Low Ductility Materials Research Articles

Finite element modeling and analysis of the self-piercing riveting forming process with the ball-shaped die

Self-Piercing Riveting (SPR) technique faces challenges as it expands to low ductile materials. In this paper, the ball-shaped die is suggested to prevent the formation of cracks in sheets during the SPR process. The SPR model with the ball-shaped die was developed using the finite element method, and simulations of the joint forming process and tensile test were conducted. The influence of die depth and diameter on SPR forming process and quality was analysed. The simulation results showed that as the die diameter increases, the undercut decreases, and the remaining thickness of bottom increases. The undercut and the remaining thickness of bottom decreases with the increase of the depth of the die. When employing a ball-shaped die with a 1 mm depth and 10 mm diameter, the SPR joint with tight interlock and excellent mechanical properties can be obtained. The results of the simulation and experimental tests are consistent.

Laser-Assisted Robotic Roller Forming of Ultrahigh-Strength Steel QP1180 with High Precision.

Laser-assisted forming provides a perfect solution that overcomes the formability of low-ductility materials. In this study, laser-assisted robotic roller forming (LRRF) was applied to bend ultrahigh-strength steel sheet (a quenching and partitioning steel with a strength grade of 1180 MPa), and the effects of laser power density on the bending forces, springback, and bending radius of the final parts were investigated. The results show that LRRF is capable of reducing bending forces by 43%, and a compact profile with high precision (i.e., a springback angle smaller than 1° and a radius-to-thickness ratio of ~1.2) was finally achieved at a laser power density of 10 J/mm2. A higher forming temperature, at which a significant decrease in strength is observed, is responsible for the decrease of forming forces with a laser power density of higher than 7.5 J/mm2; another reason could be the heating-to-austenitization temperature and subsequent forming at a temperature above martensitic-transformation temperature. Forming takes place at a higher temperature with lower stresses, and unloading occurs at a relatively lower temperature with the recovery of Young's modulus; both facilitate the reduction of springback angles. In addition, the sharp bending radius is considered to be attributed to localized deformation and large plastic strains at the heating area.

A novel two-stage friction stir-assisted incremental sheet forming method for uniform microstructure and enhanced properties in aluminum alloys

Usual single-pass friction stir-assisted incremental sheet forming (FS-ISF) can improve the formability of low-ductility metallic sheets such as AA2024 and partially produce equiaxed grains by dynamic recrystallization on the components. However, the non-uniformity of the microstructure along the thickness direction degrades the formability and component properties. To solve this problem, a novel two-stage FS-ISF method is proposed by combining intermediate pass through an optimal compensation strategy with a reverse pass. A two-stage analytical model of FS-ISF is developed to minimize the grain gradient along the thickness direction and combined with the calibrated recrystallization model to implement the two-stage FS-ISF method. Using this method, uniform grains can be obtained along the thickness direction for AA2024-T3 sheet. Grain uniformity contributes to the increasing of material ductility by allowing concordant slip deformation under consistent motions of boundaries and grains with deep and coalesced dimples. The homogeneous fine precipitates within the grains, pinning effects, and more grain boundaries are induced by the two-stage FS-ISF method, which increases the strength of the formed part. Compared with usual single-pass FS-ISF, both the ultimate tensile strength and ductility are improved for AA2024-T3 using the two-stage FS-ISF method, which are also demonstrated for AA6061-T4 sheet. The two-stage FS-ISF method has the advantage of increasing the component strengths of different aluminum alloy sheets compared with artificial aging treatment after conventional incremental sheet forming or usual single-pass FS-ISF for different aluminum alloy sheets. The multi-pass FS-ISF method with reverse pass was validated through the fabrication of vertical-wall square cup with uniform thickness. The proposed method provides an efficient alternative to increase the strength and ductility during flexibly making the sheet metal panel of low-ductility material with challenging geometry.

