Evaluation of Surface and Dimensional Phenomena in Punching of DP800 Advanced Strength Steel Utilising The Cockcroft-Latham Damage Model

The use of Advanced High Strength Steels (AHSS) in the car industry is permitting to improve vehicle safety while reducing structural weight. This is possible thanks to their very high mechanical properties. Nowadays, steels with tensile strength up to 800MPa are widely spread in industrial applications. However the use of AHSS for cold stamping with tensile strength between 1000MPa and 1500MPa is not so extended. Furthermore, when used, these steels are usually joined to other parts by resistance spot welding, the most established joining process in car industry. The work presented is focused on laser welding of dual phase and martensitic steels with tensile strength between 600MPa and 1500MPa. The aim of this work was to acquire the knowledge needed when designing new components of AHSS including laser seams. The geometries studied were butt joints, overlap joints and TWB joints with sheet thickness between 1mm and 2mm. Aspects such as microstructure, hardness, mechanical strength, fatigue strength and forming behaviour will be presented.The use of Advanced High Strength Steels (AHSS) in the car industry is permitting to improve vehicle safety while reducing structural weight. This is possible thanks to their very high mechanical properties. Nowadays, steels with tensile strength up to 800MPa are widely spread in industrial applications. However the use of AHSS for cold stamping with tensile strength between 1000MPa and 1500MPa is not so extended. Furthermore, when used, these steels are usually joined to other parts by resistance spot welding, the most established joining process in car industry. The work presented is focused on laser welding of dual phase and martensitic steels with tensile strength between 600MPa and 1500MPa. The aim of this work was to acquire the knowledge needed when designing new components of AHSS including laser seams. The geometries studied were butt joints, overlap joints and TWB joints with sheet thickness between 1mm and 2mm. Aspects such as microstructure, hardness, mechanical strength, fatigue strength and ...

<div class="htmlview paragraph">Fuel economy and federal safety regulations are driving automotive companies to use Dual Phase and other Advanced High Strength Steels (AHSS) in vehicle body structures. Joining and assembly plays a crucial role in the selection of these steels. Specifications are available for the resistance spot welding (RSW) of lower strength sheet steels, covering many aspects of the welding process from the stabilization procedure to endurance testing. Currently, specifications in the automotive industry for RSW with AHSS are limited. It is well known that welding of a thickness ratio greater than 1:2 poses a challenge. To utilize thinner gauge AHSS panels on body-in-white, welding schedules to join the thin to thick sheet steel stack-up are needed. Most of the existing published work was conducted on uncoated sheets and welded to the same thickness. This project was initiated to understand the RSW of a three-metal stack-up using AHSS - Dual Phase 600 (DP600) steel with a hot dip galvanized coating. As the use of welded joints involving thin to thick AHSS sheets become more common for vehicle body structures, the possibility of an extreme case of sheet thickness differential involving 1.9 and 0.8 mm gauges, both galvanized, is more likely. This study, through a Design of Experiments (DOE), quantified the effects of factors that included welding equipment (AC vs. MFDC), electrode shape (dome, 45º truncated and ISO 30º truncated), hold time, pulsing and weld current on joint strength (lap-shear and coach peel) for a three-metal thickness stack-up (0.8/1.9/1.9 mm). During the study a modified electrode stabilization procedure was developed and the hold-time sensitivity behavior at optimum button size was investigated. The current range, lap-shear tensile load, coach peel load and electrode life values were compared for each electrode type. The study was successful in identifying that the dome shaped electrode stabilizes faster, produces more consistent welds, and consumes less current, primarily due to its higher current density capability. The amount of weld current needed by the truncated electrodes was influenced by the electrode face angle. The dome shaped electrode produced a higher current range than the truncated cone electrodes. The dome electrode always produced the largest current range with the AC power source; no effect of the MFDC power source on the current range was observed. The number of pulses, or cool time between pulses, did not influence the current range. No hold time sensitivity was seen during the testing at 5 and 90 cycles hold time. A minimum of 2000 lbf (∼907 kgf) lap-shear tensile and 200 lbf coach peel loads were obtained with the dome electrode. Although the truncated cone electrodes produced an average load similar to the dome electrode, the results were more scattered. The endurance testing showed that the 45º truncated cone electrode provided the highest number of welds (∼1500), with the dome electrodes producing ∼1000 welds. Increasing weld current gradually and monitoring the voltage/resistance across the electrode was most effective in stabilizing the resistance spot welding electrodes. Overall, the dome electrode performed better than the truncated cone electrodes in welding a 0.8-1.9-1.9 mm DP600 HDG weld stack-up. The dome electrode is suggested for resistance spot welding of stack-ups with high thickness ratios.</div>

The automotive industry is using more and more advanced high strength steels (AHSS), aiming at reducing vehicle weight, minimizing emission, increasing fuel economy, and improving vehicle safety. This article focuses on the AHSS products recently developed at Nucor Corporation. With innovative chemistry and process designs, as well as a strictly controlled manufacturing operation carried out during the unique CSP® and down-stream processes, these newly developed steels possess finer, more uniform microstructure, with modified morphology and optimum fraction for each phase. Compared to the most commercially available AHSS, these new materials have higher strength-elongation balance, improved formability and stretch flangeability, enhanced impact toughness and crashworthiness, good coating or surface quality, as well as superior weldability and weld fatigue properties. Moreover, the property consistence of these innovative AHSS has been markedly improved.

