Mechanical performance of hot forming products is strongly related to microstructures and deformation behaviors during hot processing. Through dynamic recrystallization (DRX) kinetics analysis and ...simulation, evolution of microstructure and hot-compressive behavior of a solution-treated GH4169 superalloy at different deformation conditions were investigated. The DRX behavior confirmed by microstructure observations was promoted at higher temperatures and lower strain rates. No δ phase existed at or above 1333 K deformation temperature. Flow softening extent became more significant under lower temperatures and strain rates during compression where DRX played a dominate role in the softening behavior. A dramatic yield drop occurred within the temperature range varying from 1273 K to 1333 K due to the dissolution of δ phase and enhanced mobility of defects. The DRX kinetics model was established to calculate the volume fraction and grain size of DRX under the investigated deformation parameters. Additionally, the relationships between microstructure and deformation behavior as well as mechanical property were discussed. Excellent correlation showed a possibility of controlling microstructure and mechanical properties by selecting suitable deformation parameters. Finally, the microstructure of GH4169 superalloy in terms of grain size after hot compression was successfully predicted by finite element model integrated with the developed kinetics equations implying the excellent applicability of such model for the current study and great potential for practical applications on predicting the mechanical properties of GH4169 alloy after hot working.
Display omitted
•Dynamic fractured behaviors of Al/steel interface were observed by in–situ scan electron micrograph technology.•Interface joined with 2–3 μm τ5 exhibited the optimum bonding strength ...of 205 MPa.•Mechanisms for determination of bonding strength by metallurgical bonding, interplanar spacing mismatch and micro–defects were clarified.
Microcracks initiated and propagated behaviors in Al/steel interface determined interfacial bonding strength and this was observed by in situ scanning electron microscopy (SEM) technology. Interface without or with discontinuous intermetallic compound (IMC) had low bonding strength owing to insufficient metallurgical bonding. When interface was joined with 2–3 μm serration–shaped τ5–Fe1.8Al7.2Si, largest bonding strength of 205 MPa was obtained. Microcracks initiated at protrusion of τ5–Fe1.8Al7.2Si and then propagated to τ5–Fe1.8Al7.2Si layer and τ5–Fe1.8Al7.2Si/Al interface. When interface was joined with 3–5 μm θ–Fe(Al,Si)3 + τ5–Fe1.8Al7.2Si, microcracks initiated at root of IMC layers and the interfacial bonding strength was 150 MPa. When interface was joined with 5–10 μm θ–Fe(Al,Si)3 + τ5–Fe1.8Al7.2Si, microcracks initiated and propagated along IMC layer. Lower interfacial bonding strength (106 MPa) was produced owing to large lattice mismatch between θ–Fe(Al,Si)3 and τ5–Fe1.8Al7.2Si, microdefects and higher residual stress. When interface was joined with 10 μm η–Fe2(Al,Si)5 + θ–Fe(Al,Si)3 + τ5–Fe1.8Al7.2Si, microcracks initiated and propagated along η–Fe2(Al,Si)5 layer or steel/η–Fe2(Al,Si)5 interface. Lowest bonding strength (64 MPa) was obtained resulted from pre–generated microcracks in η–Fe2(Al,Si)5 layer, largest residual stress, crystal defects and abnormal aggregation of Si.
Shape optimization combined with Isogeometric Analysis will be an efficient design method since exact Computer Aided Design geometries can be used for numerical analysis thanks to the same basis ...functions. In this study, authors developed an in-house research software JWRIAN-IGA and applied it to optimize the weld geometries of two models, namely a T-joint fillet weld and an elongated boxing fillet weld to reduce stress concentration. A gradient-free stochastic global optimization algorithm, Bayesian Optimization was used to sought optimal geometry parameters. It was shown that the optimized geometry was efficiently designed and the stress concentration in the welds for both cases was greatly reduced.
•Integrated geometry-analysis-optimization framework was developed.•Two weld geometries were optimized to reduce stress concentration.•Multi-patch Isogeometric Analysis was utilized to analyze the weld models.•Stochastic global optimization algorithm was used to find the optimal weld shapes.
