•B2 ↔ R transformation temperature of NiTi nanograins increases without practical grain growth after aging at 573 K-2 h.•NiTi nanograins need higher stress for B2 → R transformation than coarse grains even of the same transformation temperature.•Low-temperature aging increases stress hysteresis and residual strain in NiTi nanograins but decreases them in coarse grains.•Aging increases thermal hysteresis but decreases stress hysteresis of R(B2) ↔ B19’ transformation of NiTi coarse grains.The phase transformation behaviors of nanocrystalline NiTi alloys coupling with grain size poses a challenge in functional property configuration. To realize this configuration and simultaneously avoid undesirable grain growth, the low-temperature aging (LTA) treatment at 573 K for 2 h was applied to both the nanocrystalline and coarse-grained NiTi wires in this study and the effect of LTA on both the thermally- and stress-induced phase transformations was respectively investigated. The results show that, after LTA, B2 ↔ R transformation temperature of nanograins was elevated when R → B19′ transformation was maintained suppressed. The stress hysteresis and residual strain of nanograins were increased while those of coarse grains were decreased. Nanograins required higher stress to activate stress-induced R-phase transformation than coarse grains. Aged NiTi coarse grains presented larger thermal hysteresis but smaller stress hysteresis compared with non-aged ones. To have an in-depth understanding of these differences, the microstructures and microhardness were further studied. It turns out that the nanoprecipitation, lattice recovery, as well as the preservation of the preformed grain size are responsible for the differences. This study thus suggests the potential of configuring the functional properties while simultaneously maintaining the constant grain size via LTA treatment, which may facilitate the application of NiTi nanocrystalline.Download high-res image (530KB)Download full-size image

In this study, the effect of long-term neutral salt spray (NSS) exposure (i.e., 50 g/L concentration of salt solution at 25 °C) on the retained strengths of Zr–Ti coated and bare lap-shear aluminum joints bonded with hem flange adhesive Henkel Terokal 8021 NB was investigated. A one-part toughened epoxide adhesive and Zr–Ti coated aluminum substrates (i.e., AA6014-T4 and AA6016-T4) were selected. Adhesive-bonded coated aluminum joints (ACJ) and bare aluminum joints (ABJ) were fabricated and exposed in NSS environment for various times. Quasi-static tests were conducted immediately following removal of the joints from the salt chamber at the ambient condition. It was found that while NSS exposure for 720 h degraded little the strength of ACJ, it decreased the strength of ABJ. The Zr–Ti coating protected aluminum substrates from electrochemical reaction and consequently minimized the strength degradation. As the exposure time was prolonged to 1400 h, the strength of ACJ was reduced drastically while the strength of ABJ was only degraded slightly. Fractography, differential scanning calorimetry, electrochemical potentiostatic polarization measurements, and X-ray photoelectron spectroscopy analyses revealed the strength degradation of ACJ was caused primarily by the corrosion of Zr–Ti coating. The passive film on bare aluminum provided a long time protection against NSS exposure, and consequently minimized the strength degradation for ABJ.

Forming limit curve (FLC) is an effective tool to evaluate the formability of sheet metals. An accurate FLC prediction for a sheet metal is beneficial to its engineering application. A quenched and partitioned steel, known as QP980, is one of the 3rd generation advanced high strength steels and is composed of martensite, ferrite and a considerable amount of retained austenite (RA). Martensite transformation from RA induced by deformation, namely, transformation induced plasticity (TRIP), promotes the capability of work hardening and consequently formability. Nakazima tests were carried out to obtain the experimental forming limit strains with the aid of digital image correlation techniques. Scanning electron microscopy (SEM) was employed to examine the fracture morphologies of Nakazima specimens of the QP980 steel. The observed dimple pattern indicated that tensile stress was the predominant factor which led to failure of QP980 specimens. Therefore, maximum tensile stress criterion (MTSC) was adopted as the forming limit criterion. To predict the FLC of QP980 steel, Von-Mises yield criterion and power hardening law were adopted according to the tested mechanical properties of QP980 steel. Results were compared with those derived from other three representative instability theories, e. g. Hill criterion, Storen-Rice vertex theory and Bressan-Williams model, which shows that the MTSC based FLC is in better agreement with the experimental results.

