DP600 Steel Research Articles

Modelling and Optimization of Resistance Spot Weld Responses Using RSM–GA Technique for DP590 Steel Sheets

Modelling and Optimization of Resistance Spot Weld Responses Using RSM–GA Technique for DP590 Steel Sheets

Investigation on deformation of DP600 steel sheets in electric-pulse triggered energetic materials forming

Electric-pulse triggered energetic materials forming (ETEF) is a high-speed manufacturing process, which utilizes the chemical energy released by energetic materials (EMs) triggered by underwater wire discharge to plastically shape metals. ETEF is not fully understood, particularly in research on the discharge characteristics of energetic materials triggered by metal wires and the deformation process of metal sheets. The above two problems were investigated in this paper using experimentation and numerical simulation. For the pulse discharge characteristics, the peak values of voltage and current were reduced during the triggering process of energetic materials, and the triggering energy consumption of energetic materials was quantified to be about 200 J. The matching parameters of different capacitor-voltage devices may be insensitive to triggering the energy release of energetic materials. The maximum major strain and thinning rate of the bulged specimen under ETEF conditions were significantly reduced when compared to the quasi-static specimen with the same bulging height, and the specimen’s deformation uniformity and strain distribution were improved. The simulation results showed that the addition of energetic materials significantly improved the plastic strain energy of the blank. The deformation of the blank in ETEF can be divided into two stages: the initial chemical energy action stage and the inertia action stage. The bulging height of sheet metal increased by nearly 301% in inertia action stage, accounting for 80% of the total deformation time, and the effective plastic strain distribution was more uniform.

A complex-variable finite element method-based inverse methodology to extract constitutive parameters using experimental data

This paper presents the use of full-field kinematic measurements obtained using the digital image correlation (DIC) procedure and load–displacement data to determine constitutive material properties by solving an inverse finite element optimization problem. A key ingredient in the proposed approach is computing accurate sensitivities with respect to the unknown parameters. These sensitivities were used to solve the optimization problem using an accurate, efficient, gradient-based method, and were computed using the complex-variable finite element method, ZFEM. The use of ZFEM’s gradients to inversely determine material properties is demonstrated with two examples. First, the elastic–plastic material properties of DP-590 steel are obtained using a tensile test specimen. Second, the cohesive material parameters of an adhesive are determined using a double cantilever beam test. A significant outcome of this paper is that the use of a weighted residual formulation of the interfacial strain fields and the load–displacement data within the optimization procedure provides better estimates of the constitutive properties than using only the load–displacement data. This technique minimizes the relative error in both the strain fields and the load–displacement curve, which is important to obtain accurate interfacial properties.

Elektrik direnç punta kaynağı ile birleştirilen TBF/DP600 çeliklerinin mikroyapı ve mekanik özelliklerinin incelenmesi

In the study, the effects of welding parameters on microstructure and mechanical properties of electrical resistance spot welded TBF/DP600 steel sheets of 1.2 mm thickness were investigated. TBF steel used in the study was welded both with and without electro-galvanized. Microhardness measurements and tensile shear tests were taken as basis in determining the mechanical properties, while optical microscope was used for microstructural characterization. The nugget size of the spot-welded samples was determined by image processing technique, while the indentation depths at the electrode pressure points of the samples were measured by ultrasonic technique. It was observed from the fusion zone microstructure images that the two steels did not mix completely. Increasing the welding current and welding time increased the mixing ratio in fusion zone, expanded the heat affected zone and increased the nugget size, indentation depth and load bearing capacity. TBF steel was more affected by the thermal cycle during welding than DP600 steel. At high heat input, liquid metal embrittlement based microcrack formations initiating from the surface in the heat affected zone of galvanized TBF steels were observed, while corrosion started rapidly in the welding region of ungalvanized TBF steels. The highest hardness values were observed in ITAB on the TBF steel side. However, a significant softening occurred in the transition zone between the ITAB and base metal on the TBF steel side. The nugget size, indentation depth and load bearing capacity were found to be relatively higher in the galvanized TBF steel group. In dissimilar electrical resistance spot welded TBF/DP600 steel sheets having the same thickness, DP600 steel with lower strength has determined the welding strength. In high heat input during welding process, fractures are of the fusion zone button type with higher welding strength, while fractures are of partial fusion zone button type with lower welding strength in low heat input. If TBF steel is galvanized, relatively higher welding strength was obtained in the welding parameters that provide relatively lower heat input.

