Sushanta Kumar Panda

Numerical Simulations and Experimental Results of Tensile Test Behavior of Laser Butt Welded DP980 Steels

Laser welding of advanced high strength steels for fabrication of tailor welded blanks is of increasing interest for continued improvements in vehicle performance and safety without an increase in weight. Experimental results have shown that formability of welded dual-phase (DP) steels is significantly reduced by the formation of a softened region in the heat-affected zone (HAZ). In this study, a finite element simulation of welded DP980 samples undergoing transverse uniaxial tensile testing was used to evaluate the effects of soft zone width and strength on formability characteristics. Both the strength and the ductility of laser welded blanks decreased compared with those of unwelded blanks due to the formation of a softened outer-HAZ, where strain localization and final fracture occurred during tensile testing. The magnitude of softening and the width of the HAZ depend on the laser specific energy. It was observed from tensile test experiments and numerical simulations that both a decrease in strength and an increase in width of the softened HAZ were responsible for a decrease in the overall strength and ductility of the welded blanks. DOI: 10.1115/1.2969256

Microstructure and Mechanical Performance of Friction Stir Spot-Welded Aluminum-5754 Sheets

Friction stir spot welding (FSSW) is a recent trend of joining light-weight sheet metals while fabricating automotive and aerospace body components. For the successful application of this solid-state welding process, it is imperative to have a thorough understanding of the weld microstructure, mechanical performance, and failure mechanism. In the present study, FSSW of aluminum-5754 sheet metal was tried using tools with circular and tapered pin considering different tool rotational speeds, plunge depths, and dwell times. The effects of tool design and process parameters on temperature distribution near the sheet-tool interface, weld microstructure, weld strength, and failure modes were studied. It was found that the peak temperature was higher while welding with a tool having circular pin compared to tapered pin, leading to a bigger dynamic recrystallized stir zone (SZ) with a hook tip bending towards the upper sheet and away from the keyhole. Hence, higher lap shear separation load was observed in the welds made from circular pin compared to those made from tapered pin. Due to influence of size and hardness of SZ on crack propagation, three different failure modes of weld nugget were observed through optical cross-sectional micrograph and SEM fractographs.

Finite element analysis of effects of soft zones on formability of laser welded advanced high strength steels

Abstract Applications of advanced high strength steels in tailor welded blanks are a major focus to reduce vehicle weight and manufacturing cost, and to improve part performance. In the present work, formability of two types of laser welded steels in biaxial stretch forming has been studied. Laser welded blanks of the dual phase steel showed larger reduction of formability than those of high strength low alloy steel. This was due to formation of soft zones in the outer heat affected zone of dual phase steel where strain localisation and fracture occurred. Finite element simulations of dome testing incorporating local weld zone properties were able to accurately predict experimentally observed behaviour including deformation patterns and failure locations. In particular, the simulations have shown that under matching of the softened heat affected zone leads to reduced formability by localisation of strain; and that the fracture location changes from radial to circumferential plane when the flow curve of soft zone is evenly matched with the base metal. It was also concluded that the width of the soft zone has minor influence on formability during biaxial stretch forming.

Investigations Into the Influence of Weld Zone on Formability of Fiber Laser-Welded Advanced High Strength Steel

In this study, two different dual phase steel grades DP980 and DP600, and IFHS steel sheets were laser welded by a 2-kW fiber laser. The weld quality of these three different LWBs was assessed with the help of microstructure, micro-hardness and transverse tensile tests. Tensile testing of longitudinal and miniature samples was performed to evaluate the mechanical properties of the weld zone. Formability of parent materials and LWBs were assessed in bi-axial stretch forming condition by Erichsen cupping test. To validate the weld zone properties, 3-D finite element models of Erichsen cupping test of LWBs was developed, and the failures in the deformed cups were predicted using two theoretical forming limit diagrams. It was observed that hardness of the fusion zone and HAZ in laser welded DP600 and IFHS steels was more compared to the respective parent metal. However, 29% reduction in hardness was observed at the outer HAZ of DP980 steel weldments due to tempering of martensite. Reduction of formability was observed for all the LWBs with two distinct failure patterns, and the maximum reduction in formability was observed in the case of DP980 LWBs. The presence of the soft zone is detrimental in forming of welded DP steels.

Effect of Weld Location, Orientation, and Strain Path on Forming Behavior of AHSS Tailor Welded Blanks

Use of multiple advanced high strength steel sheets for fabrication of tailor welded blanks (TWBs) is one of the current interests for automotive and steel industries as it reduces manufacturing cost and weight of the vehicle, and also improves the quality of the component. As the varieties of TWB applications are increasing, the effects of the difference in material properties, weld, and its orientation on blank formability have become important both in deep drawing and stretch forming. In this work, high strength low alloy (HSLA) grade steels were laser welded with two different dual phase steels having 980 MPa (DP980) and 600 MPa (DP600) tensile strengths to fabricate two different material combination TWBs (DP980-HSLA and DP600-HSLA). Formability of these two types of TWBs has been studied experimentally both in biaxial and plane strain stretch forming modes by performing limiting dome height (LDH) tests using a 101.6 mm diameter hemispherical punch. Five different weld locations during biaxial-stretch forming mode, and the effect of weld orientation with respect to major principal strain in plane strain stretch forming mode, have been studied. It was found that formability LDH and failure location depended on weld location, and LDH increased when weld line was positioned at the extreme positions away from the center due to more uniform strain distribution on the deformed dome. The welded blanks had lower formability in plane strain deformation mode compared with biaxial-stretch forming mode. However, influence of weld orientation on the formability depended on material combination. Changes in the fracture mode were confirmed from fractography analysis of biaxial, transverse plane strain, and longitudinal plane strain stretch formed samples.

