Steel Sheets Of Different Thicknesses Research Articles

Effect of medium-speed forming on formability of tailor welded blanks

Despite the widespread use of tailor welded blanks (TWB) in manufacturing auto-body panels and considering that different forming processes are performed at various speeds, it is necessary to examine the forming speed consequences. So, this study aims to investigate the forming speed effect on the formability of TWB and its constituent sheets. TWB consisting of St14 steel sheets of different thicknesses were welded using a fiber laser. Uniaxial tensile tests were conducted at various strain rates, and formability experiments were carried out at the quasi-static and medium-speed forming to construct forming limit curves (FLC). Tensile test results showed that some strain-hardening indicators decreased by strain rate increasing. The flow stress of base metals at different strain rates was successfully predicted by the constitutive model considering variable parameters. At medium-speed forming of base metals, the peak of major strain and fracture moved to the dome center. But, the strain concentration of most TWB samples at two forming speeds occurred in the thin sheet near the dome center. Also, the limiting dome height (LDH) at medium-speed forming decreased. Medium-speed FLC of base metals shifted downward and to the more positive minor strain compared to quasi-static FLC. Medium-speed FLC of TWB was lower than quasi-static FLC, especially on the tension-compression side. The reduction of medium-speed formability was attributed to the strain-hardening lowering, friction reduction, and inertial effects due to increased forming speed.

Method for determining of the optimal thickness of sheets of magnetic cores of electrical machines by the wattmeter method

The problem of reducing magnetic losses (no-load losses) in the steel of the magnetic cores of electrical machines is investigated. The tasc of determining the optimal thickness of steel sheets of the magnetic circuit of an electric machine is considered. The criterion for optimality is the minimum power of magnetic losses in steel. Currently, this problem does not have an exact solution due to the fact that the exact ratio of the hysteresis and eddy current components of magnetic losses in steel is unknown. Analyzed the power of magnetic losses in modern electrical machines and devices, depending on the thickness of the sheets of electrical steel. A method is proposed for determining the optimal thickness of steel sheets of the magnetic circuit of an electric machine based on the wattmeter method. In the course of the experiment, two identical magnetic circuits were selected from steel sheets of different thicknesses, for which the losses in steel were measured at different frequencies of magnetization reversal and the optimal thickness of the sheets was calculated. The proposed formula for calculating the thickness of the sheets is valid for both isotropic and anisotropic steel. The proposed technique can be used for both transformers and electric motors and generators.

Selecting Laser Cutting Modes for Engineering Steel Sheets Aiming at Provision of the Required Properties of Surface Quality

The roughness RZ, the duration of the area of laser thermal impact (ALTI) (L), and the inclination of cut wall (α) after various modes of laser cutting of steel sheets of different thickness of grades 40Kh, 20Kh13, 65G, 09G2S, 60S2KhA, and 30KhGSA have been determined. It has been established that, despite different chemical compositions of the steels, the microstructure of ALTI in surface areas after laser cutting (LC) is similar: a white layer with higher hardness, a layer with martensite structure and residual austenite, and a transient area between ALTI and initial structure not affected by LC. Mathematical models have been developed describing the influence of chemical composition, LC parameters, and sheet thickness on RZ, L, and α. The procedure of selection of a rational LC mode is proposed and exemplified by multiparametric optimization making it possible to determine LC modes to provide the best quality of cut surface in the sheets of different thickness and chemical composition.

Investigation of Welded Joints Made of Construction Steel by the Barkhausen Noise Method

Steel weld seams are characterized by heterogeneity of their microstructure. Microstructure affects the nature of the distribution, sign and magnitude of residual stresses. In combination with unfavorable factors (low temperature, metal hypoductility and an unsuccessful joint form) residual stresses lead to a decrease of load carrying capacity of a whole structure. In a weld seam residual stresses are distributed in a complex way and can affect the build quality of heavy section welded structures. Monitoring of residual stresses remains a big problem. Residual stresses in welds are often evaluated only by modeling. Unfortunately, all mathematical models describe the stress state of the welded material with low accuracy. Simple quality control, the results of which can be easy interpreted, is necessary. Welded joints made by manual arc welding and by automatic submerged arc welding were investigated. Butt seams of steel sheets of different thickness have been welded. Steel was low-carbon and low-alloyed. It is often used in welded structures for various purposes, including construction, and for pipelines manufacture. The temperature range of welded structures operation is very large – from-70 to 450С. The authors studied the structure of butt weld seams by the Barkhausen noise method, which is interesting as it represents an alternative to the known methods, which characterizes the structure and stress state of material. The relationship between the weld microstructure and magnetic noise is shown. Studies have allowed us to establish the relationship between the structure and magnetic properties and to evaluate the feasibility of applying the Barkhausen noise method to welded structures.

