Goal driven optimization of process parameters for maximum efficiency in laser bending of advanced high strength steels

Laser forming or bending is fast becoming an attractive option for the forming of advanced high strength steels (AHSS), due primarily to the reduced formability of AHSS when compared with conventional steels in traditional contact-based forming processes. An inherently iterative process, laser forming must be optimized for efficiency in order to compete with contact based forming processes; as such, a robust and accurate method of optimal process parameter prediction is required. In this paper, goal driven optimization is conducted, utilizing numerical simulations as the basis for the prediction of optimal process parameters for the laser bending of DP 1000 steel. A key consideration of the optimization process is the requirement for minimal microstructural transformation in automotive grade high strength steels such as DP 1000

Geometrical Variation from Selective Laser Heat Treatment of Boron Steels

Selective laser heat treatment is used to enhance material properties in high strength steels and finds wide range of applications in the automotive industry. However, the manufactured components also become sensitive to variation affecting functionality, esthetics, and performance of the final product. In this paper, selective laser heat treatment of boron steels is analyzed with emphasis on geometrical variation. Different manufacturing strategies are tested by varying heating direction sequence and heat treatment pattern and their influence on springback is investigated. The results indicate their significant contribution to geometrical variation and the need to consider them in various stages of the geometry assurance process

Process planning and metallurgical issues for laser asisted spin froming of dual phase automotive steel

Postprint (published version

Development of a novel Fast-Warm stamping (FWS) technology for manufacturing high-strength steel components

Hot and warm stamping are preferable sheet metal forming technologies used in manufacturing high-strength parts with the twofold objectives of reducing fuel consumption and improving automotive crashworthiness. Great efforts have been made to improve the production rate in these processes and it is difficult to further improve productivity. Therefore, the development of new forming technologies may be an alternative solution to form high-strength steels into complex shapes whilst reducing the cycle time. The present work aims to develop a novel lightweight forming technology, namely fast-warm stamping (FWS) technique, to manufacture high-strength steel components with the desired properties. The concept of this process is to utilise ultra-fast heating of a steel blank to an appropriate temperature, whilst minimising the major negative changes to microstructure which are detrimental to the post-form strength. Mechanical properties such as ductility and post-form strength (PFS) of the MS-W900Y1180T (MS1180) steel were examined via uniaxial tensile tests at various temperatures (25–500°C) and strain rates (0.01–5/s). Special attention has been afforded to the effect of heating rate on thermo-mechanical properties and microstructure of the MS1180 steel with different heating rates. The results suggest that the ductility and post-form hardness of the MS1180 steel were simultaneously improved by 25.7% and 5%, with an increase in heating rate from 1 to 150°C/s. The increased hardness is attributed to the finer precipitated carbides and lower recovery at fast heating rate conditions, which was validated by microstructural observations. The validation of the FWS technology was conducted by forming U-shaped components through a dedicated pilot production line caller Uni-form. The fast-warm stamped components exhibited over 92% mechanical strength of the original as-received material consisting of 1140MPa post-form strength and 370HV hardness. The overall manufacturing cycle time in the FWS process was within 10 seconds. Springback of the formed parts under FWS conditions IV was successfully characterized at various temperatures and forming speeds. Close agreements were achieved between the experimental and simulated results for temperature, thickness distribution and springback prediction of the formed parts which validated the accuracy of the developed finite element (FE) model. FWS technology is a promising solution to manufacture components with desirable mechanical properties and dimensional accuracy. In this work, a feasibility study of the FWS technology was extended from martensitic steels to 60Si2Mn spring steel by producing commercialized disc springs. A separate forming tool set with a replaceable forming surface was developed to reduce manufacturing cost. Experimental results showed that a disc spring was successfully formed using the proposed forming process with the required dimensional precision, post-form strength and surface roughness. This forming technique has shown to enable a tremendous reduction of overall cycle time from 30 minutes to less than 20 seconds and subsequent productivity improvement for a mass-production setting.Open Acces

