Zinc Coated Steel Sheets Research Articles

Stress and deformation of ceramic rolls to produce high quality zinc coated steel sheet

Several ceramic rolls can be used efficiently to produce high quality zinc coated steel sheet used for automobiles. Those ceramics rolls may provide a longer life and reduce the cost for the maintenance because of its large heat resistance and large wear resistance. One example may be seen in sink rolls used in molten zinc bath to manufacture zinc coated steel sheet. Since the rolls are subjected to large thermal stress and mechanical loading, care should be taken for the risk of fracture due to the ceramic brittleness. Moreover, since the sleeve and shafts can be connected only by using small shrink fitting ratio, another failure risk should be considered for the separation of those components [25, 26]. In this paper, therefore, the mechanical and thermal stress and separation condition will be investigated considering the separation of the connected portion. Here, by using the finite volume method the heat transfer coefficient is discussed and by using the finite element method the thermal stress is considered.

Analytical model of the laser welding of zinc-coated steel sheets by the aid of videography

Despite several approaches from different research groups, currently the best solution for welding of zinc-coated steel sheets in an overlap configuration is the use of a joining gap between the sheets. However, the introduction of the gap is prone to errors due to dirt or thermal distortion. A too small gap leads to the explosive zinc ejections. In contrast, a too large gap causes false friends where the melt is not able to bridge the gap due to an erroneous clamping of the sheets. Currently, there is no direct measurement of the joining gap available. The reason therefore is the lack in the understanding of the fluid dynamics inside the keyhole in laser welding. To increase the process of understanding, the authors analyzed the correlation between the well visible keyhole opening, the joining gap size, and the welding speed for uncoated and zinc-coated sheets. To scrutinize our findings, the authors present a model of the zinc degassing inside the keyhole which obtains its input parameters directly from our comprehensive high-speed videography analysis of the process. To get a deep view inside the keyhole without the use of x-rays, the authors use a glass plate flanking the keyhole which allows us to look inside the keyhole during overlap welding of steel sheets with very high spatial and temporal resolution. By the aid of tracer particles, the authors measured and modeled the pressure due to the evaporation of the zinc layer without the use of unknown thermodynamic coefficients. Our results show that the joining gap and the welding speed have an influence on the pressure balance inside the keyhole. Hence, our work can be used as a foundation for building a control system for the joining gap for the welding of zinc-coated steel sheets.

Mechanical Joining of the Combination of Steel Sheets and Aluminium Alloy Sheets

Constructions of steel sheets and sheets from aluminium alloys are widely used in car body production to reduce the total car weight. The reduction in car weight is needed to improve fuel efficiency. Aluminium alloys play a more important role in future car body generations. Properties of this material give some advantages and open the way for new applications in the automotive industry. It is not possible to join the multiply material combination by conventional joining methods such as resistance spot welding. Therefore, alternative joining methods are being developed. One of the alternatives is mechanical joining - clinching. Clinching is suitable for joining the thin sheets from ferrous and non-ferrous metals. Mechanical clinching is also well known as a speed and efficient cold joining process, mainly for joining the thin-sheet applications. In this process, the local plastic deformation of the sheets is created by the clinching tool. The paper focuses on the evaluation of the properties of mechanically clinched joints, realized on the combination of the zinc-coated steel sheets DC06 and H220PD and sheets from aluminium alloys EN AW 5754 with various hardness H11, H22 and H24 and EN AW 6082. The tensile test and a metallographic observation were used to evaluate the clinched joint properties. Mechanical clinching proved to be suitable technique for joining the observed combination of steel sheets and aluminium alloy sheets. It is necessary to consider the orientation of the sheets relative to punch and die.

Online monitoring of cold metal transfer (CMT) process using infrared thermography

Online (passive) thermographic inspection of overlap joints of aluminium and zinc coated steel sheets made by cold metal transfer weld brazing process was explored. Different experimental trials were conducted for demonstrating the feasibility of thermographic inspection to detect the porosities, improper weld bead and to differentiate the pre weld temperature. The whole process was monitored using infrared cameras in different wavelength region. Image analysis algorithms were developed to reconstruct the thermal images that contain the signatures of the weld defects and to extract the pre weld temperature and its evolution with distance from the centre of the weld torch. Post-weld radiography lends strong support to the observations.

