and FSWed sheet is estimated to be 60° and 57.5°, respectively. To explore the effects of welding and forming procedures on AA6061 sheets, experimental studies such as mechanical properties, microstructure and fracture analysis are carried out on the samples. Also, the thickness distribution of the samples is studied to investigate the effect of the welding process on the thickness distribution. Then, the numerical process was simulated by the ABAQUS commercial software to study the causes of the FSWed samples failure through analyzing the thickness distribution parameter, and major and minor strains and the strain distribution. Causes of failure in FSWed samples include increased minor strain, strain distribution and thickness distribution in welded areas, especially in the proximity of the base metal area.

Effect of Different Coupling Agents on Interfacial Properties of Fibre-Reinforced Aluminum Laminates.

Metal composite interface properties significantly affect the integrity, bonding properties, and interface structure of Fibre Metal Laminates (FMLs). Interfacial bonding strength’s effect on Carbon Fibre-Reinforced Aluminium Laminate (CARALL) mechanical behaviours was investigated via three-point bending and low-velocity impact tests. AA6061 sheets were subjected to surface pretreatments under three conditions (anodizing and A-187 and A-1387 surface modifications) to obtain different interfacial bonding strengths. The bonding interfaces of CARALL were analysed using scanning electron microscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. Interfacial bonding strength between aluminium alloy and epoxy resin was determined by the tension-shear test. CARALL’s energy absorption capacity and failure mode were analysed after low-velocity impact and three-point bending under different aluminium alloy volume contents and surface pretreatments. Upon modification of metal surfaces, the interfacial bonding strength increased, and the highest was obtained by silane coupling agent A-1387. Improved strength maintained FML’s integrity under quasi-static and dynamic loadings. A-1387 improved the bonding ability of aluminium alloy and Carbon Fibre-Reinforced Plastics (CFRP). The composite interface strongly resisted crack propagation because of its functional group characteristics. When the volume content of aluminium alloy was less and greater than that of CFRP, the energy absorption capacity of CARALL weakened and strengthened, respectively, with increasing interfacial bonding strength.

Experimental validation of the simulation of single-point incremental forming of AA7075 sheet with Yld2004-18P yield function calibrated with crystal plasticity model

Incremental sheet forming was developed several decades ago as a cost-effective forming process for low volume production. The deformation mechanism of this process is completely different from the conventional sheet metal–forming processes. Applying a three-dimensional (3D) yield function such as Yld2004-18p for the simulation of incremental sheet forming is necessary to account for the significant anisotropy and out-of-plane shears that develop in the sheet metal. Since it is difficult to experimentally measure out-of-plane shear stresses, a set of virtual experiments was conducted with crystal plasticity finite element method (CPFEM) to obtain the required data. To that end, five different representative volume elements (RVEs) were constructed and simulated with a rate-independent CPFEM model to assess which model best predicts the anisotropy of the AA7075 aluminum sheet. Of the four CPFEM models based on the associate flow rule, three used RVEs accounting for grain orientations and grain size distributions, while the fourth model used only the grain orientation information (Taylor’s assumption). The fifth CPFEM model was modified based on the Hill’s 1948 non-associated flow rule and used the Taylor’s assumption. It was verified by experimental data that the fifth CPFEM model (Taylor + Hill) provides the most computationally efficient and accurate prediction of flow stresses and R-values as a function of the accumulated plastic work. The results from the Taylor + Hill model were then used to calibrate the Yld2004 yield function used in the simulation of the single-point incremental forming (SPIF) of 45° and 67° conical shapes with AA7075 sheet metal. It was found that simulation results obtained with Yld2004 yield function well predicts the deformation characteristics of both cone shapes when compared with experimental results. Also, the yield locus of AA7075 sheet metal after the SPIF process was predicted based on evolved crystal orientations and the critical resolved shear stress.

Process parameters effect on high-temperature friction and galling characteristics of AA7075 sheets

ABSTRACT During hot forming of high-strength 7xxx series alloy, friction and galling often occur on the contact surface, degrade product surfaces quality, and affect severely by the process parameters, especially by load and speed. In the present paper, the effect of process parameters, namely applied load and sliding speed, on high-temperature friction and galling performance of AA7075 sheets has been determined using linear sliding tests at 350°C in simulated hot forming. The microscopic observations, i.e., scanning electron microscopy (SEM) and three-dimensional confocal scanning optical microscopy, were utilized to analyze hard phase distribution of the alloy as well as surface and sub-surface of the posttest specimens. A friction model was proposed to describe the friction mechanism. The results show that wear rate gradually decreased with increasing load and speed, attributing to the formed compacted oxide layer on the worn surface, which inhibited the rate of further attrition. Besides, for all test conditions, adhesion dominated the wear mechanism, and the compacted layer was damaged more serious due to more initiation of micro-cracks between hard phase and matrix and higher contact interface temperature at higher load and speed, thus the extent of adhesion is grievous resulting in a higher wear loss.

