Advanced High Strength Steel Materials Research Articles

Characterization and Parametric Optimization of EN-10149-2 Steel Welded Joints Made by MIG Welding

Advanced high strength steels (AHSS) are multiphase steels that contain various concentrations of ferrite, bainite, martensite, and retained austenite phases. The unique physical characteristics of AHSS present some challenges to welding and bonding processes. AHSS have tendency to change microstructure and mechanical properties by any welding process like other steels. Based on the significant applications in various industries, the experimental works are carried out on one of AHSS materials, EN-10149-2 S700MC, with thickness of 2 mm steel sheet metal by MIG welding process parameters such as current, voltage, and welding speed. For all combinations of process parameters, the butt joints are fabricated. The butt joints are inspected through different nondestructive testing for defects detection. All the defect-free joints are characterized through microstructure (at different zones of joint), hardness, and tensile tests. From the obtained mechanical test results, the process parameters are optimized through design of experiment techniques (DoE). The analysis from DoE method is used to identify which parameter is the most significant among all parameters. Confirmatory test is also done by taking the optimal parameters and the result shows that the improvement of the response variables is acceptable and also shows that the method that is used for optimization was valid.

An application of fully coupled ductile damage model considering induced anisotropies on springback prediction of advanced high strength steel materials

In this paper, an advanced model formulated in the framework of non-associative plasticity is used to accurately predict the springback of advanced high strength steels material. The proposed model strongly couples the isotropic ductile damage to the phenomena a combined nonlinear isotropic and kinematic hardening, initial anisotropic plastic flow and induced anisotropies due to the distortion of the yield function and the plastic potential respectively. For the application, the simulations were conducted for U-draw bending of as-received DP 780 steel sheet proposed by Numisheet’2011 Benchmark. The proposed model was able to capture adequately the Bauschinger effect, transient hardening (distortional) and ductile damage-induced softening. The results have shown that the distortion of yield function affect significantly the stress distribution especially in the sidewall curl during the forming stage thereby explaining observed variations in springback. For full coupling with isotropic ductile damage, the predicted angular springback was found in good agreement with experimental measurements and the sidewall curl was significantly improved in comparison to the simple use of hardening models. This means that isotropic ductile damage and its effect on the other physical phenomena require special attention in order to take benefit when designing newly developed advanced high strength steels (AHSS) parts for automotive and aerospace industries.

Investigations on edge quality and its effect on tensile property and fracture patterns of QP980

Advanced High Strength Steel (AHSS) sheet metals have been widely used in automotive structures due to its safety performance and lightweight potentials, and some AHSS materials have been found more sensitive to edge crack during the stamping process. The tensile property of the sheared edge has great influence on flangability for AHSS. In the present work, the effects of the blanking clearance and shear angle on the quality of the sheared edge and subsequent tensile property for QP980 have been investigated experimentally by microscope and sheared-edge tension test based on digital image correlation (DIC) technology. In addition, the tensile properties of the specimens prepared by wire electrical discharging machining (Wire-EDM) and laser cutting were tested as well. The results show that promising and stable sheared edge quality, and tension with postponed fracture can be achieved if the clearance ratio is set as 7% or 11% for QP980. During blanking with 5° or 10° oblique edge, the ratios of burnish zone and burr zone decrease simultaneously, and the subsequent tensile performance increases significantly. The oblique edge blanking obtains smaller and smoother burr, and the sheared surface benefits from less void damage. In addition, based on the fracture surface morphology and the tracking of the fracture process with DIC technology, two typical fracture patterns (oblique crack and transverse oblique crack) in tension test are illustrated. Oblique crack occurring on the specimens with good edge quality shows obvious necking phenomenon and belongs to the typical ductile fracture. While transverse oblique crack exhibiting with high frequency on the sheared specimens is typical edge fracture and shows mixed characteristics of ductile fracture and brittle fracture.

A Practical Methodology to Evaluate and Predict Edge Cracking for Advanced High-Strength Steel

With the increased application of advanced high strength steel (AHSS), the development of a reliable methodology for evaluating and predicting edge cracking is in high demand. In this study, different shear edge conditions of three different AHSS materials, TRIP 780, DP 980, and DP 1180, were evaluated. The edge cracking is evaluated in four steps: shear test, sheared edge characterization, HSDT, and prediction of edge cracking using FEA. The shear edges were prepared with five different shear clearances between 5 and 25% of the material thickness to obtain variable shear quality. In the HSDT, the strain and thinning distribution is captured using a digital image correlation system as the edge cracking limit and failure criteria in FEA. The preferred shear clearance is characterized by the largest stroke in HSDT and highest thinning value of the onset of edge cracking. FEA showed good correlations with the experiment comparing strain and load-displacement curves. The optimized shear clearance for TRIP 780 is 15% material thickness which for DP 980 and DP 1180 is 20%. By comparing test results from shear test, HSDT and FEA simulation, the peak shear load and burr height from simple experiment observation were found to be important indicators of edge condition, which can be monitored in production.