Surface investigation and tribological mechanism of a sulfate-based lubricant deposited on zinc-coated steel sheets

Phosphatation is a well-known technique to improve friction and wear behaviour of zinc coated steel, but has a variety of economic and ecologic limitations. In this study an alternative coating based on ammonium sulfate ((NH4)2SO4) is applied on skin-passed hot-dip galvanized steel sheets in order to investigate its surface chemical and tribological behaviour in a Pin-on-Disk Tribometer. Raman- and X-ray photoelectron spectroscopic results revealed a formation of ammonium zinc sulfate ((NH4)2Zn(SO4)2 * xH2O) on the surface, which is primarily located in the skin-passed areas of the steel material. Sulfate coated samples exhibited a superior friction behaviour in Pin-on-Disk Tests using squalane as a model substance for oil-like lubricated conditions and a formation of a thin lubrication film is obtained in the wear track. Squalane acts as a carrier substance for ammonium zinc sulfate, leading to an effective lubrication film in the wear track.

Effect of keyhole geometry and dynamics in zero-gap laser welding of zinc-coated steel sheets

Both zinc-coated and uncoated DP 590 steel sheets were used for laser welding experiment with a 2kW multi-mode fiber laser under the same experimental conditions. Using systematically obtained keyhole data, how the keyhole dynamically responds to an incident laser beam was studied by defining several key factors, such as keyhole expansion factor, keyhole motion range factor, average aperture diameters, mass loss fraction and melt pool volume size. The relative configuration of the keyhole and the laser beam was found to be the most influential factor for obtaining good welds, and when the beam was located away from the front keyhole wall a good weld was obtained. When the beam irradiates on the front keyhole wall, the zinc evaporation region between the two sheets can be directly heated by the beam, and the zinc vapor pressure can become extremely large, and therefore the melt pool is severely disturbed. The keyhole bottom aperture diameter and the keyhole expansion factor for the bottom aperture were both largely proportional to the mass loss for both zinc-coated and uncoated steels. Zero-gap laser welding of zinc-coated steels can be indeed successful and a theoretical basis is presented.

Electrodynamic simulation of energy absorption in laser keyhole welding of zinc-coated and uncoated steel sheets

Laser beam absorption phenomena during laser welding of zinc-coated and uncoated DP 590 steel sheets were simulated using the finite-difference time-domain (FDTD) method for the Maxwell equations. For accurate simulations, the keyhole shapes that were measured using a coaxial observation method during multi-mode fiber laser welding of the above steels were used, and the beam divergence and focusing characteristics of the laser were accurately matched. Also, the wavelength enlargement method was used to deal with a large computational domain required for laser welding simulation. Absorption mechanisms, multiple reflection patterns, and heating patterns were found to be significantly different for the two steels because of the different keyhole shapes. Zinc-coated steel has a much lower keyhole absorptance than uncoated steel at the same experimental condition, which was experimentally validated by melt pool size measurement. Ray tracing simulations were also performed for comparison purposes.

Disk Laser Welding of Car Body Zinc Coated Steel Sheets / Spawanie Laserem Dyskowym Blach Ze Stali Karoseryjnej Ocynkowanej

Autogenous laser welding of 0.8 mm thick butt joints of car body electro-galvanized steel sheet DC04 was investigated. The Yb:YAG disk laser TruDisk 3302 with the beam spot diameter of 200 μm was used. The effect of laser welding parameters and technological conditions on weld shape, penetration depth, process stability, microstructure and mechanical performance was determined. It was found that the laser beam spot focused on the top surface of a butt joint tends to pass through the gap, especially in the low range of heat input and high welding speed. All test welds were welded at a keyhole mode, and the weld metal was free of porosity. Thus, the keyhole laser welding of zinc coated steel sheets in butt configuration provides excellent conditions to escape for zinc vapours, with no risk of porosity. Microstructure, microhardness and mechanical performance of the butt joints depend on laser welding conditions thus cooling rate and cooling times. The shortest cooling time t8/5 was calculated for 0.29 s.

