High-strength Sheet Metal Research Articles

Towards improved description of plastic anisotropy in sheet metals under biaxial loading: A novel generalization of Hill48 yield criterion

Lightweight design and development of automobiles are strongly dependent on lightweight high-strength sheet metals. An accurate prediction of anisotropy and yield surface evolution of these materials under different sampling directions of biaxial tension (BT) is a necessary condition for the numerical analyses of stamping forming. In this work, the Hill48 model considering the principal stress direction is generalized to describe the anisotropic evolution and the yield loci on the normal and diagonal planes. The material parameters of the generalized M-Hill48 model (GM-Hill48) have analytical form and can be calibrated by the yield stresses and r-values expressed by the continuous equivalent plastic strain (EPS). A quadratic function interpolation method based on the near-plane strain (NPS) stress is established to control the yield locus under different plane stress states. In addition, a method for predicting the yield loci in arbitrary sampling directions under BT is proposed by tensor coordinate transformation. Different from the traditional model which can only determine material parameters by using equi-biaxial tension (EBT) or NPS stress along the rolling direction (RD) and transverse direction (TD), the yield stresses of BT in different sampling directions can be incorporated into the generalized model. The predictive capability of the GM-Hill48 model is verified by capturing the continuous evolution of yield stresses and strain for DP590 and AA6016-T4 materials. The convexity condition of the established modified model is given and discussed. Finally, the advantages of new model are further expounded in terms of the prediction accuracy and identified parameters compared with the advanced yield models.

True stress-strain identification accounting for anisotropy of sheet metals

Sheet metals for the automotive industry are subjected to continuous research efforts aiming at ever increasing mechanical performance. A remarkable feature of modern high strength sheet metals is their anisotropy, intrinsic in the technologic process of their production. When the effect of anisotropy on the mechanical response of a material cannot be neglected, specimens along different directions are usually tested, possibly under different stress states, to assess the flow curves and the deforming ratios for each direction and each loading mode. Such data are then used to calibrate many possible plastic anisotropy models available in the literature. In this work, the experimental procedures for determining the stress-strain curve and the anisotropic straining ratio are studied in detail, referring to representative tensile tests along the rolling direction of two anisotropic sheet metals, respectively PHS-1800 steel and 6181 aluminium alloy. Both alloys are ductile and exhibit remarkably long post-necking phases in tension, revealing that, in such cases, the standard procedures for the experimental derivation of the hardening curves and of the anisotropic strain ratios are limited to the very early phases of the material life and miss to cover the major part of the strain range up to failure. Different alternative procedures for the derivation of experimental data and for their postprocessing are considered and compared to each other, identifying a set of guidelines for achieving a good engineering accuracy up to failure in deriving both the stress-strain curves and the anisotropic strains ratios. The above analyses are made on the results of tensile tests at static, intermediate and high strain rate, confirming the generality of the identified procedure.

Efficient net shape forming of high-strength sheet metal parts by Transversal Compression Drawing

In this contribution, a new forming method is presented for high-strength steel sheet metal materials called Transversal Compression Drawing (TCD). For TCD, the blankholder of the conventional deep drawing process is replaced by sliders introducing a vertical force along the blank edge. Compared to conventional deep drawing, compressive stresses are thereby superimposed in the flange area of the sheet metal material, reducing the springback of formed components and the forces required during the process. Proof of this is provided here on the basis of numerical investigations in which conventional deep drawing and TCD are compared using the example of an “S-Rail” component made from DP980. Results of these investigations reveal that with TCD a maximum reduction in springback by more than 5 mm is achieved, leading to an improvement in the component´s overall shape accuracy of more than 55%. Furthermore, frictional work is decreased by − 6.92 kJ/kg, resulting in an overall reduction of maximum forming forces by 67.4%. Besides, TCD enables a trim-free net shape forming of the final part contour in a single operation and an increased material utilization of 4.5% from coil to part. Not least, no sheet thinning occurs with TCD, whereas a maximum thinning of 12% is observed with the conventional deep drawing process.

