Defect features and mechanical properties of friction stir lap welded dissimilar AA2024–AA7075 aluminum alloy sheets

5 mm-Thick dissimilar AA2024-T3 and AA7075-T6 aluminum alloy sheets were friction stir lap welded in two joint combinations, i.e., (top) 2024/7075 (bottom) and 7075/2024. The influences of process conditions (welding speed and joint combination) on defects (hook and voids) features and mechanical properties of joints were investigated in detail. It was found that the hook deflects largely upwards into the stir zone (SZ) at lower welding speeds (50, 150 mm/min) in both combinations. The process conditions significantly affect the hook geometry which in return affects the lap shear strength. In all 2024/7075 joints, voids appear and the joints fracture from the tip of hook on AS along the SZ/TMAZ (thermomechanically affected zone) interface in lap shear test (tensile fracture mode). In 7075/2024 joints, the hook on RS horizontally extends a large distance into the bottom stir zone at higher welding speeds (225, 300 mm/min). The joints fracture in three modes: shear fracture along the lap interfaces, tensile fracture and the mix fracture of both. In both joint combinations, the lap shear strength generally increases with the increase of welding speed. 7075/2024 Joints show higher failure load than 2024/7075 joints at lower welding speeds while the opposite result appears at higher welding speeds.     

Post-impact compressive strength of curved GFRP laminates

Low velocity impacts to fibre reinforced plastic composites cause a pattern of damage consisting in general of delamination, fibre breakage and matrix cracking. Such damage is accidental and may go unnoticed; therefore composite structures must be designed assuming impact damage exists. Previous work on flat composite laminates has resulted in a reasonable understanding of the mechanisms of compressive strength reduction. There are, however, many instances where curved laminates are used in structures where impact is likely. Furthermore, due to the mechanisms of strength reduction, it may be expected that curvature would have a significant effect on the behaviour of the laminates.The work described here consists of experimental measurement of the post-impact compressive strength of curved GFRP laminates. The laminates were of 8 plies of 0.3 mm thick pre-impregnated glass fibre/epoxy tape in a (0, ±45, 0°)_s lay-up. Each laminate was 200 mm in length by 50 mm wide with the plane of curvature normal to the length. Laminates were impacted on the convex surface of the laminate by dropping a steel mass from 1 m vertically above it.Impacted laminates were loaded in compression and the out-of-plane displacements of the top and bottom surfaces were recorded. Final failure was typically due to fibre breakage occurring through the centre of the impacted area of the laminate. Possible differences in the impact response, and measurable differences in the sizes of the impact damage area, were found to arise from these curvatures, and differences were observed in their post-impact buckling behaviour. However, perhaps unexpectedly, the post-impact compressive strength for a curved laminate was found to be similar to that for a flat laminate. The failure loads for the impact damage laminates are shown to be comparable with those for laminates containing artificial delaminations.     

Influence of process parameters on mechanical performance and bonding area of AA2024/carbon-fiber-reinforced poly(phenylene sulfide) friction spot single lap joints

The effects of friction spot joining process parameters on the bonding area and mechanical performance of single lap joints were investigated using full-factorial design of experiments and analysis of variance. On one hand, the main process parameters with significant influence on the bonding area were joining pressure, tool rotational speed and joining time. On the other hand, tool rotational speed and joining pressure displayed the highest influence on the lap shear strength of the joints followed by tool plunge depth, whereas the joining time was not statistically significant. The interaction between the rotational speed and joining time was the only interaction with a significant effect on the mechanical performance. Joints with ultimate lap shear forces varying between 1698 ± 92 N and 2310 ± 155 N were obtained. It was observed that generally a larger bonding area as a result of higher heat input leads to an increased mechanical performance of the joints. The generated regression model by the analysis of variance was used to identify an optimized set of parameters for increasing the lap shear strength of the joints to 2280 ± 88 N. Furthermore, the process temperature was monitored, which varied in the range of 370–474 °C.     