Behavior of Cold-Formed Steel Built-Up Sections with Intermediate Stiffeners under Bending. I: Tests and Numerical Validation

AbstractAn experimental investigation of simply supported built-up section beams with different sectional configurations has been conducted under both four-point bending and three-point bending. Intermediate stiffeners were employed to the webs of built-up sections to improve the buckling strength that is susceptible to local buckling and/or distortional buckling. In this study, the built-up sections were assembled by self-tapping screws from two channels with web stiffeners, which were brake-pressed from high-strength, zinc-coated grades G500 and G550 structural steel sheets with nominal 0.2% proof stresses of 500 and 550 MPa, respectively. The screws were located either at the flanges or webs, and the built-up sections include both open sections and closed sections. The moment capacities and observed failure modes of the beam tests are presented, and no failure was found in the screws. The local and distortional buckling behavior of the built-up section beam specimens were found to be different from the...

Nondestructive Evaluation of Friction Stir-Welded Aluminum Alloy to Coated Steel Sheet Lap Joint

Dissimilar lap joints of aluminum sheet (AA 6061) of 2 mm thickness and zinc-coated steel sheet of 1 mm thickness were produced by friction stir welding with different combinations of rotational speed and travel speed. Ultrasonic C- and B-scanning, and radiography have been used in a complementary manner for detection of volumetric (cavity and flash) and planar (de bond) defects as the defects are in micron level. Advanced ultrasonic C-scanning did not provide any idea about the defects, whereas B-scanning cross-sectional image showed an exclusive overview of the micron-level defects. A digital x-ray radiography methodology is proposed for quality assessment of the dissimilar welds which provide three-fold increase in signal-to-noise ratio with improved defect detection sensitivity. The present study clearly shows that the weld tool rotational speed and travel speed have a decisive role on the quality of the joints obtained by the friction stir welding process. The suitability of the proposed NDE techniques to evaluate the joint integrity of dissimilar FSW joints is thus established.

High-cycle fatigue behavior of MIG brazed galvanized DP600 steel sheet joint—effect of process parameters

Metal inert gas (MIG) brazing is a potential process for joining coated automotive steel sheets over conventional fusion welding process. However, understanding the process variables on the joint performance is not yet clear. This paper deals with details study on the effect of process parameters and two different modes of operation (push and pull) on bead geometry, microstructure, microhardness, nano-hardness, shear tensile strength, and high-cycle fatigue behavior of MIG brazed lap joint of 1.4-mm-thick zinc-coated (galvanized) DP600 steel sheet using Cu-Al filler wire. It has been possible to obtain 98 % joint efficiency using Cu-Al filler wire due to dispersion hardening effect of fusion zone consisting of dispersed iron from base metal in copper matrix. Also, an interface region formed (∼6 μm) in-between fusion zone and steel substrate has been characterized by X-ray and nano-hardness testing which confirmed the presence FeAl intermetallic compound. High-cycle fatigue test of all MIG brazed joints showed fatigue endurance (2 million cycles) at 10 % of tensile load and fatigue life increased with increasing heat input. Interestingly, three different fatigue failure paths were observed with different loading cycle, such as interfacial failure; fusion zone failure and fine grain heat-affected zone failure where the bead geometry played an important role in brazed joint under dynamic loading condition.

Decreasing pore formation in multiple-sheet laser joining with interfacial polymeric contaminations

Deep-penetration laser welding is an efficient joining technology for overlap joints nowadays. However, contaminations usually having a lower vaporization temperature can cause seam defects like pores and melt ejections during the process. Many approaches have been investigated, e.g., for the industrially interesting case of welding zinc-coated steel sheets. Suitable techniques for sufficient reduction or avoidance of porosity require additional process steps or time-consuming preparations. In this paper, three-sheet joints having polymeric interlayers as exemplary extensive contaminations are investigated. Seams feature extreme porosity after standard single-pass welding. The porosity can be decreased significantly when repeating the welding process (double-pass welding). This effect on formation of porosity is modified in this paper. Instead of two processes, longitudinal beam oscillation is applied. The center porosity in longitudinal sections, used as indicator for the porosity of seams, can be reduced significantly from more than 50 to 20 % by using the beam oscillation technique. As the main reason, the increased degassing time due to the lengthening of the melt pool depending on oscillation parameters is identified. Simultaneously, the results indicate that the seam width must not increase similarly to avoid that further evaporation of contamination material cancels out the achieved benefit by lengthening.