Modelling of phase transformation in hot stamping of boron steel

Knowledge of phase transformations in a hot stamping and cold die quenching process (HSCDQ) is critical for determining physical and mechanical properties of formed parts. Currently, no modelling technique is available to describe the entire process. The research work described in this thesis deals with the modelling of phase transformation in HSCDQ of boron steel, providing a scientific understanding of the process. Material models in a form of unified constitutive equations are presented. Heat treatment tests were performed to study the austenitization of boron steel. Strain-temperature curves, measured using a dilatometer, were used to analyse the evolution of austenite. It was found that the evolution of austenite is controlled by: diffusion coefficient, temperature, heating rate and current volume proportion of austenite. An austenitization model is proposed to describe the relationship between time, temperature, heating rate and austenitization, in continuous heating processes. It can predict the start and completion temperatures, evolution of strain and the amount of austenite during austenitization. Bainite transformation with strain effect was studied by introducing pre-deformation in the austenite state. The start and finish temperatures of bainite transformation at different cooling rates were measured from strain-temperature curves, obtained using a dilatometer. It was found that pre-deformation promotes bainite transformation. A bainite transformation model is proposed to describe the effects of strain and strain rate, of pre-deformation, on the evolution of bainite transformation. An energy factor, as a function of normalised dislocation density, is introduced into the model to rationalise the strain effect. Viscoplastic behaviour of boron steel was studied by analyzing stress-strain curves obtained from uni-axial tensile tests. A viscoplastic-damage model has been developed to describe the evolution of plastic strain, isotropic hardening, normalised dislocation density and damage factor of the steel, when forming in a temperature range of 600°C to 800°C. Formability tests were conducted and the results were used to validate the viscoplastic-damage model and bainite transformation model. Finite element analysis was carried out to simulate the formability tests using the commercial software, ABAQUS. The material models were integrated with ABAQUS using VUMAT. A good agreement was obtained between the experimental and FE results for: deformation degree, thickness distribution, and microstructural evolution

Laser Welding of Medium-Manganese Steel

Medium-Manganese (MMn) third generation advanced high strength steel (AHSS) was joined using laser welding. The effects of rapid heating and cooling thermal cycles imposed by laser welding on MMn steel was investigated. Microhardness profiles of large heat input diode laser bead-on-plate (BoP) welds found that the steel was not susceptible to heat affected zone (HAZ) softening. The peak temperature above the upper critical temperature (A3) was determined to have a significant effect on the microstructure and morphology of austenite in the HAZ. The solidification mode of the diode laser fusion zone (FZ) was primarily columnar dendritic in nature and microsegregation of Mn to the inter-dendritic spaces was observed. Higher energy density fiber laser welding was used to produce laser welded blanks (LWB) containing MMn steel, high strength low alloy (HSLA) and dual-phase (DP) steel. Columnar dendritic solidification was also observed in fiber laser welds of MMn steel. Dissimilar welds containing HSLA and DP980 were found to produce a martensitic FZ with a fraction of stable austenite. HAZ softening was also absent from microhardness profiles of fiber laser welded MMn steel due to the absence of pre-existing martensite and austenite grain growth. Sub-sized similar MMn steel laser welded tensile blanks were observed to exhibit high joint efficiency with respect to the BM. Tensile testing conducted on dissimilar blanks of HSLA and DP980 were observed to fracture in their respective BM due to a difference in yield and ultimate tensile strength. Formability of similar MMn steel LWB was limited, but welding to HSLA or DP980 improved the LWB formability. LWBs of MMn steel were investigated for potential as a new press hardened steel (PHS) chemistry. Standard size tensile geometries of MMn steel LWB showed that the FZ was sensitive to loading conditions where large strains begin to accumulate. Heat treating and quenching a LWB at an inter-critical annealing temperature showed that the austenite reverse transformation can occur in the laser weld FZ. Laser weld joint ductility was determined to significantly improve by heat treating at a lower temperature (700 °C) and a shorter time (3-4 minutes) compared to conventional boron press hardened steels

Processing-structure-mechanical property relationships in high carbon medium manganese steels with austenitic microstructure

A balance between strength and ductility has been one of the most important considerations in the steel industry. Austenitic steel or multi-phase steel with retained austenite has plasticity-enhancing mechanisms, which can make it achieve high strength and good formability. Due to the occurrence of twinning-based mechanisms in high Mn steels, they have improved strength without sacrificing ductility. However, high Mn steels with extraordinary mechanical properties has not been used in mass production because of its high material cost together with welding problems and so on. As a consequence, many researchers have attempted to decrease the Mn concentration of high Mn twinning-induced plasticity steels without significant sacrifice of the mechanical properties. In the present work, a novel medium Mn steel with high C is designed with the aim of obtaining comparable mechanical properties as high Mn TWIP steel. In addition to Mn, C is also common effective austenite stabilizing element. C and Mn both increase the SFE of austenite. It should be possible to substitute at least some of the Mn in high Mn steels with C and still retain the TWIP effect. If the reduction in Mn content is not compensated for by the addition of other alloying elements, the microstructure will additionally contain some ferrite or martensite. The problem with C concentration is that it will result in the formation of carbide during the cooling process. As long as the carbide formation is suppressed, the formation of ferrite/martensite in medium Mn steels can be inhibited by an increase in the C concentration. In such cases, a soft and formable austenitic microstructure can be achieved by quenching from high austenitization temperatures to retain austenite with appropriate mechanical stability. The precipitation and dissolution of cementite in austenitic medium Mn high C steels capable of deformation-induced twinning were analyzed based on the associated length changes. Al addition was found to significantly retard the kinetics of cementite precipitation, indicating its usefulness in the design of cementite-free austenitic medium Mn steels with high C concentrations. Furthermore, Al addition changes the morphology of intragranular cementite from plate-shaped to equiaxed. The tensile properties of alloy were also examined in the present study. The present contribution discusses the mechanical properties of a bulk medium Mn high C steel with special alloying additions to oppose the precipitation of cementite. In particular, it aims to justify the mechanical properties based on crack nucleation and growth mechanisms. The reported mechanical properties enable a comparison with those of the well-known high Mn and Hadfield steels