Determination of the damage threshold in woven-ply thermoplastic laminates at T > Tg: Acoustic emission and microscopic damage analysis

An original in situ measurement of acoustic emission (AE) was applied to monitor damage progress in discrete steps during gradual load/unload tensile tests on [±45°]₇ C/PPS laminates at temperatures T > Tg, when matrix ductility is enhanced. In order to understand the specific damage behavior of such materials under severe environmental conditions, AE analysis was accompanied by microscopic observations to detect the damage initiation threshold as well as the damage mechanisms within the composite material. Once the AE source mechanisms have been separated into classes thanks to the pattern recognition software Noesis, they have been identified to match physical phenomena. Earliest cracks events occur at the crimps where the rotation of warp/weft fibres takes place, followed by the intra-bundles splitting on free surface. It is observed that the onset of intralaminar cracking and debonding is affected by the presence of matrix-rich regions between the plies, because of an extensive plasticization of the PPS matrix. The study of the specific acoustic activity of neat PPS resin specimens confirms that the local plastic deformation in matrix-rich areas contributes to delay the initiation of damage, and subsequent AE signals. Finally, AE proved to be a relevant technique to investigate damage mechanisms and to determine accurately the damage threshold in TP-based composites to be used in aeronautical applications at T > Tg.     

Feasibility study of friction spot welding of dissimilar single-lap joint between poly(methyl methacrylate) and poly(methyl methacrylate)-SiO2 nanocomposite

In this work, the feasibility of friction spot welding (FSpW) of a commercial poly(methyl methacrylate) (PMMA) GS grade and a PMMA 6 N/2 wt% silica (SiO₂) nanocomposite was investigated. Single-lap joints welded at rotational speeds of 1000, 2000 and 3000 rpm were produced. The analysis of the joint microstructure and material flow pattern indicated that joints could be produced using all of the tested welding conditions. However, the joint produced at 1000 rpm displayed sharp weld lines (weak links), indicating insufficient heat input, while the welds produced at 3000 rpm displayed excessive plastic deformation (bulging of the bottom plate), volumetric defects and a lack of material mixing in the welded area, associated with higher heat input. The weld produced at a rotational speed of 2000 rpm resulted in improved material mixing, which was indicated by the absence of weld lines and volumetric defects due to the more correct heat input. This welding condition was selected for further mechanical testing. Lap shear testing of PMMA GS/PMMA 6 N/2 wt% SiO₂ nanocomposite single lap joints welded at 2000 rpm resulted in an average ultimate lap shear strength of 3.9 ± 0.05 MPa. These weld strength values are equal to or better than those obtained using state-of-the-art welding techniques for PMMA materials, thereby demonstrating the potential of friction spot welding for thermoplastic nanocomposites.     

About the influence of temperature and matrix ductility on the behavior of carbon woven-ply PPS or epoxy laminates: Notched and unnotched laminates

Could thermoplastic-based composites be used to replace thermosetting-based composites in high-temperature secondary aircraft structures? The purpose of this work is to establish the ability of a material system to be used in aircraft engine nacelles when subjected to static loadings, with a key upper temperature of 120 °C. In order to provide answers to this question, the thermo-mechanical behaviors of carbon fiber fabric reinforced PPS or epoxy laminates have been compared specifically within the temperature change with 120 °C at the upper bound. The temperature-dependent ductile behavior of laminates is more or less exacerbated, depending on polymers glass transition temperature, and laminates stacking sequence. For both materials, the degree of retention of tensile mechanical properties is quite high in notched and unnotched quasi-isotropic laminates. A Digital Image Correlation technique has been used in order to understand the influence of temperature and matrix ductility on the mechanisms of overstresses accommodation near the hole. In fabric reinforced laminates, the high-temperature results suggest a competition between the mechanisms of damage, and the mechanism of plasticization, enhanced in angle-ply lay-ups. Thus, the highly ductile behavior of TP-based laminates, at temperatures higher than their T_g, is very effective to accommodate the overstresses near the hole.     