Failure mode transition in AHSS resistance spot welds. Part I. Controlling factors

A beveled shear hole piercing process has recently been developed for advanced high strength steel (AHSS). The preliminary results have shown the new process is able to improve the quality of the sheared edge and the edge stretchability of AHSS. The goal of the current study is to optimize the beveled shearing process and identify the optimal shearing conditions for AHSS. Four different advanced high strength steels, including DP600, DP780, TRIP780, and DP980 with various thicknesses together with a conventional high strength steel, HSLA50, are selected in this study. The hole expansion test is used to evaluate the effect of shear edge conditions on the edge stretchability. The results show that an optimal selection of the die clearance and the shearing angle results in a less damaged edge, which significantly delays edge fracture in the forming process and increases the edge stretchability for AHSS. To further validate the advantages of the beveled shearing process in improving the shear edge quality of AHSS, a straight edge shearing device with the capability of adjusting the shearing variables (rake angles and die clearance) with respect to different sheet thicknesses was also developed and built. The edge stretchability of the straight edge sheared specimen was then evaluated using the sheared edge tension test. A similar trend to the beveled shear hole piercing process of AHSS is observed, and a significant improvement in the edge stretchability is also obtained with optimal shearing conditions.

The failure mechanism in stretch bending over a small die radius for Advanced High Strength Steels (AHSS), commonly referred as “shear fracture”, has rendered the Forming Limit Diagrams (FLD) fail to predict it based on the initiation of a localized neck. As shown in previous studies using a Stretch-Forming Simulator (SFS) and Bending Under Tension (BUT) test, shear fracture depends not only on the radius-to-thickness (R/T) ratio but also on the tension/stretch level applied to the sheet during bending. Although the stress-base empirical fracture limit criterion was developed for various AHSS grades, the fracture limit was not well implemented in the computer simulations to predict stretch bending fracture. In this paper, the new developed experimental analysis is conducted on the modified bending under tension test to further investigate the stretch bending fracture mechanism under the production die condition. Various AHSS grades including DP590, DP780, DP980 and DP1180 are included in the study. Based on numerous experimental results, the maximum shear stress at failure, the thinning strain and strain gradient across the die radius are obtained for all test materials. Results demonstrate that the presence of the large strain gradient is the cause for fracture in stretch bending AHSS over a small die radius. The maximum shear stress at failure and the limit thinning strain on the die radius in the stretch bending condition are determined and used as the new fracture criteria, which can be easily implemented in the computer simulations.

The automotive industry is using more and more advanced high strength steels (AHSS), aiming at reducing vehicle weight, minimizing emission, increasing fuel economy, and improving vehicle safety. This article focuses on the AHSS products recently developed at Nucor Corporation. With innovative chemistry and process designs, as well as a strictly controlled manufacturing operation carried out during the unique CSP® and down-stream processes, these newly developed steels possess finer, more uniform microstructure, with modified morphology and optimum fraction for each phase. Compared to the most commercially available AHSS, these new materials have higher strength-elongation balance, improved formability and stretch flangeability, enhanced impact toughness and crashworthiness, good coating or surface quality, as well as superior weldability and weld fatigue properties. Moreover, the property consistence of these innovative AHSS has been markedly improved.

<div class="htmlview paragraph">Rollover crash is one of the important fatal crash modes in highway accidents. To protect occupant safety, the National Highway Traffic Safety Administration (NHTSA) has proposed a higher roof strength requirement in the upcoming new federal regulation. Meanwhile fuel efficiency and environmental friendliness demands that the safety cage design should have the minimum weight while providing the sufficient roof crush strength. These requirements pose a challenge to automotive design engineers.</div> <div class="htmlview paragraph">In this paper, Advanced High Strength Steels (AHSS) are introduced as an enabler to support this challenging task. The advantages of different types of AHSS for vehicle crashworthiness are presented. The criteria to select materials to improve the roof crush performance are discussed in detail. A new steel design concept using AHSS with tensile strength ranging from 590 to 980 MPa is introduced to illustrate how steels of different strength levels were used in the design FEA simulations demonstrated that the AHSS design was capable of meeting the proposed more stringent roof crush requirement. However, a holistic approach must be taken when steels of high strength are used, including optimized use of steel strength, strength and formability compromise, part geometry modification, adopting appropriate manufacturing processes, such as roll forming, and innovative design.</div>