Analysis of the Low Cycle Fatigue Behavior of DP980 Steel Gas Metal Arc Welded Joints

Dual phase (DP) steels have high strength, while maintaining outstanding elongation capacities. This is possible using a well-controlled thermomechanical process that produces a perfect phase combination in the DP microstructures. However, automotive makers are required to weld the DP steels, which generates a soft zone in the microstructure. In this work, 1.6 mm-thick DP980 steel sheets were welded by gas metal arc welding process to analyze the response of the welded soft zone to cyclic loading conditions. Conducted macrographic and metallography analyses revealed good quality in the appearance of the welded joints, with a complete fusion of the DP980 joint and without the presence of discontinuities. Low cycle fatigue tests of the DP welded joints were conducted under a constant amplitude strain control mode. The welded joints experienced a fatigue life reduction with respect to the DP980 steel of ~16% at strain amplitudes of 0.2, 0.3, and 0.4%. For strain amplitudes larger than 0.6%, the fatigue life of the welded joint was reduced by 39%. Weld thermal cycles combined with metallography analysis indicated that a tempered process of the martensite during the welding was responsible for the soft-zone formation and the poor fatigue response.

Elucidation of intermetallic compounds and mechanical properties of dissimilar friction stir lap welded 5052 Al alloy and DP590 steel

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.

An Investigation into the Influence of Interrupted Loading in Improving the Stretch-Flangeability of Dual Phase Steel

Materials resistance to edge failure during sheet metal flanging operations is known as stretch-flangeability. It is one of the important concerns in the current automotive sector. Stretch flangeability of sheet metal is estimated by hole expansion test and commonly it is represented by hole expansion ratio (HER). The objective of the present work is to comprehend the hole expansion deformation behavior of DP600 steel. Firstly, finite element analysis was performed to understand the stress state at the edge during the hole expansion test. Thereafter, the effect of stress relaxation was studied by conducting hole expansion tests (HET) in monotonic and interrupted mode. Considerable improvement in the HER was observed. The HER was found to increase with the pre-strain. The combined effect of friction and stress relaxation played a crucial role in delaying the edge failure, resulting in the enhanced HER.

Detailed deformation behaviour analysis of DP steels at warm forming temperatures via warm tensile tests

ABSTRACT DP advanced high strength steels (AHSSs) have an important place in automotive industry for producing parts like B-pillars, door rings, etc. However, the excellent strength of AHSSs increases the issues related to their formability. Therefore, it is necessary to implement new forming processes to fully utilize the excellent properties of DP steels in more complex parts. In this study, the formability of DP steels (DP600, DP800) and their deformation behaviour at warm forming temperatures have been investigated by a series of tensile tests conducted at elevated temperatures (75, 175, 275, 375°C) and two different strain rates (0.05, 0.005 s-1). To understand the deformation behaviour thoroughly, both instantaneous and average strain hardening – n and strain rate sensitivity – m values, the change in the hardening capacity and the microstructure have been analyzed. The results have shown that the formability of DP steels at 375°C is more favourable as compared at room temperature.

Dynamic deformation behavior and corresponding constitutive model of ultra-high strength DP1000 steel

The static and dynamic mechanical properties of DP1000 steel were tested using an electronic universal testing machine controlled by a microcomputer at the strain rates of 10−4–10−2 s−1 and a high speed tensile testing machine at the strain rates of 10−1–103 s−1. The effect of strain rates on the stress-strain curves and the work hardening rate were obtained and analyzed. The modified dynamic Johnson-Cook model was established to characterize the dynamic deformation behavior. The dynamic compression characteristics and energy absorption law of thin-walled beam of DP1000 steel were determined by testing and using the Abaqus finite element simulation on account of the modified dynamic Johnson-Cook model (shortly the J-C model) to verify that the predicted values are in good agreement with the experimental ones.