A local solid model and a global shell model were employed for fast prediction of deformation in laser welded thin sheets based on inherent strain theory. Transient thermo-elastic-plastic analysis ...was performed on the local three-dimensional solid model to obtain inherent strain for the global shell model. To ensure solution accuracy, the characteristics of the laser welding heat source were considered in determination of the mesh size and time increment. The penetration shape of laser welded joint was well reproduced compared with the results by experimental observation. By applying inherent strain into shell element model, the out-of-plane welding deformation was predicted within a few minutes using elastic FEM. The predicted deformation mode and magnitude agreed with the measured ones for laser welded joints of different dimensions. By considering geometrical imperfection in the numerical model, it was reproduced that the laser welding induced deformation in thin sheets generally has the same mode as initial shape of the sheet.
•Reduction of angular distortion by jig constraint is realized by experiment and simulation.•Effect of two types of jig constraint on welding deformations is quantitatively ...investigated.•Relationships between jig position & pitch and welding deformations are explored.
Quantitative study on jig constraint effect on welding deformation was carried out. Welding deformation in a square plate with bead welding under a non-constraint free condition and a jig constraint condition was investigated by experiment. A 3D thermal elastic–plastic FEM program was employed to simulate the transient temperature and deformation occurred in the welding. It is observed that welding angular distortion has been greatly reduced by the jig constraint, and a good agreement was confirmed between simulation and experiment. Three-direction jig constraint and normal direction jig constraint were defined based on typical constraint types in practical engineering. Two parameters a and b, which represent the pitch between two jigs in the welding direction and the distance from the weld line, respectively, were focused. Effect of jig constraint on longitudinal shrinkage, transverse shrinkage and angular distortion were discussed in details.
Carbon fiber reinforced plastic (CFRP) is a prospective lightweight material in automobile industry. However, joining metal and CFRP is a great challenge. In the present study, an innovative process ...called coaxial one-side resistance spot welding (COS-RSW) is proposed to fabricate Al5052 (Al) and CFRP joints. Based upon our newly developed finite element code JWRIAN, the electric-thermal-mechanical coupled process of COS-RSW is modeled and validated with experiments. The influences of welding current, welding time and electrode force on CFRP molten zone are investigated in detail. The results show that the depth of molten zone has a strong correlation with both welding current and welding time. A relative low welding current or short welding time that results in the Al/CFRP interface temperature below the melting temperature of the resin matrix is insufficient to form a sound connection between the two sheets. While excessive current or too long welding time may lead to overheating of CFRP and decomposition of the resin matrix, which increases the risk of weak joining. Furthermore, it is found that the depth of molten zone increases even at the cooling stage, which indicates accurate simulation for both the heating and the cooling stages of the COS-RSW process is indispensable.
Display omitted
•A coaxial one-side resistance spot welding technology was proposed to join Al5052 and CFRP sheets.•The welding process was modeled based upon an in-house finite element code JWRIAN as a first try.•Different from traditional RSW method, the molten zone depth in CFRP becomes larger even in cooling stage of COS-RSW process.
Fusion-based additive manufacturing techniques such as selective laser melting, electron beam freeform fabrication cause solidification problems such as grain coarseness and high porosity. As a new ...technique, cold spraying (CS) can overcome such melting-induced drawbacks. In this study, a material model using dislocation dynamics was developed specifically for the CS process for describing the following five nanosecond-scale physical phenomena: strain hardening, normal-range strain rate hardening, ultra-high strain rate hardening, thermal softening and grain size evolution. A single Cu microparticle impact test was conducted, and a good agreement between experimental and model-predicted microparticle deformations was observed, indicating high model accuracy. The corresponding finite element model was established, and the individual effects of the above phenomena were discussed in detail to show that material deformation is mainly controlled by ultra-high strain rate hardening while jetting is controlled by thermal softening. Additionally, both simulated and actual grain size distributions indicated that grain refinement occurs only near the microparticle-substrate interface (mainly at the interface edge). Thus, the newly developed model could accurately reproduce the dynamic deformation behaviors of impacting particles and correctly predict grain refinement (particularly due to dynamic recrystallization).
Display omitted
Display omitted
•Solid-state bonding of Al sheets in F-SPR is formed by continuous dynamic recrystallization (CDRX).•CDRX of Al outside the rivet is due to the sliding-to-sticking transition caused ...by rivet rotation.•CDRX inside rivet is due to the strain rate gradient and sticking zone created by the rotation of trapped Al.•η’ & η phases coarsen in HAZ & TMAZs and completely dissolve in FGZ, resulting in Al softening.