Friction Stir Welding: Scope and Recent Development

Friction Stir Welding (FSW) is a new solid-state welding technique which finds application in various industries. This chapter introduces the process, basic mechanism, application, and recent research developments. Research work in this book chapter is broadly divided in two parts: experimental-based, and finite element modeling (FEM)-based approaches of the FSW process. In the experimental studies, three recent developments are presented in this chapter: first, a unique twin-tool concept to modify the FSW process and provide alternative to multi-pass FSW; second, feasibility of using ultrasonic coupled with FSW is studied to reduce the amount of force generated during the process and improve the process efficiency; and finally, formability study of friction stir welded blank is presented. Formability of welded blank plays a vital factor for different industrial application, especially in automobile industry. In the second part, FEM method is implemented to simulate the process. Different modeling techniques are also discussed. A case study in each case is presented with sample results, to have a better understanding on the process and development.

Microstructures, Forming Limit and Failure Analyses of Inconel 718 Sheets for Fabrication of Aerospace Components

Abstract Recently, aerospace industries have shown increasing interest in forming limits of Inconel 718 sheet metals, which can be utilised in designing tools and selection of process parameters for successful fabrication of components. In the present work, stress-strain response with failure strains was evaluated by uniaxial tensile tests in different orientations, and two-stage work-hardening behavior was observed. In spite of highly preferred texture, tensile properties showed minor variations in different orientations due to the random distribution of nanoprecipitates. The forming limit strains were evaluated by deforming specimens in seven different strain paths using limiting dome height (LDH) test facility. Mostly, the specimens failed without prior indication of localized necking. Thus, fracture forming limit diagram (FFLD) was evaluated, and bending correction was imposed due to the use of sub-size hemispherical punch. The failure strains of FFLD were converted into major-minor stress space (σ-FFLD) and effective plastic strain-stress triaxiality space (ηEPS-FFLD) as failure criteria to avoid the strain path dependence. Moreover, FE model was developed, and the LDH, strain distribution and failure location were predicted successfully using above-mentioned failure criteria with two stages of work hardening. Fractographs were correlated with the fracture behavior and formability of sheet metal.

Formability Assessment of Prestrained Automotive Grade Steel Sheets Using Stress Based and Polar Effective Plastic Strain-Forming Limit Diagram

Accurate prediction of the formability in multistage forming process is very challenging due to the dynamic shift of limiting strain during the different stages depending on the tooling geometry and selection of the process parameters. Hence, in the present work, a mathematical framework is proposed for the estimation of stress based and polar effective plastic strain-forming limit diagram (σ- and PEPS-FLD) using the Barlat-89 anisotropic plasticity theory in conjunction with three different hardening laws such as Hollomon, Swift, and modified Voce equation. Two-stage stretch forming setup had been designed and fabricated to first prestrain in an in-plane stretch forming setup, and, subsequently, limiting dome height (LDH) testing was carried out on the prestrained blanks in the second stage to evaluate the formability. The finite element (FE) analysis of these two-stage forming process was carried out in ls-dyna for automotive grade dual-phase (DP) and interstitial-free (IF) steels, and the σ-FLD and PEPS-FLD were used as damage model to predict failure. The predicted forming behaviors, such as LDH, thinning development, and the load progression, were validated with the experimental results. It was found that the LDH in the second stage decreased with increase in the prestrain amount, and both the σ-FLD and PEPS-FLD could be able to predict the formability considering the deformation histories in the present multistage forming process with complex strain path.

Experimental Investigations on Formability of Aluminum Tailor Friction Stir Welded Blanks in Deep Drawing Process

In the present work, tailor friction stir welded blanks (TFSWBs) were fabricated successfully using 2.0-mm-thick AA5754-H22 and AA5052-H32 sheet metals with optimized tool design and process parameters. Taguchi L9 orthogonal array has been used to design the friction stir welding experiments, and the Grey relational analysis has been applied for the multi objective optimization in order to maximize the weld strength and total elongation reducing the surface roughness and energy consumption. The formability of the TFSWBs and parent materials was evaluated and compared in terms of limiting drawing ratio (LDR) using a conventional circular die. It was found that the formability of the TFSWBs was comparable with that of both the parent materials without failure in the weldment. A modified conical tractrix die (MCTD) was proposed to enhance the LDR of the TFSWBs. It was found that the formability was improved by 27% using the MCTD.

PREDICTION OF FORMABILITY OF BI-AXIAL PRE-STRAINED DUAL PHASE STEEL SHEETS USING STRESS BASED FORMING LIMIT DIAGRAM

Dual phase (DP) steel is of great interest for automotive part manufacturers due to its excellent combinations of strength and formability. Complex components involving three-dimensional stampings are usually fabricated through multistage sheet forming operations. The ability of a sheet metal to be deformed into a specific desired shape by distributing strain over arbitrary tool surface depends on complex interaction of material, process and design variables. The strain-based forming limit diagram (e-FLD) is often used as a measure of formability in the press shop due to convenience of measuring the limiting strain. However, it was reported by previous researchers that the e-FLD of sheet metal shifts after pre-strain due to the initial forming operations. Hence, this work proposes a mathematical framework for constructing σ-FLD of different pre-strained sheets incorporating Barlat-89 yield criterion with different hardening laws. The formability of biaxially pre-strained DP600 was evaluated experimentally in two stages. The forming behaviour of pre-strained material was predicted by finite element model using the σ-FLD, and the predicted results matched very closely with the experimental data. It was also observed that the σ-FLD was robust and underwent insignificant changes due to the change in the pre-strain path.