Effect of pre-strain on edge formability of high strength steels for chassis applications

There is a high level of uncertainty in the prediction of edge cracking during the design and manufacturing of high strength sheet metal products. Edge ductility is a complex parameter to measure since it is not an intrinsic property of the material, but it also depends on the trimming strategy/parameters that have been applied to the edge (edge preparation method, cutting/punching clearance, etc.). Furthermore, most forming processes involve more than one operation where the material can be deformed in a first stage, then trimmed or punched and further deformed in a final forming step to obtain the desired shape. Therefore, in order to design a robust and safe forming process able to predict the risk of edge cracking, it is crucial to understand the influence of pre-strain on the residual formability of trimmed edges. This paper analyses the effect of different pre-strain levels and paths (uniaxial tension, plane strain and equi-biaxial stretching) on the sheared edge ductility of a Complex Phase (CP) steel and a high strength low alloy (HSLA) steel used for automobile chassis applications. To this purpose, rectangular specimens are cut out from pre-strained samples and punched. Hole Tension Tests (HTTs) in combination with Digital Image Correlation (DIC) are carried out to determine the edge fracture strain of undeformed and pre-strained samples. The results are discussed in terms of major strains obtained from DIC. The proposed methodology allows describing the edge cracking sensitivity of high-strength steel sheets as a function of the pre-deformation state and can be useful to predict the risk of edge cracking more accurately in forming simulations.

Investigations on the flanging characteristics of advanced high-strength sheet metals with different sheared edge quality

Investigations on the flanging characteristics of advanced high-strength sheet metals with different sheared edge quality

Numerical-experimental study of die geometry in the constrained groove pressing of 6061 aluminum sheets

Nowadays, in line with the need of different industries to the materials with high strength to weight ratio, researchers have drawn attention to the processes of severe plastic deformation. The constrained groove pressing (CGP) is one of the most effective methods of severe plastic deformation to produce high-strength sheet metal using two dies with different shapes (grooved and flat). Despite the advantages of CGP in creating high accumulative plastic strain, this process leads to non-uniformity in mechanical properties of the sheet. This non-uniformity is one of the limitations of the CGP process which is considered in this study. Experimental and numerical studies were performed to investigate CGP process on 6061 aluminum alloy sheet up to two complete passes and the mechanical properties of the sheets were studied. Hardness testing was conducted on the worked material and the increase in the hardness of tested material in both longitudinal and thickness directions were observed. Also, it was shown that the non-uniformity in hardness distribution is correlated to the non-uniformity in the strain distribution. So, validated numerical model was used to develop the response surface method (RSM) and consequently to investigate the effect of geometric parameters in the grooved die on the strain uniformity. Finally, die geometry was optimized using the developed RSM model to produce a sample with more uniformity in strain and hardness distribution.

Yüksek Mukavemetli Çeliklerde Geri Esnemenin Kalınlık Azaltma Yöntemi ile Telafisi

The sheet metal bending process plays an important role in sheet metal production and springback is an unintended consequence of this operation. If the bending die process is performed without knowing the amount of springback, it is difficult to produce parts within acceptable tolerance values. The springback behavior of the part to be bent also affects the size of the bending die. In particular, in the case of a 90° U bend die, the manufacturing cost of the bending die with the springback compensation method is high. Therefore, it is important to estimate the springback value at the design stage. In this work, the estimation of the amount of deformation to avoid springback value was performed using the thickness reduction method for high strength sheet metal. According to this study, the thickness reduction method can be used effectively in springback compensation. The estimation of the measurement accuracy at the design stage reduces the cost of the bending die and it is possible to produce parts within tolerance limits without much experimentation. The results showed that thickness reduction at the 0,2 t value was suitable to eliminate the springback value in the U-bending process. It has been found that experimental, empirical and finite element results are close to each other.