Investigations on the forming characteristics of a novel flexible incremental sheet forming method for low-ductility metals at room temperature

• Proposed a novel flexible ISF method without edge constraint on target blank for low-ductility materials at room temperature. • Proposed analytical model of sheet thickness for undesired severe thinning region. • Investigated deformation characteristics and advantages by comparison with other ISF processes. • Fabricated several freeform panels with typical low-ductility materials to evaluate formability and geometric deviation. As important structural freeform panels (FFPs) for medical implants and aerospace system, lightweight thin wall panels with lower ductility are conventionally produced by hot forming process by utilizing the die on the press, which is not efficient or cost-effective for small batch or customized manufacturing. The dieless incremental sheet forming (ISF) provides a promising solution with increased formability, but making sheet metal panels with low-ductility at room temperature is still a challenging work. Targeting to solve this puzzle, a novel ISF without edge constraint on the target blank between the two edge-fixed supporting layers is developed to get flexible and free plastic deformation for the target blank. To reveal the deformation mechanism of this new method, an analytical prediction model of sheet thickness in ISF process is developed to calculate the deformed thickness in severe thinning region, which indicates that the undesired severe thinning region of conventional ISF process is caused by different stress states near the constrained edge, and can be completely eliminated by this proposed method to obtain uniform plastic deformation and improved formability. Different FFPs of magnesium alloy AZ31 and titanium alloy TC4 are fabricated by the novel flexible free ISF (FFISF) to validate the improved formability, and compared with conventional ISF and three-sheet incremental forming. Experimental studies demonstrate that the proposed method provides potentials for flexible fabrication of FFPs with low ductility at room temperature. More importantly, the promising geometric accuracy without trimming can be obtained by this new method, and multiple axisymmetric parts can also be formed within a cycle time.

Contact-induced vibration tool in incremental sheet forming for formability improvement of aluminum sheets

Formability improvement is extremely crucial in incremental sheet forming (ISF) of low-ductility material and high wall angle 3-dimensional parts. The contact-induced vibration from the tool rotation provides an alternative route for optimizing formability; however, the underlying mechanism induced by the contact-induced vibration effect has not yet been fully elucidated. This study designed a contact-induced vibration tool (V-Tool) with a flat elliptical shape tip and applied it into the ISF of a 0.5 mm Al5052 sheet. Hyperbola shapes were manufactured by the ISF with V-Tool and conventional tool (C-Tool) with different rotation speeds to experimentally investigate the forming limit, temperature, hardness, fracture morphology, and material deformation. The main results show that the vibration effect and the resultant shear deformation by V-Tool result in the lower friction heat verified by the peak temperature from 180 °C to 80 °C and higher formability from the thinning rate of 54%–84% as compared to those of the conventional tool (C-Tool). Further, the deformation observation with the drilled holes on initial sheets indicates that the contact-induced vibration effect in the ISF with V-Tool may produce more shear deformation on the inner surface to delay the continuous enlargement of the drilled hole along the meridional direction.

Investigation of the joinability of single- and multi-layered AA6014 sheets produced by accumulative roll bonding in the shear-clinching process

Accumulative roll bonding belongs to the severe plastic deformation processes and is suitable for producing multi-layered as well as high-strength sheets through repeated rolling. After four accumulative roll bonding-cycles, the strength of the investigated AA6014-T4 alloy is elevated due to strain and fine grain hardening by 170% in yield strength and by 37% in ultimate tensile strength. This is accompanied by a significant decrease in material ductility which is disadvantageous for joining processes based on forming operations. Shear-clinching, however, can be used to join high-strength and low-ductile materials on the die side. Thus, it is a promising method for the mechanical joining of accumulative roll bonding components. The multi-layered and more brittle material is located on the die side whereas the single-layered base material with higher ductility is positioned on the punch side. At present, there is no scientific knowledge how the multi-layered material behaves in the shear-clinching process. Since the material is cut in the process, delamination effects could occur at the interfaces produced via accumulative roll bonding. Additionally, the investigation aims at the influence of the accumulative roll bonding process on the properties of the joints. They are expected to depend on the number of process cycles and thus on the change of mechanical properties of the multi-layered material. The bond strength of the shear-clinched joints is determined by shear and tensile tests. The areas of material failure are analysed via metallographic methods. From the failure behaviour and load-bearing capacity it is determined which number of accumulative roll bonding cycles is ideal for joining multi-layered sheets via shear-clinching.