Friction self-piercing riveting (F-SPR) is an emerging technique for low ductility materials joining, which creates a mechanical and solid-state hybrid joint with a semi-hollow rivet. The severe plastic deformation of work materials and localized elevated temperatures during the F-SPR process yield complex and heterogeneous microstructures. The cut-off action of the work materials by the rivet further complicates the material flow during joint formation. This study employed the F-SPR process to join AA7075-T6 aluminum alloy sheets and systematically investigated the microstructural evolutions using electron backscatter diffraction (EBSD) techniques. The results suggested that as the base material approached the rivet, grains were deformed and recrystallized, forming two distinct fine grain zones (FGZs) surrounding the rivet and in the rivet cavity, respectively. Solid-state bonding of aluminum sheets occurred in the FGZs. The formation of FGZ outside the rivet is due to dynamic recrystallization (DRX) triggered by the sliding-to-sticking transition at the rivet/sheet interface. The FGZ in the rivet cavity was caused by the rotation of the trapped aluminum, which created a sticking affected zone at the trapped aluminum/lower sheet interface and led to DRX. Strain rate gradient in the trapped aluminum drove the further expansion of the sticking affected zone and resulted in grain refinement in a larger span.
The friction stir lap welding (FSLW) was employed to join the dissimilar 5052 Al alloy and DP590 steel. The intermetallic compound (IMC) formation area and thickness increased linearly with ...increasing rotation speeds at 300 mm/min welding speed. Microvoid and gap defects existed on low-rotational-speed-produced lap interfaces, disappearing with increasing rotation speeds. However, cavities defects neighbouring the large-sized steel fragment appeared in the stir zone of the Al matrix with rotation speeds higher than 750 rpm. The negative influences from the Al-rich IMCs layer exceeding 1.5 µm thick and the aggravating internal cavities defects significantly reduced the joint strength. Correspondingly, the fracture mode of the friction stir lap welded 5052 alloy and DP590 steel was converted from the semi-brittle fracture into the full-brittle fracture. The maximum tensile shear force of 4.52 kN reached 75.2% of the Al 5052 base metal, obtained at the rotation speed of 750 rpm. However, from the combined perspective of integral bearing load capacity and local shear strength, the rotation speed of 500 rpm was recommended to achieve sound FSLW joints of Al5052/DP590.
•Well-formed dissimilar Al5052/DP590 joints were obtained by friction stir lap welding (FSLW).•IMCs layer distribution was correlated to interfacial strength and fractured behaviour.•Material flow characterization and IMCs formation sequence were discussed.•IMCs layer with thickness exceeding 1.5 µm was found to deteriorate the FLSWed Al 5052/DP590 joint significantly.
Safety evaluation of resistance spot welds necessitates the accurate measurement of local constitutive properties. This study employed miniature mechanical tests to investigate the deformation and ...failure behaviors of nugget, heat affected zone (HAZ), and corona bond of resistance spot welded JSC980YL steel. A novel mini-peel test was developed to enable local fracture in HAZ for numerical inverse calibration of constitutive parameters. The fracture constants of weld zones calibrated using Cockcroft-Latham ductile failure criterion were incorporated in finite element models to predict the failure modes of spot welds in tensile-shear and cross-tension coupon tests. The result indicates that the ultimate tensile strengths of the nugget and the corona bond were 37.6% higher and 5.8% lower, respectively, than that of the base material. The nugget and HAZ exhibited ductile fracture, whereas the corona bond was brittle fracture with only 1.2% elongation. In the coupon tests, the increase of nugget diameter slowed down the damage accumulation rate in the nugget and accelerated that in the HAZ, resulting in the transition of failure mode from interfacial to pullout. The failure load of corona bond in coupon tests increased with the increase of nugget diameter while its contribution to the peak load decreased.
Display omitted
•Mini-peel test is a novel way to enable local fracture in heat-affected zone for numerical calibration of fracture parameter.•The increase of nugget size accelerates damage accumulation in heat-affected zone, causing interfacial-to-pullout transition.•The nugget exhibits 74.9% higher yield strength and 37.6% higher ultimate tensile strength than the JSC980YL base material.•The corona bond has yield strength and ultimate tensile strength similar to those of base material, but only 1.2% elongation.
PDF