Acoustics and manufacture of Caribbean steelpans

The Caribbean steelpan is a pitched percussion instrument that originated in Trinidad and Tobago during the Second World War. Despite several research initiatives to improve the making of this relatively new instrument, several areas remain unaddressed. This thesis presents new approaches to help improve the making of the instrument. These approaches are situated in the production, vibration and material aspect of the steelpan. A novel sheet forming technology termed Incremental Sheet Forming (ISF) is applied to the production of miniature steelpan dishes. The thickness distribution in the wall of the ISF dishes is compared to the wall thickness distribution in a traditionally formed steelpan dish and a wheeled dish. Unlike traditional forming and wheeling, ISF produces stretching in only a portion of the walls of the formed dishes. Multi-pass ISF is used to extend the stretched zone but this extension is minimal. A break even analysis is also applied to investigate the fiscal viability of ISF application to the production of miniature and full size steelpan dishes. The application of ISF to steelpan making is found to be commercially profitable but could be jeopardised by the tuning stage of the steelpan making process. A preliminary study on the effect of impact on tone stability is conducted on a pair of notes on a full size steelpan and detuning is found more likely to occur by repeated impact of the note at its centre. Mode confinement in test-pans is also investigated. ISF is used to produce miniature test-pans with test-notes that are geometrically identical to notes on full size pans. It is possible to confine modes by varying the curvature of the bowl surrounding the test-note. The number of localised modes in the test-note increases as the radius of curvature of the surrounding bowl increases. The natural frequency of the first confined mode in the test-notes is sensitive to material springback in ISF and the mechanism of confinement appears to be due to the change in geometry that occurs between the flat test-note region and the bowl wall. This control of mode confinement may find use in future efforts to completely or partially automate the steelpan making process. Material damping and mechanical properties in low-carbon steel used to produce steelpans are researched. Damping and mechanical properties are extracted from low-carbon steel that is subjected to identical stages to the steelpan production process. Material damping trends suggest that an annealing temperature between 300°C and 400°C would be appropriate for the heat treatment of steelpans. Air-cooled and water-quenched low-carbon specimens exhibit comparable damping trends. Hardness increases in cold formed low-carbon specimens is attributed to strain hardening and not strain ageing. Investigation of damping trends and mechanical properties in ultralow bake-hardenable and interstitial-free steels reveals that a wider range of low-carbon steels maybe suitable for steelpan making.This work was supported by the University of Trinidad and Tobag

Towards Geometry Assurance of Laser Processed Sheet Metal Components

Stricter emission norms have propelled the automotive industry towards lightweight transportation solutions to reduce the environmental impact. The industry has adapted to cost effective lightweight sheet metal solutions to control the vehicle weight as well as maintain the safety standards of the vehicle. Also, the industry continues to explore for innovative manufacturing techniques to achieve further vehicle weight reduction. Novel laser processing techniques for sheet metals have slowly gained prominence in the automotive industry. Specifically, the selective laser heat treatment process has gained interest for its ability to locally modify material properties to enhance formability and strength. Consecutively, it widens the horizon for lightweight design. However, some challenges remain in order to utilize this process to its fullest. Geometrical variation related effects from local heating is an area of concern. Also, integrating them into the virtual product development setup is necessary to enable accurate decision making.In every manufacturing process the component produced varies from the desired values. This is further affected by variation in fixtures and subsequent assembly processes. It affects aesthetical characteristics and functionality of the product in its operating environment. It is expensive to either make adjustments in the final stages of the production process or to totally eliminate the variation sources. This could be overcome by making the design insensitive to the effects of such variation through the concept of robust design. Implementing the robust design concept requires adequate understanding of geometrical variation related effects due to local heating, an area which remains scarce. As a result, this phenomena is unaccounted for in the methods and tools that are in practice today.The goal of this research is to understand the phenomena and gain sufficient knowledge to develop methods and tools for geometrical variation simulation that considers selective laser heat treatment effects. In this thesis, boron steels are the material in focus. Through literature studies, the geometrical variation influencing factors are identified and further investigated through experimental studies for deeper understanding. Sufficient knowledge on the influencing factors are developed which forms the major outcome of this thesis. This lays the foundation for developing methods and tools to perform accurate robust design assessment for selective laser heat treated sheet metal components