Study on the wrinkling behavior of perforated metallic plates using uniaxial tensile tests

The Yoshida buckling test (YBT) is a commonly used method to evaluate the wrinkling behavior of sheet metals; however, it is difficult to reflect the actual in-plane stresses in many sheet forming processes owing to the limited variation in the stress state. Furthermore, the test is impracticable for some low-ductility materials because fracture typically occurs before wrinkling. This paper proposes an approach for examining the wrinkling properties of sheet metals using uniaxial tension of plates with a central cut-out; a relevant wrinkling model is established using stress analysis of a finite perforated plate and an energy method. The results show that the typical stress distribution modes around the central hole before wrinkling are biaxial compression in the top/bottom domains and biaxial tension in the lateral domains, and the mode changes with the hole size and tensile displacement. The ratio of transverse to axial stresses increases with the diameter/width ratio; this leads to a larger influence of transverse nonuniform compressive stress, consequently decreasing the critical wrinkling stress. The wrinkling tendency in perforated sheets is more sensitive than that in a conventional YBT specimen; thus, it can be used to capture the subtle distortion of wrinkling for low-ductility materials. A second wrinkling phenomenon that occurred in plates with large holes under a large tensile displacement was observed and addressed.

Microstructural evolution in friction self-piercing riveted aluminum alloy AA7075-T6 joints

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.

Self-Piercing Riveting of Medium- and High-Strength Al and Mg Alloy Sheets Enabled by In-Process Electric Resistance Heating

Abstract Enduring joints of high-strength materials are becoming increasingly important for the manufacturing of highly loaded body-in-white structures, particularly for automobiles, which require structural light-weighting without sacrificing passenger safety. While self-piercing riveting has been proven suitable for various material classes, its application to high-strength, low-ductile materials is hindered by the occurrence of cracks and insufficient penetration. In this work, we demonstrated that these restrictions can be overcome by localized one-sided short-time electric resistance heating. The treatment can be integrated into the riveting process, allowing for short process times. The heating lowers the hardness of the materials to be joined and enables piercing and penetration of the sheets to yield crack-free joints. The local mechanical properties are hardly changed in case of peak-aged sheet material conditions but increase significantly in the close vicinity of the rivet in case of under-aged sheet material conditions. The outstanding static and dynamic mechanical properties of the resulting joints evidence their robustness.

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.

A new workflow for the effective distinction between necking induced spilts and direct fracture phenomena

The predominant mode of failure in stamping applications is still splitting due to localized necking. This kind of failure is very effectively captured by the forming limit curve, especially if nonlinear deformation effects are modelled. With the sharp increase in low-ductility (weight efficient) materials, direct fracture phenomena (surface cracks, edge cracks, shear cracks, etc.) increasingly enter the daily industrial stamping practice, especially in follow operations. Contrary to the classical FLC analysis, there exists nowadays no industry-wide standardization for characterizing these phenomena. This drives up modelling costs and reduces the expected improvement in the overall modelling uncertainty. It therefore becomes crucial to understand and detect the circumstances in which an advanced fracture characterization is necessary, such that limited resources can be effectively allocated. The present contribution aims to propose such a workflow. Firstly a reasonable lower bound for the fracture limits is estimated based on theoretical considerations and evidence from scientific literature. This estimation is used to conduct a first analysis step which is used to categorize the process based on whether a standard modelling approach is sufficient to deliver the required accuracy. A second step of accurate fracture identification is therefore only recommended for the subset of geometries with particularly high risk. The approach is tested on data for AA6016 and DP980 from literature and validated based on published experimental results.

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.