Longitudinal strain and wrinkling analysis of variable depth flexible roll forming

Flexible roll forming (FRF) enables the forming of high-strength sheet metal materials into complex shapes relevant to the transportation industry until recently, however, has been limited to the forming of variable-width profiles where the web section is flat with a curved flange. Recent developments in 3D flexible roll forming now enable forming of variable depth components that have a curved web with flat flanges; these shapes are better suited for automotive applications. However, wrinkling in the flange due to excessive compressive longitudinal stress remains a major challenge. Deformation strain and stress analysis has been applied in previous studies to estimate the wrinkling tendency, but current analytical equations are limited to variable-width shapes with limited accuracy, particularly regarding the level and distribution of longitudinal strain. In the current study, a new analytical model is developed to analyse the distribution and level of longitudinal edge strain for a variable depth component shape. The analytical predictions are compared with experimental measurements for key conditions. This shows that the new analytical equations developed in this study provide higher accuracy compared to existing models and enable for the first time the prediction of the distribution of longitudinal strain in the flange of a flexible roll forming component. For some of the tested conditions, flange wrinkling was observed and there the model correlation was poor given the theory assumes a wrinkle-free flange. The strain deviation observed here is associated with the shape deviation due to wrinkling. The analytical model predicts the strains that are required to form a perfect part shape. Wrinkling reduces the compressive longitudinal strains that are formed into the flange, and this leads to a poor correlation between the experimental and the predicted strain values if the wrinkling severity is high. The model brings some insights into the wrinkling behaviour in variable depth flexible roll forming by predicting the variation of the peak strain and overall distribution with key geometric parameters. The new analytical model for longitudinal edge strain presented in this study will enable a more rapid process design and optimisation before time-consuming FEA analysis is performed. It, therefore, presents a vital step towards a wider spread in the application of flexible roll forming in the automotive and transport industries.

Additive Manufacturing of Tool Steels*

Abstract Additive manufacturing of tool steels represents a great challenge, yet it offers new possibilities for the tool manufacture of, for example, complex forming tools with conformal cooling. First, this contribution gives an overview of the most relevant additive manufacturing processes, the materials and processing concepts. By means of a hybrid manufactured press hardening tool for high-strength sheet metal parts, an example of practical implementation is presented subsequently.

Numerical simulation and experimental investigation: Metal spinning process of stepped thin-walled cylindrical workpiece

Many equipment and devices utilized in the aerospace industry are formed as symmetric parts through high plastic deformation of high strength sheet metal alloys with low thickness. Considering the inherent advantages of the spinning process of simple tooling and concentrated deformation loading, this process can be considered as one of the main options in producing these thin-sectioned lightweight parts. In this study, a Finite Element (FE) model has been developed to simulate the formation of a stepped thin-walled cylindrical workpiece of AISI 316 stainless steel alloy by spinning process. The FE simulation results were employed to investigate the effects of process parameters, including feed rate of the roller and rotational velocity of the mandrel on the distribution of stress and strain in the sheet metal, wrinkling failure, and thinning of the sheet metal during deformation. Experiments were carried out using selective input parameters based on the results of FE simulations. The comparison between FE simulations and experiments revealed that the developed model could predict the thinning of the sheet metals with over 93 % accuracy. Additionally, a good agreement between the experimentally deformed sheet configurations with those resulting from finite element simulations has been observed.

Stating Failure Modelling Limitations of High Strength Sheets: Implications to Sheet Metal Forming.