•Excellent bond performance was achieved on aluminum AA6022-T4 substrates using fiber laser ablation.•Surface topography and chemistry of the aluminum was modified by the laser, which contributed the bonding performance.•Prepared aluminum surfaces appeared stable after aging in ambient conditions.This study was conducted to investigate the effect of laser ablation surface treatment on the joint performance of AA6022-T4 2.0 mm adhesively bonded to itself. It was found that laser ablation treatment at higher energy fluence (i.e., 19.01 J/cm2) improved the joint strength by 25% versus untreated material as measured after water soak exposure. Furthermore, a greater proportion of fracture surface exhibited pure cohesive failure following laser ablation treatment. Results revealed that while laser ablation treatment at lower energy fluence had little influence on the surface topography (though it did act to remove any sheet metal lubricant or other contaminants), treatment at higher energy fluence increased both surface roughness and surface area. This is considered a contributing factor to the improved adhesive bond performance. However, the higher energy fluence were also found to modify the surface chemistry and created a more uniform and thicker aluminum oxide layer, which was likely to be another contributing factor to the improved bond performance. Additionally, the effect of aging (up to 6 weeks) in ambient conditions of laser ablated aluminum on adhesive bonding performance appeared to have no statistical influence.

The mechanical properties of boron steel 22MnB5 is determined by phase fractions. In order to obtain the mechanical properties and service performance of car component by simulation accurately, a microstructure based constitutive model needs to be developed. In this paper, different area fractions of martensite, bainite and ferrite in boron steel 22MnB5 were obtained via control of cooling rate and the different microstructures were determined quantitatively by metallographic image processing. A modified Katsuro Inoue’s constitutive model was then proposed, which is a function of effective plastic strain, strain rate and the phase fractions of martensite, bainite and ferrite. On the other hand, crash simulation of B-pillar with tailored mechanical properties was conducted as a case study to investigate the relationship between microstructure and property configuration by using the modified Katsuro Inoue’s model. The relationship among phase fraction, the height of tailored segment, and the internal energies absorbed by different segments, the maximum acceleration and displacement of B-pillar during the collision process was studied, and the empirical equations between microstructure and dynamic response of crash performance were also established and formulated.

•Friction stir blind riveting (FSBR) is a new method to dissimilar-material joining.•FSBR combines friction stir welding and conventional blind riveting.•Mechanics analysis was performed for frictional penetration by a blind rivet.•A ratio of torque to penetration force was proposed to evaluate contact conditions.•Penetration force increases linearly with material removal rate in sliding contact.The mechanics of frictional penetration driven by a blind rivet to sheet metals is analyzed for a friction stir blind riveting process. Analytic models are deduced to calculate the material removal rate, penetration force and torque during the frictional penetration process. Frictional penetration tests with modified rivets and an Al alloy sheet were carried out at various rotation speed–feed rate combinations, where the penetration force and torque were recorded with a data acquisition system. An analysis of the contact condition between the rivet tip and the work material based upon the assumption of pure sliding contact in the initial penetration to partial sticking contact beyond a critical penetration depth of the rivet is completed, and the results are discussed based on the comparison of the analytically calculated and experimentally measured torque–force ratios.