Local and global tensile deformation behavior of AA7075 sheet material at 673oK and different strain rates

The constitutive behavior of AA7075 aluminum sheet like other metallic sheet materials is highly temperature and strain rate dependent at elevated temperatures. The uniaxial tensile test is commonly employed to characterize the temperature and strain rate dependent deformation behavior. However, the traditional uniaxial tensile testing and data analysis procedure treats the gauge length of sample as a uniformly-deforming (or homogeneous) region. This may not be accurate due to early localized deformation that is initiated in the gauge region at elevated temperatures. Treating the post-necking hardening behavior of uniaxial tensile samples has been an important task to expand the effective range of true stress-true strain curves at elevated temperatures for metal and alloys. To explore this issue, uniaxial tensile tests on AA7075-F sheets (with as-fabricated temper) were conducted at 673o K and four different test speeds. The test procedure also utilized an on-line 2-D digital image correlation (DIC) system method to obtain full-field strain map from the deforming gauge region of the test specimen. Local stress-strain curves at different points along the gauge length were then calculated based on the recorded local in-plane principal stress and strain values from macroscopic force data and current cross-section of the specimen at the chosen points. In this manner, the DIC strain analysis not only enabled the determination of stress-strain response but also the determination of strain rates at specific chosen points along the gauge length. The local stress-strain curves revealed significant deviation at an earlier stage of deformation from the global (or macroscopic) stress-strain response based on the entire gauge region. To estimate the constitutive behavior at different strain rates, a correction method to traditional stress-strain curves was applied to obtain a family of stress-strain curves at different constant strain rates based on the local stress-strain curves. Additionally, the material deformation response in the stress, strain and strain rate space in the 3D Cartesian coordinate system was also constructed for tests conducted at different stretching speeds. Further, a verification test was conducted to see if the above methodology could provide an accurate estimation of local deformation behavior of AA7075 sheet. Furthermore, the above experimental method was also coupled with the Gurson-Tvergarrd-Needleman (GTN) ductile void damage model and incorporated into ABAQUS-Explicit general purpose finite element (FE) analysis code via a VUMAT material subroutine. Good agreement was obtained for both the local and global stress-strain curves between the FE test simulation and experimental stress-strain results. The proposed experimental methodology and the resulting stress-strain-strain rate behavior is believed to be applicable in describing the strain rate-dependent constitutive behavior of AA7075-F sheet metals at 673o K.

Mechanical properties of rolled and aged AA6061 sheets at room-temperature and cryogenic environments

We prepared AA6061 sheets by room-temperature rolling and low-temperature aging. We tested the tensile mechanical properties of the sheets at room temperature and cryogenic (173 K) temperature. The maximum difference between the cryogenic ultimate tensile strength and room-temperature ultimate tensile strength was 50 MPa for the sheets subjected to a rolling reduction ratio of 50% and aging at 393 K for 10 h. As the rolling reduction ratio increased, the lattice friction stress, dislocation strengthening, and precipitation strengthening of the material gradually approached a limiting value, thus reducing the difference between the strengths of the samples tensile tested at room temperature and cryogenic temperature.

Effect of Cavitation Peening on Fatigue Properties in Friction Stir Welded Aluminum Alloy AA5754

Friction stir welding (FSW) is an attractive solid-state joining technique for lightweight metals; however, fatigue properties of FSWed metals are lower than those of bulk metals. A novel mechanical surface treatment using cavitation impact, i.e., cavitation peening, can improve fatigue life and strength by introducing compressive residual stress into the FSWed part. To demonstrate the enhancement of fatigue properties of FSWed metal sheet by cavitation peening, aluminum alloy AA5754 sheet jointed by FSW was treated by cavitation peening using cavitating jet in air and water and tested by a plane bending fatigue test. The surface residual stress of the FSWed part was also evaluated by an X-ray diffraction method. It was concluded that the fatigue life and strength of FSWed specimen were improved by cavitation peening. Whereas the fatigue life at σa = 150 MPa of FSWed specimen was about 1/20 of the bulk sheet, cavitation peening was able to extend the fatigue life of the non-peened FSW specimen by 3.6 times by introducing compressive residual stress into the FSWed part. This is the first paper to demonstrate the improvement of fatigue properties of FSWed metallic sheet by cavitation peening.