Effect of Rotation Speed and Steel Microstructure on Joint Formation in Friction Stir Spot Welding of Al Alloy to DP Steel

In this work, friction stir spot welding of 5754 aluminum alloy to dual phase steel was investigated using two different ratios of martensite and ferrite (0.38 and 0.61) for steel sheet initial microstructure and varying tool rotation speed (800, 1200 and 2000 rpm). The effect of these parameters on the joint formation was evaluated by studying the plunging force response during the process and the main characteristics of the joint at (i) macrolevel, i.e., hook morphology and bond width, and (ii) microlevel, i.e., steel hook and sheet microstructure and intermetallic compounds. The plunging force was reduced by increased tool rotation speed while there was no significant effect from the initial steel microstructure ratio of martensite and ferrite on the plunging force. The macrostructural characterization of the joints showed that the hook morphology and bond width were affected by the steel sheet initial microstructures as well as by the tool rotation speed and by the material flow driver; tool pin or shoulder. At microstructural level, a progressive variation in the ratio of martensite and ferrite was observed for the steel hook and sheet microstructure. The zones closer to the tool presented a fully martensitic microstructure while the zones away from the tool showed a gradual increase in the ferrite amount until reaching the ratio of ferrite and martensite of the steel sheet initial microstructure. Different types of FexAly intermetallic compounds were found in three zones of the joint; the hook tips, in the hooks close to the exit hole and in the corner of the exit hole. These compounds were characterized by a brittle behavior with hardness values varying from 456 to 937 HV01.

Microstructure and Fatigue Properties of Resistance Element Welded Joints of DP500 Steel and AW 5754 H22 Aluminum Alloy

The modern concept of lightweight design (LW) requires the application of different materials in one structure (multi-material structures). The structure of different materials has a good perspective for application in the automotive and aerospace industries but only if it is possible to achieve a quality joint between different materials. The most used technology for joining different materials in the automotive industry is Resistance spot welding (RSW). Due to different mechanical, physical, and chemical properties, the joining of different materials by RSW technology does not provide a quality joint, and accordingly, alternative technologies for joining different materials have emerged. Resistance element welding (REW) was developed to enable joint of different materials. This paper presents the welding of AW 5754 H22 Al alloy (1.0 mm-thick) and DP500 steel (1.5 mm-thick) using novel REW. The peak load, absorption energy, microstructure, microhardness and fatigue strength of the REW joint has been investigated. The joint of the same materials has been done also using conventional RSW to compare some results. The results that will be presented in this paper show that that REW can achieve reliable joining of the two materials at relatively low welding currents compared to RSW. Using REW process with a significantly lower welding current, satisfactory mechanical characteristics of the weld joint can be achieved, so peak load is between 2300–2500 N, displacement is between 2.5–3 mm and the absorption energy is between 3.3–5.7 J. REW joints showed fatigue strength with the fatigue limit of 882 N.

Improvement of welding stability and mechanical properties of galvanized DP800 steel lap joint by high-speed tandem beam laser

Dual beam laser welding has shown promise to improve the weldability of galvanized dual phase steels. In this study, the lap joints of galvanized dual phase steel were welded with tandem beam laser at speed up to 100 mm/s. The power ratio for the dual beams was set to 50%:50%, 40%:60%, 30%:70%, and 20%:80%, respectively. A high-speed camera was used to observe the dynamic process of zinc vapor, molten pool, and keyhole behaviors under different welding conditions. The corresponding metallographic and mechanical analysis were conducted to reveal the effects of variable welding thermal cycles on the microstructures and properties of the welded joints. The results indicate that the fusion zone (FZ) is composed of blocky-lath martensite, whereas the heat-affected zone (HAZ) is characterized by the lath microstructure containing tempered martensite (TM) and ferrite (F). The power ratio of dual beams significantly affects the microstructures and hardness profiles of welded joints. It also influences the zinc-vapor and molten pool dynamic behaviors, thus affecting the weld bead formation. The weight of spatter decreases with the decrease of leading beam power in tandem beam laser welding process. In addition, the mechanical properties of the welded lap joints are significantly affected by the high-speed tandem dual beam laser welding process parameters. The failure load of tensile-shear testing reaches the maximum value when the beam power ratio is 30%:70%.