Impact of Stack Orientation on Self-Piercing Riveted and Friction Self-Piercing Riveted Aluminum Alloy and Magnesium Alloy Joints

Self-piercing riveting (SPR) is a mature method to join dissimilar materials in vehicle body assembling. Friction self-piercing riveting (F-SPR) is a newly developed technology for joining low-ductility materials by combining SPR and friction stir spot welding processes. In this paper, the SPR and F-SPR were employed to join AA6061-T6 aluminum alloy and AZ31B magnesium alloy. The two processes were studied in parallel to investigate the effects of stack orientation on riveting force, macro-geometrical features, hardness distributions, and mechanical performance of the joints. The results indicate that both processes exhibit a better overall joint quality by riveting from AZ31B to AA6061-T6. Major cracking in the Mg sheet is produced when riveting from AA6061-T6 to AZ31B in the case of SPR, and the cracking is inhibited with the thermal softening effect by friction heat in the case of F-SPR. The F-SPR process requires approximately one-third of the riveting forces of the SPR process but exhibits a maximum of 45.4% and 59.1% higher tensile–shear strength for the stack orientation with AZ31B on top of AA6061-T6 and the opposite direction, respectively, than those of the SPR joints. The stack orientation of riveting from AZ31B to AA6061-T6 renders better cross-section quality and higher tensile–shear strength and is recommended for both processes.

Net section tension strength of bolted connections in ultra-high strength sheet steel during and after fire

This paper investigates the net section tension strength of bolted connections in ultra-high strength sheet steel (yield stress greater than 1000 MPa) under various fire scenarios through laboratory testing. The specimens are tested at room (ambient) temperature and elevated temperatures up to 700 °C, and are subjected to air or water cooling from temperatures as high as 1000 °C. The degradations of the tensile strength at elevated temperatures are much more severe than design code predictions. Substantial recovery of the tensile strength is feasible using water cooling if the steel has been exposed to 1000 °C, by almost 90%. Based on finite element analyses incorporating tensile fracture simulation, the paper determines that elongation at fracture is the most relevant parameter of material ductility for a structural steel connection. Tension fractures in very low ductility materials tend to be inclined to the loading direction of the bolted connection. This paper also examines the alternative formulae for determining the net section tension strength of a stagger bolted connection. Their accuracy is verified through the use of a certain geometry, which eliminates any potential errors due to the use of incorrect material tensile strength in the calculations. Cochrane's original formula is confirmed to be significantly more accurate than the simplified version used in the literature and some design codes, for the tested specimens.

Effect of mechanical and solid-state joining characteristics on tensile-shear performance of friction self-piercing riveted aluminum alloy AA7075-T6 joints

Friction self-piercing riveting (F-SPR), as a combination of traditional self-piercing riveting and friction stir spot welding processes, has been proposed to solve the cracking issues in riveting low ductility materials. The F-SPR process creates mechanical and solid-state hybrid joints, which are quite different from existing spot welding/joining methods. However, the roles that mechanical interlocking and solid-state bonding play upon the mechanical behavior of the joints are unknown. To fill in this gap, the current body of work investigated the tensile-shear behavior and fracture mechanism of the F-SPRed AA7075-T6 aluminum alloy sheets. The results reveal that the ring-shaped solid-state bonding between aluminum sheets enhances the overall stiffness of the joints but shows a minor contribution to the tensile-shear performance. The solid-state bonding formed between the captured aluminum in the rivet shank and the lower sheet serves as an “anchor” to hinder the rotation of the rivet, which delays the action of rivet pull-out from the lower sheet and thus strengthens the joints significantly. The combination of a hard rivet, large rivet flaring and solid-state bonding inside the rivet shank is necessary to achieve the preferred rivet pull-out fracture mode, which shows a higher peak load and larger energy absorption compared to other fracture modes.

Single-sided joining of aluminum alloys using friction self-piercing riveting (F-SPR) process