Process and performance characteristics of an improved friction-stir riveting process

Friction-stir riveting is a new mechanical fastening technique developed in recent years. It combines the spinning and extruding actions of friction-stir spot welding, and the interlocking created by embedding a rivet in the sheets in self-piercing riveting, producing a joint which strength stems from a compact stirred or mixed zone, an embedded solid rivet, and solid bonding. In this study a set of improved rivet design and tooling, based on the understanding of friction-stir riveting process and the joint formation gained in previous investigations, was used to join aluminum alloy sheets. The evolution of microstructure during riveting was investigated using the new rivet and tooling, and the joint structural characteristics were analyzed. A detailed study was also conducted on the effect of rivet depth of feed, the joint structures created, and the performance of these joints, compared with those using the previous rivet design and tooling. Afterwards, a statistical analysis was conducted to optimize the riveting process. Using the new rivet and tooling, and optimized riveting parameters, joints with peak load consistently exceeding 8 kN were obtained in joining 2-mm AA5052 sheets. The strength of the new friction-stir riveted joints is much higher than those obtained through other mechanical joining techniques, and even higher than resistance spot welding in joining aluminum sheets.

Experimental investigations on deformation characteristics in microstructure level during incremental forming of AA5052 sheet

Abstract Revealing the deformation mechanism of incremental sheet forming (ISF) in microstructure level is crucial in verifying the enhanced formability, which is helpful for fabricating the panels with low-ductility materials or with complex geometry. In the present work, grain fragmentation, grain orientation, void evolution and dislocation accumulation during ISF were observed by EBSD, SEM, TEM and X-Ray microscope. Results show that the grains are elongated along the meridianal direction with obvious grain fragmentation, and the special grain gradient along thickness direction was observed. The distinct grain orientation along {111} are gradually accumulated with the increasing of plastic deformation. Meanwhile, the voids are coalesced and elongated basically along meridianal direction into strip-shaped voids. The observation of TEM micrographs shows that coarse grains are transformed into fine sub-grains/grains with obvious misorientation. Multiple sets of dislocation pile-up stacked at the sub-grain boundaries is believed to increase stress concentration and tendency of micro-crack.

An insight into the vibration-assisted rolling of AA5052 aluminum alloy: Tensile strength, deformation microstructure, and texture evolution

Abstract Lightweight metals and alloys should have high strength and moderate ductility if they want to meet the requirements of practical applications. For this aim, a continuous severe plastic deformation technique entitled vibration-assisted rolling (VAR) was introduced to produce high-strength AA5052 aluminum alloy. Also, the effect of annealing heat-treatment was investigated on mechanical properties, microstructure characterization, and texture evolution of the VAR-processed samples. It was found that this process can severely deform the sheets through two rollers, the bottom roller of which is associated with vibration, using plastic pressure, and shear deformation. The results showed that the strength of the VAR-processed sample (369 MPa) is much higher than when using a conventional rolling process (181 MPa). Additionally, at annealing temperatures of 150 and 200 °C, the yield strength of the VAR-processed AA5052 sheet reached 324 and 302 MPa, respectively. In this regard, the elongation to failure of the above sample at 200 °C annealing temperature reached about 8%. Besides, the ultrafine-grained microstructure was observed in the VAR-processed samples even after annealing conditions. Eventually, a strong Cube texture was obtained at the 150 °C annealing temperature in the VAR-processed aluminum; whereas, the annealed alloy under the conventional rolling condition did not show this strong Cube texture.

A pressure-coupled Drucker function for plasticity and fracture modelling of AA5182

In this paper, a pressure-coupled Drucker function is proposed to model the plastic deformation and fracture from shear to plane strain tension of AA5182 sheet. Experiments are conducted for AA5182 in shear, uniaxial tension and plane strain tension, and the force-stroke curves are measured during the tests. The plastic deformation is modeled by the Drucker function. The Drucker function is modified to consider the pressure effect for fracture limit stress prediction of the alloy. The pressure-coupled Drucker function for plasticity and fracture is calibrated by an inverse engineering approach. The calibrated plasticity models are then applied to numerical prediction of plastic deformation of the alloy under various loading conditions. It is observed that the pressure coupled Drucker function accurately describes the plastic deformation under large plastic deformation as well as the onset of ductile fracture under these loading conditions.