•Friction stir blind riveting is used to join two Al alloy sheets.•Quality issue and failure modes of the joints were observed and discussed.•The penetration force and torque depend on rivet's spindle speed and feet rate.•Over 95% of the consumed energy is due to rivet's rotational motion during FSBRSingle lap-shear joints of two Al alloy sheets: 2.0 mm AA6111-T4 and 2.0 mm AA6022-T4 were fabricated by applying the friction stir blind riveting (FBSR) process using either a 4.8 mm or 6.4 mm shank diameter blind rivet. The process window was investigated using three spindle speeds: 3000, 6000 and 9000 rpm and three feed rates: 120, 420 and 780 mm min−1. Force and torque FSBR process data was captured and used in part to analyze the mechanical process during frictional penetration of the rivet. Over 95% of the total energy was consumed by the rotational motion during frictional penetration of the rivet. The combination of Al sheet alloy rivet, spindle speed, and feed rate have statistically significant effects upon the peak force, the peak torque, the energy input and the peak power; all of which need to be properly selected for an optimized process

Tailor-welded blanks (TWBs) have been considered as a productive sheet forming method in automotive industries. However, formability of TWBs is reduced due to different properties or thicknesses of the blanks and is a challenge for manufacturing designers. The plastic capacity of TWBs is decreased even when the material and thickness are the same. The constraint effect of the laser weld (including weld and heat-affected zone) material in the forming process of similar TWBs is a key problem to be solved in the research, development and application of thin-sheet TWBs. In this paper, uniaxial tensile tests with full-field strain measurement by digital image correlation and Erichsen tests are performed to investigate the constraint effect on deformation behavior and explore the mechanism of decreasing formability of similar TWBs. In addition, finite element models are conducted under ABAQUS code to further reveal the phenomenal behavior of the constraint effect. The results of the base material and welded blanks are compared for characterizing the differences. Furthermore, in order to better understand this mechanism, theoretical and numerical investigations are employed and compared to interpret the constraint effect of laser weld on the deformation behavior of TWBs. An index is proposed to quantify the constraint effect. Results show that the constraint effect of laser weld appears in both stretch forming and drawing of TWBs. Strain paths are approaching the plane strain condition as compared to the monolithic blank due to the constraint effect. Constraint effect is a major factor affecting the formability of TWBs when the failure occurs away from the weld seam.

22MnB5 steel specimens were deformed at 923 K and 693 K to three strain levels to study the effect of applied strain level on the microstructure and secondary-deformation behavior. As the steel was deformed at 923 K, deformation induced ferrite transformation (DIFT) occurred even when a small strain of 0.044 was applied, and the volume fraction of deformation induced ferrite (DIF) increases with increasing applied strain level. When deformed at 693 K, deformation induced bainite transformation (DIBT) was observed when the applied strain was larger than 0.109. The incubation period for DIFT is shorter than that for DIBT, but the DIBT proceeds much faster than DIFT. Sub-size tensile specimens were cut from the hot deformed 22MnB5 steel specimens, and digital image correlation technique was employed to investigate the secondary-deformation behavior of the sub-size tensile specimens at room temperature. It is found that the appearance of DIF or DIB (deformation induced bainite) decreases the yield strength and ultimate tensile strength (UTS) but increases the elongation and strength–ductility product of the hot deformed 22MnB5 steel specimens compared with the as-quenched 22MnB5 steel specimen with full martensite.Highlights► 22MnB5 steels were deformed at 923 K and deformation induced ferrite was observed. ► 22MnB5 steels were deformed at 493 K and deformation induced bainite was observed. ► RT deformation behaviors of hot deformed 22MnB5 steels were studied by DIC.

Recent trends toward economically fabricating lightweight vehicle structures while ensuring structural performance have led to the implementation of crash-toughened adhesive bonding in automotive industry. Adhesive bonding has been shown to offer better fatigue performance and greater flexibility in joining dissimilar materials when compared with resistance spot welding. While a great deal of effort has been focused on studying the performance of adhesive, there is an urgent need to understand the effect of curing conditions encountered in paint baking environment on the mechanical properties of adhesive-bonded workpieces.This study was carried out to determine the effects of curing temperature, curing time and enclosure conditions in the paint oven on the strength of adhesive-bonded structural steels. The enclosure conditions, simulating the exposure of the vehicle interior and exterior parts in paint baking environment, were classified into three types: fully-enclosed, semi-exposed and fully-exposed. Tests were designed and implemented to examine how the adhesive-bonded interior parts would cure in an electro deposition (ELPO) environment. The results showed that enclosure conditions influenced significantly the strength of the bonded steel because of variations in heat convection. The optimum curing temperature and time significantly depend upon the enclosure conditions of the joints. The increase of the curing time or curing temperature is beneficial for improving the strengths of the adhesive-bonded steel joints.