This article discusses the fracture modelling accuracy of strain-driven ductile fracture models when introducing damage of high strength sheet steel. Numerical modelling of well-known fracture mechanical tests was conducted using a failure and damage model to control damage and fracture evolution. A thorough validation of the simulation results was conducted against results from laboratory testing. Such validations show that the damage and failure model is suited for modelling of material failure and fracture evolution of specimens without damage. However, pre-damaged specimens show less correlation as the damage and failure model over-predicts the displacement at crack initiation with an average of 28%. Consequently, the results in this article show the need for an extension of the damage and failure model that accounts for the fracture mechanisms at the crack tip. Such extension would aid in the improvement of fracture mechanical testing procedures and the modelling of high strength sheet metal manufacturing, as several sheet manufacturing processes are defined by material fracture.

A new cracking resistance index based on fracture mechanics for high strength sheet metal ranking

Driven by current safety and weight reduction policies in the automotive sector, the development of new high strength sheet metal products has experienced unprecedented growth in the last years. With the emergence of these high strength materials, new challenges related to their limited ductility and higher cracking susceptibility have also raised. Accordingly, the development of new fracture criteria accounting for the material’s cracking resistance has become unavoidable. In this work, a new cracking resistance index (CRI) based on fracture mechanics is proposed to classify the crack propagation resistance (i.e. the fracture toughness) of high strength metal sheets. The index is based on the fracture energy obtained from tensile tests with sharp-notched specimens. The procedure is very fast and simple, comparable to a conventional tensile test, and it may be used as routine testing for quality control and material selection. The CRI is investigated for several advanced high strength steel (AHSS) sheets of 0.8-1.6 mm thickness with tensile strengths between 800 and 1800 MPa. The results show that the proposed index is suitable to rank high strength steel sheets according to their crack propagation resistance and it can be correlated to the material’s crashworthiness and edge cracking resistance.

Comparative study of ASS 316L on formability at room temperature and super plastic region

ABSTRACT In the sheet metal industry, hot forming of high strength sheet metal alloys such as austenitic stainless steels (ASSs) are in great demand. Identifying the safe strain region is essential to avoid failures and reduce the wastage. Forming limit diagram (FLD) is a vital tool which is used to describe the maximum limit of principal strains that can be sustained by sheet metals till the onset of localised necking. These diagrams are also useful in comparing the changes in forming characteristics of the material with changes in process parameters like temperature, orientation and speed of deformation. The present study aimed at comparing the experimental FLD for ASS 316L at room temperature and 900°C by stretch forming. These specimens are cut in three different orientations (0°, 45° & 90°) to the rolling direction and experiments are conducted at a strain rate of 0.1/s. It is reported that, the specimens from rolling direction are among the stronger samples. The change in formability characteristics of ASS 316L with changes in temperatures using FLD has been observed. The optical microscopy and scanning electron microscopy is analysed to understand the fracture morphologies. The comparison between these set of results indicates that there is significant increase in formability of material. Abbreviations ASS: Austenitic Stainless Steel; RT: Room Temperature; FLD: Forming Limit Diagram; FFLD: Fracture Forming Limit Diagram; SEM: Scanning Electronic Microscopy; LDH: Limiting Dome Height; UTS: Ultimate Tensile Strength; YTS: Yield Tensile Strength; K: Strength Factor; n: Strain Hardening Exponent

Investigation of the Mechanical Punch Loads during Fine Blanking of High-Strength Steels with Cemented Carbide