Friction self-piercing riveting (F-SPR) process has shown advantages over traditional self-piercing riveting (SPR) process in joining magnesium alloys and aluminum alloys. Frictional heat produced by the rotation of the rivet plays important roles to the inhibition of cracking in riveting low ductility materials and the formation of solid-state bonding. However, the current F-SPR process needs a matching die to facilitate rivet deformation and formation of mechanical interlock between lower sheet and the rivet, which poses a big challenge in riveting large thin-walled structures, where a matching die is hard to apply. In this research, single-sided F-SPR process was proposed to join 2.2 mm-thick aluminum alloy AA6061-T6 to 3.0 mm-thick aluminum alloy AA6061-T6, and effects of process parameters, e.g., rotation speed and feed rate, on the morphology and mechanical properties of the joints were studied experimentally. It was found that higher rotation speed and lower feed rate which signified higher heat input could effectively reduce defects but lead to insufficient undercut. As such, a two-stage process was proposed to reduce frictional heat generation in lower sheet and thus increase the deformation resistance acted at the rivet tip. A preliminary study showed that the two-stage F-SPR process increased the tensile-shear and cross-tension strengths by around 25% and 81% respectively, compared to the one-stage process.

Determining the strain to fracture for simple shear for a wide range of sheet metals

There is a particularly high degree of experimental uncertainty associated with the determination of the strain to fracture for simple shear. In most shear specimens, different stress states coexist and it is usually impossible to prove experimentally that fracture indeed initiated in the sheared zones. Peirs’ simple shear specimen is analyzed in detail for nine generic materials covering different combinations from low to high strain hardening and from low ductility to high ductility. It is found that the validity of the shear fracture strain measurement depends on the strain hardening capability and the ductility of the material being tested, with a higher likelihood of validity for low hardening and low ductility materials. An extensive parametric study evaluating more than 600 distinct specimen geometries is performed in an attempt to identify a universal specimen geometry. It turns out that no single geometry exists which provides reliable measurements for all types of materials. While a single geometry can be found for different hardening behaviors, it seems to be necessary to use several distinct geometries to cover all levels of ductility. Since the ductility is usually not known prior to testing, we recommend testing three different types of specimen per material, reporting the highest measured strain among all specimens as strain to fracture for simple shear. Experiments are performed on specimens extracted from aluminum AA2024-T351, as well as DP800 and DP980 dual phase steels to demonstrate the validity of the proposed experimental methodology for identifying the strain to fracture for simple shear.

Modeling and experimental validation of friction self-piercing riveted aluminum alloy to magnesium alloy

Friction self-piercing riveting (F-SPR) process has been proposed to achieve crack-free joining of low-ductility materials by combining SPR process with the concept of friction stir processing. The inhibition of cracking in an F-SPR joint is related to the in-process temperature as well as plastic deformation of materials, which are controlled by the process parameters, i.e., spindle speed and feed rate. However, the relationship between F-SPR process parameters and the temperature characteristics within the joint has not been established. In the current study, a coupled thermal-mechanical model based on solid mechanics was setup to study the F-SPR process of aluminum alloy and magnesium alloy. Temperature and strain rate-dependent material models and preset crack surface method were integrated in the model and geometry comparisons were conducted for model validation. Based on this model, the evolutions of temperature and plastic deformation in the rivet and the sheets of an F-SPR joint were obtained to reveal the formation mechanism of the joint. The temperature distribution and evolution of the sheet materials were correlated with F-SPR process parameters, and a critical spinning speed of 2000 rpm at a feed rate of 1.35 mm/s was determined capable of inhibiting cracking in the magnesium sheet.

Titanium Alloys 2017

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Power-energy conditions of round item hydro-mechanical pressing

One of the main widespread methods of metal forming is pressing characterized by a favorable plastic deformation pattern with the predominant effect of all-round compressive stresses. This allows deforming low-ductile materials and alloys with sufficiently high degrees of deformation. This paper studies plastic deformation conditions at hydro-mechanical pressing as one of pressing types. A distinctive feature of hydro-mechanical pressing as compared to other pressing types is the ability to control the movement of the billet and prevent its ejection at the final process stage. The study covers the conditions of hydro-mechanical pressing which combines the use of high-pressure working fluid and the mechanical impact of the tooling on the pressing die. Formulas for the components of the total hydro-mechanical pressing stress are derived to serve the basis for determination of the optimal process tool geometry. Taper angles of the hydro-mechanical pressing die are optimized depending on the main pressing process parameters. The dependency graphs are plotted for the ratio of pressing stress to the resistance of pressed material deformation as a result of drawing that confirmed the presence of optimum taper angles of pressing dies.