Study on quarter-wave generation mechanism in DP980 steel during cold rolling

Study on quarter-wave generation mechanism in DP980 steel during cold rolling

Simultaneously improved the strength dramatically and eliminated HAZ softening of DP980 steel pulsed-arc welding joints by PWHT

A pathway of post-weld heat treatment (PWHT) to eliminate heat affected zone (HAZ) softening and improve the strength of the welded joints of DP980 steel was developed. It is found that the high temperature isothermal transition at 750–950 °C followed by water-quenching significantly influences the strength and fracture location by changing the microstructures of the welded joint. The block martensite is distributed discretely in the island ferrite after PWHT, which causes ferrite to deform continuously in space. At the same time, the microstructure of joints is more uniform after PWHT. As a result, the highest strength (1146 MPa) can be obtained by PWHT at 950 °C, which is about 1.35 times that of the as-welded joint.

Effects of rotation tool-induced heat and material flow behaviour on friction stir lapped Al/steel joint formation and resultant microstructure

Reliable joining of Al/steel via friction stir lap welding (FSLW) is strongly dependent on interfacial macro-/micro-structures. However, how the underlying thermo-mechanical process affects them and how they can be controlled remain unclear. In this study, a 3D coupled Eulerian–Lagrangian finite element model was integrated with a tracer particle technique to simulate the FSLW of Al alloy 5052 and high-strength DP590 steel, to elucidate the role of tool-induced heat and material flow. The results showed that the material flow recirculated near the shoulder and pin, which mainly originated from the advancing side of the Al and the retreating side of the steel. Increasing the rotational velocity intensified the overall material flow, resulting in increased migration on the steel side. Insufficient steel migration around the pin at lower rotational velocities was mainly responsible for the micro-voids and non-bonding defects at the lap interface. The asynchronous convergence of Al and steel flowing from the advancing and retreating sides, respectively, refilled the instantaneous gap, thereby normally bonding the lapped interface behind the pin. The rotational pin affected the hook structure and steel fragment formation through the shearing and squeezing of the steel migration flow. Material inter-migration produced an intercalated structure at the lapped Al/steel interface. The extremely high temperature, approximately above Al 5052 solidus temperature, under the pin bottom caused the formation of a thicker intermetallic compound (IMC), thereby considerably reducing the interfacial strength. Multi-scale mechanical strength assessments indicated that an intercalated interfacial structure with a thickness of Al-rich IMC layer less than ∼1.0 μm was desirable because it exhibited a higher local interfacial strength than the Al matrix.

Analytical model of the in-plane torsion test

In research and industry, the in-plane torsion test is applied to investigate the material behaviour at large plastic strains: a sheet is clamped in two concentric circles, the boundaries are twisted against each other applying a torque, and simple shear of the material arises. This deformation is analysed within the scope of finite elasto-plasticity. An additive decomposition of the Almansi strain tensor is derived, valid as an approximation for arbitrary large plastic strains and sufficiently small elastic strains and rotations. Constitutive assumptions are the von Mises yield criterion, an associative flow rule, isotropic hardening, and a physically linear elasticity relation. The incremental formulation of the elasticity relation applies covariant Oldroyd derivatives of the stress and the strain tensors. The assumptions combined with equilibrium conditions lead to evolution equations for the distribution of stresses and accumulated plastic strain. The nonzero circumferential stress must be determined from the equilibrium condition because no deformation is present in tangential direction. As a result, a differential-algebraic-equation (DAE) system is derived, consisting of three ordinary differential equations combined with one algebraic side condition. As an example material, properties of a dual phase steel DP600 are analysed numerically at an accumulated plastic strain of 3.0. Radial normal stresses of 3.1% and tangential normal stresses of 1.0% of the shear stresses are determined. The influence of the additional normal stresses on the determination of the flow curve is 0.024%, which is negligibly small in comparison with other experimental influences and measurement accuracies affecting the experimental flow curve determination.