The effect of cold rolling, double aging and overaging processes on the tensile property and precipitation of AA2024 alloy

In this investigation, the effect of the amount of cold rolling on the single stage and double stage aging responses of an AA2024 aluminum alloy was assessed. Two cold deformation amounts were applied on the solution heat treated AA2024 sheets. The effect of these deformation on a two-step short time aging performed at low, and subsequently at high temperatures, were studied in terms of precipitation and mechanical properties variations. As well, the effect of rolling percentage on the singe stage of overaging was assessed. Microstructural evolution was studied by optical microscope (OM) and Field Emission Scanning Electron Microscope (FE-SEM) and Transmission Electron Microscope (TEM). Mechanical properties were assessed through tensile testing and microhardness variation. Electrical conductivity (EC) measurements were also made to track the changes of precipitation condition. Results showed that applying cold rolling substantially increased the strength and slightly reduced ductility. In addition, performing the cold rolling created better condition, through dislocation generation, for the strengthening precipitates. Under such a condition, a short time aging through double aging process provided higher strength level compared with the long aging process of the overaging treatment. TEM studies showed the development of sub-structure through dislocation cells formation for the cold deformed cases. It also showed that very small size S′ hardening precipitates are present for the highest level of strength/hardness achieved in the double aging process. Also TEM studies suggested that no morphological changes occurred in the highest strength level achieved for the double aged conditions. Microhardness and electrical conductivity variation followed the same trend as the strength variation, for both the double aging process and the overaging condition.

Optimization of pyramid shaped single point incremental forming of AA5052 alloy sheet

This paper presents a comprehensive experimental study on Single Point Incremental Forming (SPIF) of Aluminium alloy AA5052 sheet of 1.5 mm thickness. In order to investigate the response variables such as surface roughness and wall angle were measured for analysis. Feed rate, Step depth and Spindle speed were used as process parameters. Three different diameters of the carbide tools were employed. Also, to inspect the property of incremental forming constraints, a pyramid shaped component with fixed wall angle of 660 was considered using L9 orthogonal array (OA). These multi-objective responses have been regularized with the help of Grey Relational Analysis (GRA). Thus, the Taguchi method (TM) was adopted to verify the optimal grouping of forming parameters. Tool diameter and spindle speed is identified as most influencing effect on surface roughness and wall angle respectively. The grey relational grade value of confirmation run is improved by 6.3% from the predicted value.

Improving the properties of AA7075 resistance spot-welded joints by chemical oxide removal and post weld heat treating

The aluminium alloy studied in this paper is the heat treatable AA7075 which has zinc as the main alloying element. 7xxx aluminium alloys have the best strength performance among all commercial series and AA7075 has tensile strength above 500 MPa. The outstanding strength properties open the possibility to use this alloy in automotive industry as a possible alternative material for car body elements instead of steel. However, their limited formability properties mean obstacle when a complicated shape car body panel is formed, since the elongation is limited only between 5 and 11%. In order to successfully form these sheets into the demanded geometry, hot forming should be used, and thus the body panels should be formed when the AA7075 is in a solution heat treated condition, such as using the HFQ® process. Then, after assembling the body elements, when resistance spot welding is among the most common joining methods, the AA7075 body parts should get the artificial ageing. It means that welding is followed by artificial ageing as a kind of post weld heat treating which can partially compensate the softening of this alloy during welding. Softening is considered among the most challenging weldability issues similarly to the high melting point oxide layer at the surface which also reduces the load bearing capacity of RSW joints. During the performed experimental program, three routes were investigated. The RSW experiments are performed on 1-mm-thick AA7075 sheets. The spot-welded joints were examined by macroscopic testing, tensile shear and hardness tests.

Corrosion behavior of novel AA1050/ZnO surface composite: A potential material for ship hull

Friction stir processing is one of the effective surface treatments which was employed to process the AA1050 sheets in bare and reinforced condition. The primary objective of the investigation was to expand the applications of AA1050 as a ship hull element in shipbuilding with the least corrosion rate to withstand the harsh marine environment. The base material processed with a rotational speed of 1200 rpm resulted in the highest corrosion rate of 0.173622 mpy. The formation of Al-Fe intermetallic phases was responsible for pitting corrosion. Further, processing by embedding zinc oxide with a rotational speed of 1000 rpm exhibited ~6.68 times improvement in corrosion resistance compared to as-received material. The corrosion rate was found to be 0.003390 mpy. The Al2O3 passive film hinders the initiation and propagation of pits. This study coins a novel composite material and future investigations are emphasized on the same lines.