Abstract The increasing ecological demands require emissions reduction of automotive vehicles. This challenges the automotive industry for the increased application of light-weight components made of high-strength steels. The processing of high-strength steels brings many manufacturing processes to its mechanical limitations. In fine blanking, conventional tool materials lack on compressive strength in order to separate high-strength sheet metal. Nowadays, high-speed steels used for tool applications must be substituted with materials having a higher compressive strength, such as cemented carbides. However, with the increased compressive strength the carbides show a significantly lower ductility. During a repeated stripping off process, the low ductility of cemented carbides leads to a premature punch fracture and with this reduced economic feasibility of the process. The stripping off force required for the final punch ejection is generated by the clamping force applied by elastic deformation of the scrap material. Currently, phenomenological models exist allowing only approximate prediction of the clamping force. The precise prediction is not possible, since the clamping force depends on plenty parameters, such as sheet material strength and thickness, as well as punch geometry, ambient temperature, nest positioning, etc. A comprehensive investigation of the influence of these parameters on the punch load is strictly required. In order to enhance the applicability of cemented carbides as an alternative punch material in fine blanking of high-strength steels, a tool for process forces quantification, such as clamping and stripping off forces during one cutting-ejection cycle, must be provided. To achieve this goal, a comprehensive numerical study on the influence of process parameters such as sheet metal strength, sheet thickness and cemented carbide grade on the cutting, clamping and stripping off forces is performed. The developed three-dimensional numerical model of a fine blanking process with an elastic punch is validated by means of the experimentally measured punch force during an industrial fine blanking process of spacer discs. Based on the obtained numerical results, a basic correlation between the clamping force during the cutting step and the stripping off force during the ejection step are analyzed. The identified tendencies are used to create an analytical model of the clamping force and a regression model of the stripping off forces. The developed models allow for improved prediction of the expected process forces during fine blanking of high-strength steels and with this for the selection of the appropriate grade of cemented carbide as a punch material.

Integration of Press-Hardening Technology into Processing of Advanced High Strength Steels

Development of high strength or even ultra-high strength steels is mainly driven by the automotive industry which strives to reduce the weight of individual parts, fuel consumption, and CO2 emissions. Another important factor is the passenger safety which will improve by the use of these materials. In order to achieve the required mechanical properties, it is necessary to use suitable heat treatment in addition to an appropriate alloying strategy. The main problem of these treatments is the isothermal holding time. These holding times are technologically demanding which is why industry seeks new possibilities to integrate new processing methods directly into the production process. One option for making high-strength sheet metals is press-hardening which delivers high dimensional accuracy and a small spring-back effect. In order to test the use of AHSS steels for this technology, a material-technological modelling was chosen. Material-technological models based on data obtained directly from a real press-hardening process were examined on two experimental steels, CMnSi TRIP and 42SiCr. Variants with isothermal holding and continuous cooling profiles were tested. It was found that by integrating the Q&P process (quenching and partitioning) into press hardening, the 42SiCr steel can develop strengths of over 1800 MPa with a total elongation of about 10%. The CMnSi TRIP steel with lower carbon content and without chromium achieved a tensile strength of 1160 MPa with a total elongation of 10%.

Process-induced bottom defects in clinch forming: Simulation and effect on the structural integrity of single shear lap specimens

Lightweight and high strength sheet metal such as aluminum alloy sheets are used in a variety of industrial applications. Due to the limited weldability of these materials, mechanical joining techniques such as clinching are of interest. The challenge in this regard is that conventional round clinch forming locally induces large plastic deformations which potentially cannot be accommodated by materials with limited ductility. Ductile damage models are used to predict the occurrence of bottom cracks during conventional round clinch forming of EN AW-6082 T6 sheet. It is shown that cracks in the bottom of a clinched joint and the final static strength of a single lap shear specimen can be numerically reproduced provided that the post-necking strain hardening behavior and damage behavior of the base material are properly identified. The observed bottom cracks did not have a detrimental effect on the static strength and fatigue life of single shear lap specimens. It is hypothesized that fatigue cracks in single shear lap tests initiate due to fretting.

Wear behaviour of thermally oxidised tool surfaces as low-friction separation layers for dry sheet metal forming

This paper presents the results and analysis of wear investigations carried out on a wear test bench with high strength sheet metal DP600+Z drawn over selectively oxidised α-Fe2O3 tool steel surfaces. Wear investigations were carried out with several selectively oxidised specimens by varying the pulling and counter force, and thus the surface pressure on the samples. This allowed for a systematic study of the wear behaviour of the oxide layers under different loads. The specimens were characterised using microscopy (light microscopy, scanning electron microscopy and topography analysis) and scratch testing on a nanoindenter. The study shows that it is possible to realise a dry metal forming process while using α-Fe2O3 oxide layers on the tool steel surface. The oxidised surface acts as a friction reducing separation layer and protects the tool against wear. Simultaneously, it was found that the surface of sheet metal drawn over oxidised specimens showed lower zinc abrasion than sheet metal drawn over non-oxidised reference specimens at the same surface pressure. In addition, the oxidised specimens have a reduced Zn pick-up affinity.