Friction Spot Joining of aluminum AA2024/carbon-fiber reinforced poly(phenylene sulfide) composite single lap joints: Microstructure and mechanical performance

Friction Spot Joining is a promising alternative joining technology for polymer–metal hybrid structures. In this work, the feasibility of Friction Spot Joining of aluminum AA2024-T3 (bare and alclad)/carbon-fiber reinforced poly(phenylene sulfide) is reported. The process temperature and the microstructure of the joints were investigated. Lap shear tensile strength as high as 27 MPa was achieved by using aluminum bare specimens. Sand blasting was also performed as an effective mechanical surface pre-treatment on aluminum surfaces, which resulted in higher surface roughness and accordingly improved mechanical performance for the selected conditions. In addition, the alclad specimens exhibited promising mechanical performance (lap shear strength of up to 43 MPa) that justifies further investigations. Finally, the bonding and failure mechanisms of the joints are briefly discussed.     

Influence of the laminate lay-up on the fatigue behaviour of SiC-fibre/BMAS-matrix composites

The influence of the laminate lay-up on the mechanical response and the damage development during fatigue loading of SiC fibre toughened BMAS glass-ceramic matrix composites was studied at room temperature. Uni-directional, cross-plied, angle-plied, and quasi-isotropic laminates were investigated. The different materials survived 10⁶ fatigue cycles for maximum fatigue stresses below a stress level dependent on the laminate lay-up. The thus defined fatigue limit decreased from 375 MPa for uni-directional to 70 MPa for angle-plied SiC/BMAS. Finally, the fatigue damage mechanisms were identified and the mechanical response related to the fatigue damage.     

Interlaminar shear behavior of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid and non-hybrid composite laminates

The interlaminar shear behavior of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid composites was studied with short beam shear bending test. Random glass fiber (R)/epoxy means chopped fiber composite having short discontinuous fiber randomly dispersed in epoxy matrix. The effect of stacking sequence and unidirectional glass fiber relative volume fraction (V_fU/V_fT) on the interlaminar shear strength (ILSS) of the manufactured composites has been investigated experimentally and theoretically. The laminates were fabricated by hand lay-up technique with 5 plies. Two non-hybrid composite laminates [R]₅ and [U]₅ were fabricated using the same fabrication technique for the comparison purpose. The average thickness of the manufactured laminates is 5.5 ± 0.2 mm and the total fiber volume fraction (V_fT) is 37%. Failure modes of all specimens were investigated. Experimental results indicated that the ILSS of [U]₅ is higher than those of hybrid and [R]₅ composite. Hybrid composites have higher ILSS than that of random composites. The stacking sequence and (V_fU/V_fT) ratio have a detectable effect on ILSS of the investigated composites.     

Off-axis tensile fatigue assessment based on residual strength for the unidirectional 45° carbon fiber-reinforced composite at room temperature

The tensile fatigue behavior of unidirectional carbon fiber-reinforced thermoplastic and thermosetting laminates was examined at room temperature. Tension-tension cyclic fatigue tests were conducted under load control at a sinusoidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limits of carbon fiber-reinforced thermoplastic laminates (CF/PA6) and thermosetting laminates (CF/Epoxy) were found to be 28.0 MPa (48% of the tensile strength) and 56.2 MPa (63% of the tensile strength), respectively. Two types (in constant and incremental loading way) of loading-unloading low cycle fatigue tests were employed to investigate the modulus history of fatigue process for announcing the fatigue mechanism. The residual tensile strength of specimens that survived fatigue loading maintained with the increase of fatigue cycles and applied stress. Examination of the fatigue-loaded specimens revealed that the more flexible/ductile trend of resins and the formation of micro-cracks at the interface between fiber and matrix was facilitated during high fatigue loading (⩾fatigue limit stress), while no interfacial/matrix damage in resins was detected during low fatigue loading (<fatigue limit stress), which was consider to be the governing mechanism of strength maintain during fatigue loading.     