Examining Dent Formation Caused by Hailstone Impact

Hailstorms pose significant risk for exposed building cladding materials. Steel sheeting is the most important cladding material used. The understanding of steel sheets behavior under hail impact loading is not sufficient for the manufacturing of hail-resistant sheets. With the purpose-built equipment, artificial hailstones of different sizes were launched to impact at steel sheets of different thicknesses and yield stresses as targets. A theoretical approach for the problem of predicting the dent size due to hailstone impact was developed and compared to the test results. The expressions developed in the theory can predict the dent depth before the impact, assuming the ratio between the dent depth and dent diameter is constant. The expression is not able to predict the depth of dents smaller than 0.75 mm and cannot predict whether the denting will occur or not. All hailstone sizes lead to visible dent on steel sheet of thicknesses 0.35 mm, 0.42 mm, and 0.55 mm. Visible denting was also obtained for the 0.75 mm steel samples with 45 mm and 55 mm hailstones; however, no denting occurred using 40 mm hailstones. It was found that the dent depth was inversely proportional with thickness and yield stress, while the dent diameter was found to be proportional to yield stress. As the yield stress of the steel sheet increased, the dent depth decreased for G300 and G550 steel. The dent diameter however increased as the yield stress increased. When the artificial hailstone shatters on impact, significant energy is lost and less energy is available to cause plastic deformation of the impacted material.

Residual stress profiles in riveted joints of steel sheets

The residual stress profiles in two different self-piercing riveted (SPR) joints were characterised using the neutron diffraction technique. The joints were produced using semi-tubular steel rivets and carbon steel sheets of different thicknesses and hardnesses. The residual stress in the sheet material inside the bore of the rivet was compressive at the centre and the stress became tensile away from the centre. The stresses found in the semi-tubular rivet were compressive, with a lower magnitude in the rivet head compared with those in the rivet leg. For the SPR joints examined, the compressive residual stress in the rivet leg was greater for the thin joint than the thick joint, and this was due to the higher force gradient encountered during the rivet flaring stage. The originality of this work was to relate the residual stress profile to the physical event that occurred during the SPR process.

Finite Element Simulation of the Drawability of Tailor-Welded Blank

Tailored welded blank (TWB) consists of steel sheets of different thicknesses and strength which welded into one sheet to reduce weight and production costs, to increase dimensional accuracy and strength. Since a tailored blank is composed of different sheets of metals having different thicknesses and properties, the forming of tailored blanks then creates a lot of technical problems especially in the scheme of deformation. The objective of this work was to evaluate the drawability of tailored blanks. In order to assess the forming behavior of the tailor-welded blanks under the influence of weld orientations, a finite element and experimental methods were conducted. Three welded specimens with weld lines oriented at an angle of 0°, 90° and 45° with respect to the direction of load were investigated. The finite element result showed in a good agreement with the experimental result. The result of the experiment showed that a welded part was characterized by a higher strength and lower plasticity compared to those of the base material. Hence, the weld influences the strain distribution of the tailor-welded specimens. This phenomenon depends strongly on the direction of the weld against the direction of tensile load. The lowest strains occur in the specimen with a longitudinal weld.

Miniature supercapacitors based on nanocomposite thin films

This paper reports on the development of a miniature supercapacitor for portable applications such as mobile and wearable energy storage. In a primary embodiment, the developed supercapacitor consists of two flexible electrodes fabricated on thin metal base substrates. The electrodes sandwich a solid polymeric separator doped with an appropriate ionic material acting as electrolyte. Different electrode materials including carbon nanotubes (CNTs), silver nano-particulate ink, and nanocomposite of CNTs and polyvinyl alcohol (PVA) were investigated in fabricating the flexible electrodes. Ultrathin and flexible stainless steel sheets of different thicknesses ([email protected], [email protected]) were evaluated as base substrate material. The integrated separator and electrolyte layer was made of polyvinyl alcohol doped with phosphoric acid. The evaluation of different electrode and substrate materials allowed for selection of a combination of materials that yielded optimal supercapacitor performance. Devices with optimal combination of materials exhibited planar capacitance value (>[email protected]/cm^2) significantly higher than the range reported in similar devices. Our investigation of electrode and substrate materials clearly demonstrated that planar capacitance is influenced by factors such as surface area of the electrode and its materials, and the thickness of the base substrate that functions as a current collector.