The applications of Advanced High-Strength Steels (AHSS) in the automotive industry are rapidly increasing due to a demand for a lightweight material that significantly reduces fuel consumption without compromising passenger safety. Automotive industries and material suppliers are expected by consumers to deliver reliable and affordable products, thus stimulating these manufacturers to research solutions to meet these customer requirements. The primary advantage of AHSS is its extremely high strength to weight ratio, an ideal material for the automotive industry. However, its low ductility is a major disadvantage, in particular, when using traditional cold forming processes such as roll forming and deep drawing process to form profiles. Consequently, AHSS parts frequently fail to form. Thereby, in order to improve quality and reliability on manufacturing AHSS products, a recently-developed incremental cold sheet metal forming technology called Chain-die Forming (CDF) is recognised as a potential solution to the forming process of AHSS. The typical CDF process is a combination of bending and roll forming processes which is equivalent to a roll with a large deforming radius, and incrementally forms the desired shape with split die and segments. This study focuses on manufacturing an AHSS top-hat section with minimum passes without geometrical or surface defects by using finite element modelling and simulations. The developed numerical simulation is employed to investigate the influences on the main control parameter of the CDF process while forming AHSS products and further develop new die-punch sets of compensation design via a numerical optimal process. In addition, the study focuses on the tool design to compensate spring-back and reduce friction between tooling and sheet-metal. This reduces the number of passes, thereby improving productivity and reducing energy consumption and material waste. This numerical study reveals that CDF forms AHSS products of complex profiles with much less residual stress, low spring back, low strain and of higher geometrical accuracy compared to other traditional manufacturing processes.

Triggered by the recent popularity of advanced high strength steels (AHSS) and aluminium alloys for weight-reduction in automotive components, industrial interest in deformation-induced ductile damage in sheet metal is increasing in the last decades. Severe deformation during forming or service triggers different damage micro-mechanisms in the multi-phase microstructures of these materials, leading often to unpredicted failures. These failures can be avoided by (a) the optimization of metal microstructures to be less susceptible to damage-induced failures, which requires experimental characterization of damage micro-mechanisms or (b) the incorporation of continuum damagemodels in forming simulations to design forming operations within safe deformation limits, which requires experimental quantification of damage accumulation. However, both strategies are hampered by the limitations of the currently available experimental diagnostics. Therefore, the aim of this work is to develop new experimental methodologies that allow for (i) characterization of damagemicro-mechanisms and (ii) accurate quantification of damage accumulation, with a focus on industry-relevant sheet metal. As a starting point, the influence of damage evolution on localization and fracture is investigated by deforming two steels of different microstructure in different strain paths. The results revealed that for microstructures with many damage mechanisms (e.g. AHSS), damage accumulation significantly affects both necking and fracture limits, verifying the strong need for thorough characterization of damage micromechanisms in different strain-paths. The analysis of these mechanisms requires the development of a miniaturized testing setup that could fit within a scanning electron microscope (SEM) to track deformation-induced microstructure evolution in real time. To this end, a miniaturized Marciniak test setup is designed, built and tested, which allows the real-time, multi-axial testing of industrial sheet metal to the point of fracture within a SEM. A major benefit of the in-situ analysis with miniaturized equipment is the possibility of obtaining evolution of local strain distribution at the microstructure level, as demonstrated in a case study that clarifies the mechanical influence of the morphology and properties of microstructural banding in steels. The effect of band continuity and hardness are elucidated, yielding a clear detrimental influence especially for hard bands with a continuous morphology. Finally, an improved experimental methodology is developed to analyse 3D features of ductile deformation, with minimum specimen preparation artifacts. For the damage quantification problem a wide variety of experimental methodologies have been proposed in the literature, without a thorough evaluation with respect to measurement accuracy, precision, practicality, etc. To determine the most suitable damage quantification strategy for continuum damage models, damage morphology-based damage quantification methodologies (the volume fraction methodology, area fraction methodology, or density measurement methodology) and material property-based damage quantification methodologies (the indentationbased methodology, modified indentation methodology and micropillar compression methodology) are comparatively analyzed. The obtained results clearly indicated that methodologies that quantify ductile damage through its morphology have limited accuracy and probe a narrow damage spectrum, revealing the need for accurate material-property based damage quantification techniques. The indentation based methodology is a widely used example of such methodologies, however, a numerical-experimental analysis revealed that it cannot be used for this purpose, as the damage-induced degradation of both hardness and modulus is masked by other deformation-induced microstructural mechanisms (e.g. grain shape change, texture development, etc.). To this end, two original mechanical property-based damage quantification methodologies are proposed in this work. A new indentation-based methodology is developed and evaluated, that eliminates the influence of the microstructural heterogeneity to properly capture the damageinduced degradation of indentation hardness and modulus. And finally, an elastic compression-based methodology is developed where the elastic damage parameter is obtained through the deformation-induced degradation of the compression modulus of electro-discharge machined micropillars. The results from these two methodologies clearly indicate that methodologies that quantify ductile damage through its influence on mechanical property (e.g. hardness, modulus) have significantly higher accuracy, and therefore more suitable for identifying damage parameters for continuum damage models.