New ductile laminate structure of Ti-alloy/Ti-based metallic glass composite with high specific strength

Bulk laminate structure of Ti-alloy/Ti-based metallic glass composite(MGC) was prepared by melting a preform of alternate stack-up foils in the high vacuum atmosphere. The composite demonstrates a good combination of yield strength(~1618 MPa), plasticity(~4.3%) and specific fracture strength(384 × 10~3 Nmkg~(-1)) in compression. The maintained yield strength results from the unique microstructure composed of the Ti layer, the solution layer with gradient structure and the MGC layer. Such a multilayer structure effectively inhibits the propagation of shear band, leading to the enhanced plasticity. Those extraordinary properities suggest that combining ductile lamella with brittle metallic glass(MG) by such a lay-up method can be an effective way to improve mechanical properties of MG.     

Evaluation of microstructures and mechanical properties of friction stir welded lap joints of Inconel 600/SS 400

The microstructures and mechanical properties of friction stir welded Inconel 600 and SS 400 lap joints were evaluated in this study. Friction stir welding was carried out at a tool rotation speed of 200 rpm and a welding speed of 100 mm/min. Application of friction stir welding was notably effective in reducing the grain size of the stir zone, as a result, the average grain size of Inconel 600 was reduced from 20 μm in the base material to 8.5 μm in the stir zone. The joint interface between Inconel 600 and SS 400 was soundly welded without voids and cracks, and MC carbides with a size of 50 nm were partially formed in the region of the lap joint interface in Inconel 600. In addition, a hook from SS 400 was formed on the advancing side of the Inconel 600 alloy, which directly affected an increase in the peel strength of the weld. In this study, we systematically discussed the effect of friction stir welding on the evolution of the microstructures and mechanical properties of friction stir lap jointed Inconel 600 and SS 400.     

Achieving superior mechanical properties in friction lap joints of copper to carbon-fiber-reinforced plastic by tool offsetting

It is a challenge to achieve a sound welded metal/carbon-fiber-reinforced thermoplastic (CFRTP) joint with high strength and few bubbles. In this study, sound lap joints of Cu and CFRTP were obtained by friction lap joining (FLJ) directly at rotation rates of 600–2000 rpm, with the welding tool at the joint center and offsetting the tool 7 mm away from the center toward the retreating side, respectively. Tool offsetting reduced the non-uniform temperature distribution in the lap joints resulting from the high conductivity of Cu, which not only enhanced the tensile shear force from 0.89–2.25 kN to 1.71–3.54 kN, with the maximum increasing rate of 135%, but also reduced the bubble area to only 19% of the original level of 2000 rpm. It is the first time to report a high-quality Cu/CFRTP joint with a high strength and few bubbles. The large increase of the strength after tool offsetting was attributed to the increase of the joining area, the decrease of bubbles and the decrease of the CFRTP degradation. The details on the generation, quantitative distribution and expulsion of the bubbles in the FLJ joints were discussed.     

Mechanical properties of a medical β-type titanium alloy with specific microstructural evolution through high-pressure torsion