Comparative Study by Numerical Analysis on the Formability of Deep-Drawn Tailor-Welded Blanks

The realising of complex parts by welding together and then deep-drawing metal sheets made of different steel types or even different metal types can have many advantages, such as decreasing the part’s weight, reducing the manufacturing costs and an increased dimensional precision. In order to benefit from these results, a detailed knowledge of the metal sheets’ behaviour during deep-drawing is necessary. The current paper aims to compare, using numerical simulations based on the finite element method, the deep-drawing behaviour of parts made of steel sheets of different thicknesses welded together in three different manners and also of a part made of a monoblock blank.

Investigations on Incremental Forming of Low Carbon Steel Sheets

Low carbon steel sheets are used invariably in automobile body panels and construction structural applications. Conventional forming techniques require forming dies and heavy duty presses to make required shapes, which are costly in manufacturing. Incremental forming is a recently developing die less sheet metal part production technique in which the necessary part is obtained by gradually tracing its contours on a sheet of required thickness using stepwise tool indents. In this investigation, Low carbon steel sheets of different thicknesses were incrementally formed using a hemispherical tool under varying step depths. The final thickness of the sheet, forming angle, roughness and microhardness of the formed shapes were characterized. The microstructure of the formed materials was studied using optical microscope.

Effect of microstructure on void nucleation and coalescence during forming of three different HSLA steel sheets under different stress conditions

The microstructure of any metal influences the void formation statistically. The geometrical nature and the distribution of the second phase particles viz. carbides, sulphides, etc. and inhomogeneities present in the microstructure play a vital role in determining mode of void formation during fracture. The analysis of microstructure and its impact on void nucleation, their growth, and coalescence during the fracture of three different HSLA steel sheets of different thicknesses is presented in this work. The analysis shows that the criterion of void nucleation and coalescence depends on geometrical shape factor or aspect ratio ( L/ W) of both ferrite grains and second phase particles.

Analysis of fracture limit curves and void coalescence in high strength interstitial free steel sheets formed under different stress conditions

Void formation, which is a statistical event, depends on inhomogeneities present in the microstructure. The analysis on void nucleation, their growth and coalescence during the fracture of high strength interstitial free steel sheets of different thicknesses is presented in this article. The analysis shows that the criterion of void coalescence depends on the d-factor, which is the ratio of relative spacing of the ligaments (δd) present between the two consecutive voids to the radius of the voids. The computation of hydrostatic stress (σm), the dominant factor in depicting the evolution of void nucleation, growth and coalescence and the dimensional analysis of three different types of voids namely oblate, prolate and spherical type, have been carried out. The ratio of the length to the width (L/W) of the oblate or prolate voids at fracture is correlated with the mechanical properties, microstructure, strains at fracture, Mohr’s circle shear strains and Triaxiality factors. The Lode angle (θ) is determined and correlated with the stress triaxiality factor (T), ratio of mean stress (σm) to effective stress (σe). In addition, the Void area fraction (Va), which is the ratio of void area to the representative area, is determined and correlated with the strain triaxiality factor (To).

Forming, fracture and wrinkling limit diagram for if steel sheets of different thickness

In this work, forming limit, fracture limit and wrinkling limit diagrams have been experimentally evaluated for various interstitial-free (IF) steel sheets of different thicknesses namely 0.6 mm, 0.9 mm, 1.2 mm and 1.6 mm, 0.85 mm noncoated and 0.85 mm coated. With the help of the above diagrams the suitability of the different steel sheets for press forming operations under different strain combinations were examined. The microstructural aspects, tensile properties and formability parameters were studied. Forming, fracture and wrinkling limit diagrams were evaluated for the above sheet metals and they were compared. The tensile properties and formability parameters were correlated with the forming, fracture and wrinkling limit diagrams.

Study on wrinkling limit of interstitial free steel sheets of different thickness when drawn through Conical and Tractrix dies

This paper deals with the wrinkling limit diagrams of cold rolled and close annealed of different grades of IF Steel sheets, which are used in Automotive Industries. The study pertains to deep drawing into cylindrical cups through Conical and Tractrix dies using a flat bottom punch. It is well established that, the use of either a Conical die or a Tractrix die can enhance the limiting draw ratio compared with that obtainable in a conventional drawing operation. When a Conical or a Tractrix die is employed the need for a hold down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early stages of a drawing process in which thin sheet blanks are used. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (d ε r/d ε θ ) reaches a critical value during the drawing process. This value, which could be, determined experimentally over which the wrinkling takes place has been shown in the form of wrinkling limit diagrams for the above different grades of IF Steel sheet. An attempt is also made to rank the IF Steel sheet grade that appears to suppress the wrinkling based on results obtained in the form of wrinkling limit diagrams, established for the above different IF Steel sheet grades, in terms of the strain increments ratio and the stress ratio parameters. Further, it is noticed that as a thickness of IF Steel sheet increases the safe region also increases. This means that higher is the thickness better is the resistance against wrinkling. Drawing through the Tractrix die shows better resistance against wrinkling comparing with the Conical die.