Effect of inter-cycle heat treatment in accumulative roll-bonding (ARB) process on planar isotropy of mechanical properties of AA1050 sheets

The accumulative roll-bonding (ARB) process was applied on the strips of aluminum alloy 1050 in two processing conditions: cold ARB and warm ARB. The results of tensile tests and microhardness measurement show that the warm ARB process exhibits the lower tensile strength and microhardness, more homogeneous distribution of the microhardness, higher elongation, and especially superior planar isotropy of the tensile properties in comparison to the cold ARB, because of the intermediate heat treatment as well as the elevated temperature rolling in the warm ARB process. Furthermore, with increasing the cycles of both processes, the planar isotropy decreases progressively.

The effect of plastic deformation on mechanical properties of aluminium matrix composites reinforced with 2D crystals

Abstract In this study, AA6061 matrix composites reinforced with multilayer graphene and MoS2 were analyzed. The composites were prepared by powder metallurgy using the spark plasma sintering and spark plasma texturing methods. Microstructure, physical and mechanical properties were investigated and compared with unreinforced AA6061 sinter and AA6061 sheet plate. The results showed that the application of spark plasma texturing positively influences the relative density and compressive yield strength of AA6061 matrix composites. Moreover, in composites with MoS2, significant differences in compressive yield strength between the centre and the edge of the sintered compacts were noticed. These differences are related to the formation of the MoAl12 phase as a result of the temperature gradient generated in the graphite die during sintering by the spark plasma texturing.

A new tool path with point contact and its effect on incremental sheet forming process

Incremental sheet forming (ISF) is a promising and flexible sheet metal forming process, which is appropriate for small batch production of complex parts and customized sheet metal parts. Although the formability of sheet metal in ISF can be significantly improved compared with conventional stamping, some low-ductility materials still cannot be formed to specific geometry through ISF at room temperature, such as magnesium alloy and titanium alloy. Besides, geometric deviation affects industrial applications. In the present work, a novel point-contact tool path (PCTP) method free of circumferential friction is developed. For better understanding the forming characteristic and deformation mechanism of this new tool path, the surface quality, geometric accuracy, and forming limit of the sheet metal are investigated through experiment and numerical simulation for different sheet materials and compared with conventional spiral tool path. Experimental results demonstrate that PCTP can obviously improve the surface quality and geometric accuracy by using small interval length. Besides, the forming limits of AA2024 and AA7075 sheets are improved dramatically due to the absence of circumferential friction, and an updated analytical model of stress triaxiality is developed based on membrane analysis to explore the increased forming limit of PCTP. This novel tool path provides a simple and feasible solution for forming low-ductility sheet metal components at room temperature.

Influence of Intermediate Heating in Cross Accumulative Roll-Bonding Process on Planar Isotropy of the Mechanical Properties of Commercial Purity Aluminium Sheet

In this study, the cross accumulative roll-bonding (CARB) process, as an improved technique of the conventional accumulative roll-bonding (ARB), was conducted on the AA1050 sheet up to 10 passes in two processing modes. The specimens were either preheated before the rolling step of CARB passes and promptly roll bonded (warm CARB) or were processed at room temperature (cold CARB). The microstructure of samples was characterized using a scanning electron microscope. The effect of intermediate heat treatment on the planar homogeneity of the mechanical properties in the CARB (and even ARB) deformed sheets has not been examined yet. Hence, the present research focuses on the comparison of the planar isotropy of tensile properties between warm and cold CARB products. Also, the uniformity of microhardness distribution throughout the thickness of samples was evaluated. The warm CARB presented higher elongation, lower tensile strength and microhardness with more uniformity of the microhardness distribution, and also premier planar isotropy of the mechanical properties than the cold CARB.

Investigating Mechanical and Corrosion Behavior of Plain and Reinforced AA1050 Sheets Fabricated by Friction Stir Processing

The present investigations help in improving the bendability and corrosion resistance of AA1050 rolled sheets by selective friction stirring. The processing of AA1050 with a tapered square pin at a tool rotation speed of 1200 rpm yielded the highest strain of 0.345 at ultimate tensile strength compared with 0.054 in as-received material. The identified processing conditions produced an ultimate tensile strength of 89.23 MPa with a toughness of 34.451 × 106 J/m3 and a lower corrosion rate with Icorr of 0.324 × 10−6 A/cm2. Further, processing with a simple tapered circular pin resulted in maximum ultimate tensile strength of 102 MPa with a toughness of 33.990 × 106 J/m3. However, it came at the expense of least resistance to corrosion with Icorr of 4.813 × 10−6 A/cm2. Consequently, the addition of zinc oxide during friction stirring showed a remarkable improvement in corrosion resistance with Icorr of 0.094 × 10−6 A/cm2. Future studies are planned on these lines.