A numerical study on chain-die forming of the AHSS U-channel and contrast with roll forming

Advanced high strength steels (AHSS) pose great challenges to sheet metal forming processes in vehicle components manufacturing with their increasing high strength and surface hardness. Chain-die forming is a recently proposed manufacturing method alternative to roll forming, aiming at reducing the redundant strain by enlarging the deformation length through a remarkably large rotation radius. In this work, the performance of AHSS in chain-die forming is investigated by experiment and simulation. The mechanical tests and chain-die forming tests on four AHSS materials are performed. The Chaboche hardening model to describe the Bauschinger effect of the AHSS is established and implemented into the finite element model of chain-die forming process. The evolution of longitudinal strain and springback are elaborated and discussed. Furthermore, a systematic comparison between chain-die forming and roll forming of the U-channel are performed based on the verified FE model. The chain-die forming has obviously smaller and less fluctuate longitudinal strain on the flange, mainly because chain-die forming has a smoother transitional surface. Chain-die forming can alleviate web bow; the flange width of the chain-die formed U-channel presents a monotonic increase trend along the longitudinal direction, while the flange width of the roll-formed shows a fluctuate trend. On the other hand, the springback after chain-die forming is larger than that after roll forming, which implies more compensation is needed to obtain a desired product.

Formability prediction for AHSS materials using damage models

Advanced high strength steels (AHSS) are seeing an increased use, mostly due to lightweight design in automobile industry and strict regulations on safety and greenhouse gases emissions. However, the use of these materials, characterized by a high strength to weight ratio, stiffness and high work hardening at early stages of plastic deformation, have imposed many challenges in sheet metal industry, mainly their low formability and different behaviour, when compared to traditional steels, which may represent a defying task, both to obtain a successful component and also when using numerical simulation to predict material behaviour and its fracture limits. Although numerical prediction of critical strains in sheet metal forming processes is still very often based on the classic forming limit diagrams, alternative approaches can use damage models, which are based on stress states to predict failure during the forming process and they can be classified as empirical, physics based and phenomenological models. In the present paper a comparative analysis of different ductile damage models is carried out, in order numerically evaluate two isotropic coupled damage models proposed by Johnson-Cook and Gurson-Tvergaard-Needleman (GTN), each of them corresponding to the first two previous group classification. Finite element analysis is used considering these damage mechanics approaches and the obtained results are compared with experimental Nakajima tests, thus being possible to evaluate and validate the ability to predict damage and formability limits for previous defined approaches.

Evolution of elastic modulus in roll forming

Roll forming is a continuous process in which a flat strip is incrementally bent to a desired profile. This process is increasingly used in automotive industry to form High Strength Steel (HSS) and Advanced High Strength Steel (AHSS) for structural components. Because of the large variety of applications of roll forming in the industry, Finite Element Analysis (FEA) is increasingly employed for roll forming process design. Formability and springback are two major concerns in the roll forming AHSS materials. Previous studies have shown that the elastic modulus (Young’s modulus) of AHSS materials can change when the material undergoes plastic deformation and the main goal of this study is to investigate the effect of a change in elastic modulus during forming on springback in roll forming. FEA has been applied for the roll forming simulation of a V-section using material data determined by experimental loading-unloading tests performed on mild, XF400, and DP780 steel. The results show that the reduction of the elastic modulus with pre-strain significantly influences springback in the roll forming of high strength steel while its effect is less when a softer steel is formed.

Pulsed LASER-(Micro) TIG Welding of Automotive Zn Coated Advanced High Strength Steels Thin Sheets in Overlap Configuration

The wider implementation into automotive sector of new materials categorized as Advanced High Strength Steels (AHSS) in order to reduce costs and weight require the development of appropriate joining technologies. The paper presents the results of applying of a new hybid welding process, i.e. Pulsed LASER-(micro)TIG, for realizing overlap joints for Zn-coated AHSS materials. The statistical influence of main hybrid welding process parameters was investigated by means of factorial experimental design and weighted against the shear strength results of the realized joints. The influence of laser pulse peak power, the average welding TIG current and the joint gap was investigated for three types of Zn coated AHSS with different thicknesses. The results revealed that sane joints could be obtained with the new welding process and that the new process is fit to be applied for these materials. Each material did behave different in respect to the investigated process parameters and the results did reveal rather strong interaction between the influence factors (IF), however the joint gap does play an important role in respect to the maximum shear strength of the joint, which could be correlated to the specifics of the laser beam welding of Zn coated steels. The strong interaction between the studied IF imply further study of the new process by including other process parameters.

Springback Investigation in Roll Forming of a V-Section

To have fuel efficient vehicles with a lightweight structure, the use of High Strength Steels (HSS) and Advanced High Strength Steels (AHSS) in the body of automobiles is increasing. Roll forming is used widely to form AHSS materials. Roll forming is a continuous process in which a flat strip is shaped to the desired profile by passing through numerous sets of rolls. Formability and springback are two major concerns in the roll forming of AHSS materials. Previous studies have shown that the elastic modulus (Young’s modulus) of AHSS materials can change when the material undergoes plastic deformation and the main goal of this study is to numerically investigate the effect of a change in elastic modulus during forming on springback in roll forming. Experimental loading-unloading tests have been performed to obtain the material properties of TRIP 700 steel and incorporate those in the material model used in the numerical simulation of the roll forming process. The finite element simulations were carried out using MSC-Marc and two different element types, a shell element and a solid-shell element, were investigated. The results show that the elastic modulus diminution due to plastic strain increases the springback angle by about 60% in the simple V-section roll forming analyzed in this study.