The effect of high-pressure torsion (HPT) processing on the microstructure and mechanical biocompatibility includes Young's modulus, tensile strength, ductility, fatigue life, fretting fatigue, wear properties and other functionalities such as super elasticity and shape memory effect, etc. at levels suitable for structural biomaterials used in implants that replace hard tissue in the broad sense (Sumitomo et al., 2008 [4]). In particular, in this study, the mechanical biocompatibility implies a combination of great hardness and high strength with an adequate ductility while keeping low Young's modulus of a novel Ti–29Nb–13Ta–4.6Zr (TNTZ) for biomedical applications at rotation numbers (N) ranging from 1 to 60 under a pressure of 1.25 GPa at room temperature was systematically investigated in order to increase its mechanical strength with maintaining low Young's modulus and an adequate ductility.TNTZ subjected to HPT processing (TNTZ_HPT) at low N exhibits a heterogeneous microstructure in micro-scale and nano-scale consisting of a matrix and a non-etched band, which has nanosized equiaxed and elongated single β grains, along its cross section. The grains exhibit high dislocation densities, consequently non-equilibrium grain boundaries, and non-uniform subgrains distorted by severe deformation. At high N which is N > 20, TNTZ_HPT has a more homogeneous microstructure in nano-scale with increasing equivalent strain, ε_eq. Therefore, TNTZ_HPT at high N exhibits a more homogenous hardness distribution. The tensile strength and 0.2% proof stress of TNTZ_HPT increase significantly with N over the range of 0 ≤ N ≤ 5, and then become saturated at around 1100 MPa and 800 MPa at N ≥ 10. However, the ductility of TNTZ_HPT shows a reverse trend and a low-level elongation, at around 7%. And, Young's modulus of TNTZ_HPT decreases slightly to 60 GPa with increasing N and then becomes saturated at N ≥ 10. These obtained results confirm that the mechanical strength of TNTZ can be improved while maintaining a low Young's modulus in single β grain structures through severe plastic deformation.     

Dissimilar joining of Ti3Al-based alloy to Ni-based superalloy by arc welding technology using gradient filler alloys

In this study, Ti–Al–Nb, Ti–Ni–Nb and Ni–Cr–Nb system alloys were designed and incorporated in order to construct a gradient structure at the surface of the joined Ti₃Al base material. And the Ti₃Al-based alloy and Ni-based superalloy were successfully joined together using gas tungsten arc (GTA) welding technology. The microstructure evolution, mechanical properties and fractured behaviors of the joints were investigated. The gradient structure remarkably decreased the formation tendency of brittle phases within the joints compared with a single filler alloy and thus improved the joint strength effectively. The average room-temperature tensile strength of the Ti₃Al/In718 dissimilar joint reached 353 MPa, and the strength value at 873 K was 245 MPa. At the Ti–Ni–Nb/Ni–Cr–Nb interface, some Ni₃(Nb, Ti) + (Nb, Ti)Cr₂ and TiNi₃ phases were detected in the Ti–Ni–Nb matrix. It was believed that their presence decreased the room-temperature strength of the Ti–Ni–Nb alloy but improved its high-temperature strength.     

The optimization of friction spot welding process parameters in AA6181-T4 and Ti6Al4V dissimilar joints

Friction spot welding is a relatively new solid-state joining process able to produce overlap joints between similar and dissimilar materials. In this study, the effect of the process parameters on the lap shear strength of AA6181-T4/Ti6Al4V single joints was investigated using full-factorial design of experiment and analyses of variance. Sound joints with lap shear strength from 4769 N to 6449 N were achieved and the influence of the main process parameters on joint performance was evaluated. Tool rotational speed was the parameter with the largest influence on the joint shear resistance, followed by its interaction with dwell time. Based on the experimental results following response surface methodology, a mathematical model to predict lap shear strength was developed using a second order polynomial function. The initial prediction results indicated that the established model could adequately estimate joint strength within the range of welding parameters being used. The model was then used to optimize welding parameters in order satisfy engineering demands.     

Effects of welding parameters and post-heat treatment on mechanical properties of friction stir welded AA2195-T8 Al-Li alloy