Influence of laser welding power on steel/CFRP lap joint fracture behaviors

Dual-phase steel DP590 and carbon fibre reinforced polymer (CFRP) were successfully joined by laser welding. Experiments and numerical simulations were performed to clarify the influence of laser power on the joint characteristic, bonding mechanism and fracture behavior. The results indicate that the melting width and depth of CFRP increased with the enhancement of laser power. With relatively low laser power, un-bonded steel/CFRP interface was formed due to the insufficient melting of resin matrix and interfacial residual stress resulted from thermal expansion mismatch between steel and CFRP. Compact-bonding was produced with the laser power range of 400–700 W. Decomposition of CFRP at the interface occurred with the further increase of laser power to 800 W due to excessively high peak temperature. The highest tensile-shear peak load of 3855 N was produced at 700 W laser power. Three different fracture modes were found in the joints with different laser powers, i.e., interfacial fracture (IF) at 300 W and 400 W, cohesion fracture (CF) in CFRP at 500 W and 600 W, and interface fracture at inner region together with cohesion fracture at outer region of the bonding interface (IF + CF) at 700 W and 800 W. X-ray photoelectron spectroscopy results showed that C-M and O-M chemical bonds were formed at the interface due to the reaction between resin matrix and DP590. Stronger O-M and C-M chemical bonds produced at higher laser power were beneficial for the improvement of fracture load.

Joining sheets made from dissimilar materials by hole hemming

This research work presents a new joining process based on the hemming process for attaching sheets made from dissimilar materials with very different mechanical properties. The process is termed ‘hole hemming’ and consists in producing a mechanical interlock between pre-drilled holes which can be made anywhere on the sheets. The process is carried out in a two-stage operation including flanging the hole of an outer sheet and bending the flange over the hole of an inner sheet. First, the joining stages and the required tools are designed. Then, the joining of DP780 steel and AA6061-T6 aluminium alloy sheets, which are applied to manufacture lightweight structures in the automotive industries, is investigated using finite element analysis. Results show that the hole hemming process is able to successfully join these materials without fracture. The hole-hemmed joint withstood the maximum forces of 2.5 and 0.5 kN in single-lap shear and peel tests, respectively, and failed with hole bearing mode which is known as a gradual failure mode. The results demonstrate the applicability of the hole hemming process for joining dissimilar materials.

Taguchi optimization in machining dual-phase steel (DP980) with WEDM using brass wire

Dual-phase steels are manufactured from low and medium carbon steels. DP steels are high in strength and can be machined using non-traditional techniques. The challenge in machining the dual-phase steel is to obtain the minimum surface roughness and the highest material removal rate. WEDM is chosen for machining the DP 980 steel, as it is capable of maintaining low surface roughness, high accuracy, quick processing and is very cost-efficient. For this experimental study 27 pieces of DP (980) steel of dimensions 20 mm diameter and 20 mm length are used for conducting 27 experiments on WEDM. The experiment layout is based on Taguchi's L-27 orthogonal array. The experiment was executed considering the parameters like voltage (V), pulse on (TON) and pulse off (TOFF). It was noticed that the optimum parameters for the minimum surface are voltage (30 V), pulse on (110 µs), pulse off (54 µs) and for maximum material removal rate are voltage (20 V), pulse on (120 µs), pulse off (50 µs).

Local formability of medium-Mn steel

• Local formability of medium-Mn steel is described by forming limits at fracture. • Ductile fracture occurs prior to plastic localization in MMnS. • Large uniform elongation indicates superior global formability of MMnS. • Local formability of MMnS is not enhanced by pronounced strain hardening. Global formability and local formability are critical in different metal forming processes. Edge cracking, controlled by the local formability, is a dominant factor limiting the application of advanced high strength steels (AHSS) in automotive industries. The local formability of a medium-Mn steel (MMnS), a promising candidate of the third generation of AHSS, is evaluated based on forming limit curves at fracture and compared with a dual-phase DP1000 steel using the damage mechanics approach. The superior tensile properties of the investigated MMnS, high tensile strength, pronounced strain hardening, large uniform and total elongation, lead to a very good global formability, which is an indicator of necking resistance. However, the local formability of the investigated MMnS, which is an indicator of fracture resistance and quantified by the plastic strain at fracture under different stress states, is worse than the DP1000 steel. By comparing the local and global formability of the two AHSS, it is confirmed that ductile fracture is the dominant failure mode in the MMnS and the onset of localized necking occurs prior to ductile fracture in the DP1000 steel. To achieve an accurate determination of the local formability, the effects of stress states need to be considered, which cannot be derived explicitly from uniaxial tensile tests. In addition to tensile properties, more attention should be paid to the local formability of new AHSS to assess their potential application in automotive industries.