Evaluation of limiting strains and strain distribution for interstitial free steel sheets while forming under different strain conditions

In this work, forming limit diagrams (FLDs) were experimentally evaluated for the interstitial free steel sheets of different thickness namely 0.6, 0.9, 1.2 and 1.6 mm. The limiting strains for these sheets under different strain conditions were studied. The forming limit diagrams for the above sheet metals were compared. The microstructural aspects, tensile properties and formability parameters were experimentally determined and studied. The tensile properties and formability parameters were correlated with FLDs. The longitudinal major strain and minor strain distribution profiles were plotted from the forming experiment. The thickness strain distribution profiles in longitudinal, transverse and diagonal directions were also plotted and detailed analysis of strain distribution profiles was attempted. The fractography of the formed samples was taken using scanning electron microscope (SEM). Using SEM fractography, the fracture behaviour and formability were analyzed in three different strain conditions namely tension–tension, plane strain and tension–compression strain conditions. From the fractography and experimental data, the average size of voids present in the fractured surface, strain ratios and the shear strain developed in the sheet metal at fracture were determined. An attempt was made to relate the void size to the shear strain developed in the sheet metal at fracture. When comparing all the sheets, the limiting strains are higher for the sheet having 1.6 mm thickness.

Laser weldability of high-strength steel sheets in fabrication of tailor welded blanks

Tailor welded blanks (TWBs) – pressing blanks arranged to combine, by welding, multiple steel sheets of different thicknesses and properties, and which may or may not have been surface-treated – are now extensively used in the automotive industry for fabrication of car body panels. TWBs were initially used at mass production level in Germany in the early 1980s and have since seen rapidly increasing engineering applications worldwide.1, 2 TWBs were originally viewed as a way of boosting material yields through reuse of pressed-off edge material as well as of cutting manufacturing costs by reducing the number of component spot welds, number of welding operations, and number of pressing dies through integral forming of multiple pressings after pre-welding of blanks from conventional spot welding production.

Development of an Electromagnetic Acoustic Transducer with Multi-Wavelength for Lamb Wave

Lamb waves are normally utilized for inspecting thin metal sheets, and a wheel type probe with piezoelectric oscillators is used as the sensor, although it has a few serious disadvantages such as a dramatic change in sensitivity. We then studied a useful electromagnetic acoustic transducer (EMAT) without a couplant. The trial EMAT consists of a meandering coil with a narrow distance between the intervals which can generate Lamb waves of variable wavelengths corresponding to the frequency range from approximately 150 kHz to 4.0 MHz. This transducer, for example, can select Lamb waves with optimum wavelengths on the factory line that produces thin sheets with variable thicknesses. The described EMAT can be used to inspect steel sheets of different thicknesses. It is also shown that the S0-mode Lamb wave with the longer wavelength is the most effective for a thick sheet (up to 6 mm).

‘Tailored blanks’ for automotive components. Evaluation of mechanical and metallurgical properties and corrosion resistance of laser-welded joints

Summary The main aim of this research is to investigate the suitability of ‘tailored blanks’, fabricated with laser welding technology, prior to forming. It is also a review of their fatigue and corrosion behaviour. During the experiments, laser-welded joints were performed using different steel/thickness combinations, most commonly used in today's automobile industry. Joints were assessed by metallographic examination, as well as by pressing, fatigue and corrosion tests. In order to evaluate the suitability of ‘tailored blank’ technology on an actual automotive component, a suspension mounting was produced by joining, using both laser and roller resistance welding, two FePO4 bare steel sheets of different thickness (1.5 and 2 mm). Fatigue strength of the lighter component has generally produced comparable results to the standard component made in a single thickness (2 mm). This confirms the suitability of ‘tailored blank’ technology to save body weight and optimise material utilisation.

Texture, Yield Locus and Plastic Anisotropy in Steel Sheets

A comprehensive analysis of texture in the commercially produced low carbon aluminium-killed steel sheets of different thicknesses, by orientation distribution functions (ODF) technique, has been carried out. To characterise the plastic anisotropy behaviour of these sheets, studies about the evolution of Lankford parameter and yield locus prediction are performed, on the basis of Taylor theory using the {hkl} pencil glide {112} glide and {110} glide. The predicted r-values in the sheet plane are compared with the experimentally determined r-values.

Effect of grain size on formation and development of brittle cracks in steel sheets of different thicknesses

Effect of grain size on formation and development of brittle cracks in steel sheets of different thicknesses