In this study, the effects of main welding parameters(rotation speed(ω) and welding speed(v)) on the microstructure, micro-hardness distribution and tensile properties of friction stir welded(FSW)2195-T8 Al-Li alloy were investigated. The effects of T6 post-treatments at different solution and aging conditions on the mechanical properties and microstructure characteristics of the FSW joints were also investigated. The results show that with increasing and v, both strength and elongation of the joints increase first, and then decrease with further increase of and v. All the joints under varied welding parameters show significant strength loss, and the strength reaches only 65% of the base metal. The effect of T6 post-heat treatment on the mechanical properties of the joints depends on the solution and aging conditions. Two heat treatment processes(480℃× 0.5 h quenching + 180℃× 12 h,520℃× 0.5 h quenching + 180℃× 12 h aging) are found to increase the joint strength. Furthermore,low temperature quenching(480℃) is more beneficial to the joint strength. The joint strength can reach 85% of the base metal. Whereas both low temperature aging(140℃× 56 h) and stepped aging(100℃× 12 h + 180℃× 3 h) processes decrease the joint strength. After heat treatment all the joints show decreased ductility due to the obvious grain coarsening in the nugget zone(NZ) and thermo-mechanically affected zone(TMAZ).     

Evaluation of microstructure and mechanical properties of Al/Al2O3/SiC hybrid composite fabricated by accumulative roll bonding process

In this investigation, a new kind of metal matrix composites with a matrix of pure aluminum and hybrid reinforcement of Al₂O₃ and SiC particles was fabricated for the first time by anodizing followed by eight cycles accumulative roll bonding (ARB). The resulting microstructures and the corresponding mechanical properties of composites within different stages of ARB process were studied. It was found that with increasing the ARB cycles, alumina layers were fractured, resulting in homogenous distribution of Al₂O₃ particles in the aluminum matrix. Also, the distribution of SiC particles was improved and the porosity between particles and the matrix was decreased. It was observed that the tensile strength of composites improved by increasing the ARB passes, i.e. the tensile strength of the Al/1.6 vol.% Al₂O₃/1 vol.% SiC composite was measured to be about 3.1 times higher than as-received material. In addition, tensile strength of composites decreased by increasing volume fraction of SiC particles to more than 1 vol.%. Scanning electron microscopy (SEM) observation of fractured surfaces showed that the failure mechanism of broken hybrid composite was shear ductile rupture.     

In-plane shear properties of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid and non-hybrid composites

The in-plane shear properties (shear strength τ_xy and shear modulus G_xy) of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid and non-hybrid composites have been investigated experimentally and theoretically. The effect of stacking sequence and random fiber relative volume fraction (V_fR/V_fT) in hybrid composites were reported. Laminates were fabricated by hand lay-up technique with a total of 5 plies, by varying the number and position of random glass layers so as to obtain four different hybrid laminates; i.e. [0.5R/U/U]_S, [U/0.5R/U]_S, [U/U/0.5R]_S, and [U/R/U/R/U]. All unidirectional fiber laminate [U]₅ and another of all random fiber laminate [R]₅ were also fabricated for comparison purpose. The average thickness of the manufactured laminates is 5.5 ± 0.2 mm and the total fiber volume fraction (V_fT) is 37%. Failure modes of all specimens were investigated. Results indicated that the in-plane shear properties (shear strength τ_xy and shear modulus G_xy) of unidirectional fiber composite can be considerably improved by incorporation of random glass fiber and forming hybrid composites.     

Mode I and Mode II fracture toughness of high-performance 3000 g mole−1 reactive poly(etherimide)/carbon fiber composites

A phenylethynylphthalic anhydride terminated poly(etherimide) oligomer with a molecular weight of M_n=3000 g mole⁻¹ has been synthesized for use in high-performance composite and adhesive applications. This reactive poly(etherimide) exhibits excellent thermo-oxidative stability and solvent resistance upon curing at high temperatures. Towpreg has been produced by coating unsized, G30-500, 12k carbon fiber tow with this oligomer in a powder form. A minimal dry powder prepregging technique was employed for rapid processing to circumvent the problems associated with high viscosity and to perform the operation in an organic solvent-free environment. This towpreg has been manufactured into flat composite laminates via manual lay-up and hot press consolidation and cure. The Mode I and Mode II fracture toughness of these composite panels has been investigated via the double cantilever beam (DCB) and end-notch flexure (ENF) test methods. Fracture surface and microstructure features are correlated with the measured mechanical properties.