Experimental approach for quantification of fluid dynamics in laser metal welding

Laser metal welding is widely applied in industry. Nevertheless, a complete process understanding which leads to a full control of the process is not available yet. To contribute a solution for this task, we use two high-speed cameras with recording rates of up to 250 kHz and measure the velocity and direction of the fluid flow inside the keyhole with very high precision. Furthermore, we use tracer particles to measure the fluid dynamics in the keyhole. To get a deep view into the keyhole without the use of x-rays, we use a setup which allows us to look inside the keyhole during overlap welding of zinc-coated steel sheets with a spatial resolution of 25 μm per pixel and a temporal resolution of 5.3 μs per frame. Moreover, we quantify the influence of this approach on the process dynamics. Our findings show how the fluid dynamics inside the keyhole are linked to the laser power, the feed rate, and a gap between two zinc-coated steel sheets.

Anticorrosive Property of Potassium Dichromate Film on Galvanized Coating in Distilled Water

Hot Dip Galvanized (HDG) coating protects steel from corrosion by providing a thick, tough metallic zinc envelope, which completely covers the steel surface and seals it from the corrosive action of its environment. The anti-corrosion ability of this sacrificial anode cathodic protection can be improved upon through the protective nature of passivation films on zinc like potassium dichromate.This study investigated the anti-corrosion effect of potassium dichromate (K2Cr2O7) film on freshly galvanized coating in distilled water (pH=7.92). The K2Cr2O7 film passivity on the hot-dip zinc coated steel sheets (taken as treated in the context of this study) was used in direct comparison with their untreated counterparts. The test was run for 30 days at 120 hours interval during which the corresponding weight losses, corrosion rates, inhibitor efficiencies, and pH of the final solutions were obtained of the coupons. Analysis of results was made using Microsoft office applications. The surface morphology of the samples was obtained using Optical microscope. The results obtained revealed the greater influence of the action of the K2Cr2O7 film on selected and examined HDG steel sheets corrosion performance. Weight losses increased with increase in immersion time. Inhibitor efficiency of 4.1% was achieved. The photomicrographs confirmed the occurrence of corrosion on untreated coating more than the treated ones. Conclusively, potassium dichromate was effective in passivating galvanized coating from white rust.

Laser beam oscillation for fillet welding

In today’s automotive frame-and-body construction, laser welds are typically carried out as square butt welds in lap joints. These welds are increasingly produced by remote laser welding optics with working distances of 500 mm and more. This enables simple path programming because clamping devices are traversed over and therefore low cycle times with improved productivity is achieved. However, workpiece tolerances lead to part displacements and varying joint positions over time. These displacements have to be acted against by appropriate flange length that contain for positioning deviations. Using this concept, aspired light-weight optimisations, e.g. reducing flange length, are difficult to achieve. By using seam tracking sensors, part tolerances are automatically detected and counteracted for. In addition, joint edge detection allows constructive changes on the type of weld. Fillet welds reduce flange lengths’, improve force flow and open up possibilities for visual quality monitoring. Apart from that, vertical displacements in the form of height tolerances still occur. This needs consideration by adapting the set of welding parameter to the current welding situation. In this respect, one main welding parameter is the lateral beam offset to the upper sheet. Since body-in-white welding applications mainly comprise of zinc-coated steel sheets, special requirements for the welding process are given. Especially zero-gap-welding and welding of joints with gaps larger than 0.2 mm are critical. Using a laser beam oscillation process can stabilise these situations. Approaches to finding parameter sets are presented in this paper.