A significant barrier to broader implementation of magnesium alloys is their poor room temperature formability, a consequence of the anisotropic response of the Mg hexagonal closed-packed (hcp) crystal structure. Additions of rare earth (RE) elements, such as in the ZEK100 alloys, weaken the texture and improve formability. Room temperature forming limit analyses of RE-containing Mg alloys, particularly Mg ZEK100, have not been explored to any significant extent in the literature. In this paper, strain-based forming limit diagrams (FLDs) are derived for an Mg ZEK100-O alloy (Zn1.34Zr0.23Nd0.182, wt.%) using an analytical method that combines the vertex theory of Storen and Rice (J Mech Phys Solids, 23:421-441, 1979), the anisotropic yield criterion of Barlat and Lian (Int J Plast, 5:51-66, 1989), and a hardening law. The method does not rely on assumptions about pre-existing defects, is broadly applicable to sheet alloys exhibiting in-plane anisotropy requiring a higher-order yield criterion, and requires only minimal experimental inputs. Results from the analytical method are compared with experimentally derived FLDs based upon the well-known Nakajima test and tensile deformation, and with predictions from an existing analytical method for FLDs. Close agreement between the experimentally derived FLDs and the present theoretical method was obtained. Sheet materials where the theoretical method does not apply are also discussed.

A lot of research has been focused on the necking process during the plastic deformation of sheet metals, but the localized necking is rarely distinguished form diffused necking by experiments, due to the limit of measurement equipment and method. Quenching and Partitioning (Q&P) steel is a 3rd generation advanced high strength steel (AHSS). Its good combination of high strength and ductility ensures potential application in automobile industry. Uniaxial tensile tests of QP980 steel sheet at five strain rates are performed to investigate the necking process and the effect of strain rate on necking behavior of Q&P steel. Digital image correlation (DIC) method is applied during tensile tests, and evolutions of major strain, minor strain and normal strain distributions along gauge section of the tensile specimens are obtained. The diffused and localized necking strains are determined according to SWIFT necking theory and HILL necking theory respectively. The test results indicate that with the increasing of strain rate in the investigated range, the diffused necking strain decreases from 0.152 to 0.120 and localized necking strain decreases from 0.245 to 0.137. Meanwhile, the difference of the two strains decreases form 0.096 to 0.017. Thus it can be concluded that strain rate has an influence on both necking strains during the deformation of QP980 steel sheet. Diffused and localized necking strains are determined by uniaxial tensile tests with the aid of DIC technique and the effect of strain rate on necking strains is evaluated.

Cyclic tensile loading–unloading tests were carried out with a cyclically incremental strain of 0.03% in a strain range of 0–2% and with cyclically incremental strain of 1% till fracture to investigate the anelastic behavior of annealed Mg ZEK100 alloy sheet. Large hysteresis stress–strain loops were observed in all cyclic loading–unloading tests, which indicate that the annealed Mg ZEK100 alloy sheet exhibits obvious anelastic effect. Due to the fact that twins are easier to produce in transverse direction (TD) than in rolling direction (RD), the alloy shows larger hysteresis loops in TD when applying cyclic loading–unloading. Both true stress and true plastic strain have an effect on the anelastic effect. At a stress, ∼15 MPa higher than the yield strength, the anelastic effect reaches the maximum and the hysteresis loops add an anelastic strain of 0.0018 in RD and 0.003 in TD to the total elastic strain, and consequently lead to a decrease of 30% and 54% in the apparent elastic modulus in RD and TD, respectively. This phenomenon is discussed in terms of the roles of partially reversible twinning and dislocation slip upon unloading and reloading. A novel phenomenological model is developed to describe the anelastic behavior in the Mg alloy, and the calculated hysteresis loops agree well with the experimental ones. Cyclic hardening in the alloy is observed after a certain number of cycles.