Study the Effects of Spring Back on Sheet Metal Bending using Mathematical Methods

In this study mathematical analysis of spring back was done for optimization of sheet metal bending process. The influence of sheet metal thickness, sheet metal type, friction, tool radius and tool shape on spring back for Aluminium, copper, mild steel and High strength steels, sheet metal have been used as variable to conducted this study. Since, the effects of each variable on the formation of spring back on sheet metal forming are investigated the following way. This empirical result shows that increasing sheet metal thickness from 0.8 mm to 4.5 mm the spring back is reduced 16% and 20.35%. When increasing of sheet metal strength spring back increases because spring back of the sheet should depend on the yield strength of the material. As the materials yield strength increase the spring back after un-loading condition also increases. The effects of material type as shows that, using Aluminium sheet metal instead of high strength sheet metals spring back is reduced by 56.%. Also, for decreasing of the tool radius leads reducing spring back. Spring back of deferential die 12% lower than edge bending die. In addition to these, if increasing the friction coefficient from 0.01 to 0.50 the spring back also increases by 52% because of increasing of friction force and this force generates higher amount of spring back. Thus the effects of material type, tool geometry, working condition and the thickness of sheet metal types were studied and investigated. Since, ultimately utilizing and compensation of tool is considered for prevention of spring back and optimizing of sheet metal bending process as well.

Approaches for springback reduction when forming ultra high-strength sheet metals

Nowadays, the automotive industry is challenged constantly by increasing environmental regulations and the continuous enhancement of standards with regard to passenger's safety (NCAP, Part 1). In order to fulfil the aforementioned requirements, the use of ultra high-strength steels in research and industrial applications is of high interest. When forming such materials, the main problem results from the large amount of springback which occurs after the release of the part. This paper shows the applicability of several approaches for the reduction of springback amount by forming of one hat channel shaped component. A novel approach for springack reduction which is based on forming with an alternating blank draw-in is presented as well. In this investigation an ultra high-strength steel of the grade DP 980 was used. The part's measurements were taken at significant cross-sections in order to provide a qualitative comparison between the reference geometry and the part's released shape. The obtained results were analysed and used in order to quantify the success of particular approaches for springback reduction. When taking a curved hat channel shaped component as an example, the results achieved in the investigations showed that it is possible to reduce part shape deviations significantly when using DP 980 as workpiece material.

Hot single-point incremental forming assisted by induction heating

High-strength sheet metal materials are hardly formed by conventional single-point incremental forming; the formed parts produced using this process exhibit decreased geometrical accuracy and low formability values. In the present study, a new process is investigated for hot single-point incremental forming. The method was simplified by combining incremental sheet metal forming on the front side of the sheet and induction heating on the backside of the sheet. The process setup was based on elementary tooling; it utilizes synchronized movement of the tool with the inductor. The locally heated zone allows smooth forming of advanced high-strength steels. The appropriate heating rate was controlled by optimizing the correct relationships between the process variables by using DC04 as a reference material for comparison. The process variables affecting the operation have been investigated and are the step size, induction power, and feed rate. This method has been tested and successfully improved. The results show that the formability improved and that the residual stress of DP 980 steel decreased. On the other hand, it was possible to diminish the springback and to increase the geometrical accuracy for 22MnB5 steel. Furthermore, a major reduction of the forming forces was noticed while incremental forming was performed and supported by induction heating. The significant results demonstrated a method to solve the forming problems of high-strength metals by induction heating at forming.