The development of modern automotive vehicles with improved environmental, safety and vehicle performance has driven the development of new steel grades that are lighter , safer , greener and more cost effective. As a result, conventional low carbon steels and high strength steels are increasingly replaced with adv anced high strength steels (AHSS) due to their high strength and good uniform elongation. This unique combination in mechanical properties is achieved by carefully designing the microstructure by adding special alloying elements and controlled heat treatments. In automotive manufacturing processes, fusion welding is an important process employed for the joining of steel parts and components. However , the thermal cycle of a welding process destroys the carefully designed microstructure of AHSS. In order to use these materials effectively , it is necessary to have a sound understanding of the influence of weld thermal cycles and alloying additions on the evolution of microstructure in the fusion and heat affected zones. In this paper , the current understanding and recent developments in the welding of current generation advanced high strength steels for automotive applications are discussed. The paper concludes with the assessments and possible solutions to improve the weldability of advanced high strength steels for automotive applications.

Due to increasing demands to reduce C02-emission and to augment occupant’s safety new modern materials are developed ongoing. Because of relatively low production costs, high strength and simultaneously good formability the advanced high strength steels (AHSS) are applied among others for the lightweight design of body-in-white components in the automotive industry. Their already mentioned properties follow from the presence of mixed mild and hard ferrous phases. Due to this multiphase microstructure of the most AHSS steels, a complex material and damage behavior is observed during forming. The damage grows in a ductile manner during plastic flow and the cracks appear without necking. They are often characterized as the so called shear cracks. The damage predictions with standard methods like the forming limit curve (FLC) lack accuracy and reliability. These methods are based on the measurement of linear strain paths. On the other hand ductile damage models are generally used in the bulk forming and crash analysis. The goal is to prove if these models can be applied for the damage prediction in sheet metal forming and which troubles have to be overcome. This paper demonstrates the capability of the Gurson-Tvergaard-Needleman (GTN) model within commercial codes to treat industrial applications. The GTN damage model describes the existence of voids and they evolution (nucleation, growth and coalescence). After a short introduction of the model the finite element aspects of the simulative damage prediction have been investigated. Finally, the determination of the damage model parameters is discussed for a test part.

Predicting instability at die radii in advanced high strength steels

Manufacturing protocols or processing parameters in a coiling mill affect multiple desired properties of advanced high-strength steel (AHSS) coils. These properties include yield strength (YS), ultimate tensile strength (UTS), elongation percent, hardness, etc. In this work, attempts were made to maximize YS, UTS, and elongation percent for AHSS coils while determining the operating parameters that can be helpful in achieving those properties. Additionally, operating parameters were also determined for a few specific grades of AHSS steel with respect to desired properties of interest. Actual plant data from a coiling mill was analyzed through a set of statistical and artificial intelligence (AI) based algorithms. Predictive models were developed through k-Nearest Neighbor (k-NN) algorithm. Optimization of multiple properties was performed through a non-dominated sorting genetic algorithm (NSGA2). The concept of parallel coordinate chart (PCC) was used for visualization as well as identifying operational parameter that can be helpful in achieving a desired property. The research methods and findings presented in this article are of industrial significance and can be applied to other manufacturing processes.

Laser additive manufacturing (LAM) technology applied to the 316L stainless steel is attracting interest in the machining industry since it can shorten the production cycle and reduce numbers of machining steps. It still needs to be machined because of the poor surface quality, so the tool wear of post milling process after LAM needs to be studied. However, there are a few studies about the tool wear in milling of laser additive manufacturing stainless steel alloy. The aim of the paper is to evaluate the tool wear performance and surface quality when post milling the 316L stainless steel under different post milling (PM) time. The tool wear behavior was investigated using different analysis techniques. 3-D surface contour profilometer was used to measure the surface roughness and observe the morphology of small area; digital optical ultra-depth microscope analysis was carried out to evaluate tool wear and broken; the wear width of the blade was an indicator of the degree of the tool wear; different post milling methods were also compared. The obtained results demonstrate that the height of the LAM parts were connected with the heat dissipation effect; there are three stages of tool wear: initial wear stage, normal processing stage, and severe broken stage; usually the down-milling is better than up-milling at the bottom surface roughness, quality and edge morphology.