As part of a cooperative research program to develop and implement crash-resistant toughened adhesives targeted for future vehicles, this paper summarizes a study of the influence of pre-exposure of uncured adhesive and steel sheets in a humid and elevated temperature environment on quasi-static strength of bonded hot dipped galvanized SAE1006 steel joints.In this study, we use a DOE (design-of-experiment) program called DEXPERT to design the experiment and to analyze the effects of exposure temperature, exposure time, curing temperature and curing time on joint strength of adhesive-bonded galvanized SAE1006 steel. Prior to adhesive curing, the adhesive and galvanized steel coupons were pre-exposed to various relative humidity levels and temperatures. The experimental results were then analyzed by DEXPERT and the relative contributions of each factor on variance in joint strength were calculated. It was found that curing temperature is the most influential factor affecting the strength of adhesive-bonded galvanized SAE1006 steel joints. The curing of a joint at 180 °C can increase the robustness of the process and provides the greatest strength regardless of the variation of other factors. The joint strength curing at 150 °C shows a strong sensitivity to the curing time, while the adhesive cannot cure at 130 °C at all under all conditions. It has also been found that the pre-exposure of adhesive and steel for an hour can slightly decrease the joint strength at high temperature and humidity. Therefore, the effect of long time exposure of the uncured adhesive and steel still needs to be further investigated.

The effect of hot-humid exposure (i.e., 40 °C and 98% R.H.) on the quasi-static strength of the adhesive-bonded aluminum alloys was studied. Test results show that the hot-humid exposure leads to the significant decrease in the joint strength and the change of the failure mode from a mixed cohesive and adhesive failure with cohesive failure being dominant to adhesive failure being dominant. Careful analyses of the results reveal that the physical bond is likely responsible for the bond adhesion between L adhesive and aluminum substrates. The reduction in joint strength and the change of the failure mode resulted from the degradation in bond adhesion, which was primarily attributed to the corrosion of aluminum substrate. In addition, the elevated temperature exposure significantly accelerated the corrosion reaction of aluminum, which accelerated the degradation in joint strength.

•Nucleation, propagation, orientation and kinematics of PLC bands in a TWIP steel.•An algorithm is proposed to include the elongating effect in kinematics analysis.•Applied strain rate has little effect on PLC band width.•PLC band width increases with true strain when considering the elongating effect.•PLC band propagating velocity decreases with increasing true strain.Uniaxial tensile tests at three strain rates are performed with the aid of the digital image correlation (DIC) technique to experimentally investigate the spatio-temporal behavior of PLC bands in a twinning induced plasticity (TWIP) steel. The whole strain fields of tensile specimens are acquired throughout the tests. Significant serration crests corresponding to band nucleation are observed on the true stress vs. true strain curves derived from DIC results beyond a critical true strain. The work hardening exponent (n-value) increases from ∼0.08 to ∼0.5 when true strain increases to the critical true strain, and beyond that, the n-value exhibits serrations with increasing true strain. Two typical nucleation modes of Type-A Portevin–Le Châtelier (PLC) bands are observed in all tests. Nucleation and propagation of PLC bands are described in details based on these two nucleation modes of Type-A PLC bands. The PLC band orientation, which indicates the angle between the normal direction of a PLC band and tensile direction, fluctuates during propagation, and the fluctuation amplitude increases during the development of a localized necking band from a PLC band before fracture. In particular, the effect of strain rate on the kinematics of Type-A PLC bands (band strain, band width and band propagating speed etc.) in the TWIP steel is quantitatively analyzed, and a new algorithm based on the DIC results is presented which includes the elongating effect of tensile specimens during deformation to show the actual kinematics of Type-A PLC bands.