Remote Laser Welding of Zinc Coated Steel Sheets in an Edge Lap Configuration with Zero Gap

Abstract Remote Laser Welding (RLW) of zinc-coated steel sheets is a great challenge for the automotive industry but offers high potentials with respect to flexibility and costs. In state of the art applications, sheets are joined in overlap configuration with a preset gap for a stable zinc degassing. This paper investigates RLW of fillets without a preset gap and conditions for a stable process. The influence of process parameters on weld quality and process stability is shown. Experimental data give evidence, that the degassing of zinc through the capillary and the rear melt pool are the major degassing mechanisms. Furthermore the paper gives experimental validation of the zinc degassing in advance of the process zone to the open side of the fillet. Chemical analysis of the hot-dip galvanized zinc coating proof the iron-zinc-alloys to be the reason for a limited effectiveness of this mechanism in comparison to pure zinc as intermediate.

The Role of Zinc Layer During Wetting of Aluminium on Zinc-coated Steel in Laser Brazing and Welding

Abstract The zinc layer of zinc-coated steel is known to be a crucial factor for the spreading of liquid aluminium on the coated surface. For industrial brazing and welding processes these zinc-coatings enable a fluxless joining between aluminium and steel in many cases. Yet, the reason for the beneficial effect of the zinc to the wetting process is not completely understood. Fundamental investigations on the wetting behaviour of single aluminium droplets on different zinc-coated steel surfaces have revealed a distinct difference between coated surfaces at room temperature and at elevated temperature regarding the influence of different coating thicknesses. In this paper the case of continuous laser brazing and welding processes of aluminium and commercial galvanized zinc-coated steel sheets are presented. It is shown that in the case of bead-on-plate laser beam brazing, the coating thickness has a measureable effect on the resulting wetting angle and length but does not have a significant impact in case of overlap laser beam welding. This might be linked to different heat transfer conditions. The results also strongly indicate that proper initialbreakup of oxide layers is still required to accomplish good wetting on zinc-coated surfaces.

Soldering steel sheets using soft solders

The paper contains strength tests results of joints soldered using lead and leadless soft solders. For tests lead solders types Sn63Pb37, Sn60Pb40, Pb60Sn40 and Pb48Sn32Bi and leadless soft solders types Sn96Ag4, Sn99Cu1, Sn70Zn30 and Sn96Ag3Cu1 were used. As basic joint material steel sheet of 1.0 mm thickness and zinc-coated steel sheet of 1.0 mm thickness were used. The size of test specimen was 100 × 20 mm. Two sheets were always cleaned and jointed together. For heating the propane-butane plus air flame was used. The tested assemblies were loaded using the universal tensile-strength testing machine until their failure. At the tests the force needed for assemblies’ failure and failure type (in the soldered joint, in the basic material) was recorded and the solder strength was calculated from the measured data. The test results show that for soldering of steel sheets as well of zinc-coated steel sheets of 1.0 mm thickness the joints soldered using the lead soft solder type Sn63Pb37 and the leadless soft solder type Sn96Ag4 were of the highest strength.  

Soldering sheets using soft solders

The paper contains strength tests results of joints soldered using lead and leadless soft solders. For tests lead solders types Pb60Sn40 and Sn60Pb40 and leadless soft solders types Sn95.5Ag3.8Cu0.7 and Sn96Ag4 were used. As basic materials steel sheet, zinc-coated steel sheet, copper sheet and brass sheet 100 x 20 x 1 mm was the test samples size. Always two sheets were cleaned and jointed together. For heating the propane-butane + air flame was used. Then the tested assemblies were loaded using the universal tensile-strength testing machine till to failure. At the tests the force needed for assemblies failure and failure type (in soldered joint, in basic material) were recorded. From measured data the solder strength was calculated. From the experiment results it follows that from the point of view of the soldered joints strength as well of the solder strength relatively small differences were found. At the same time it is evident that the joint strength and solder strength depend on soldered material type and on soldered joint lapping length. On the basis of carried out experiments it can be stated that the substitution of lead solders by leadless solders is possible without risk of soldered joints strength decrease.

Modeling and numerical simulation of the resistance spot welding of zinc coated steel sheets using rounded tip electrode: Analysis of required conditions

This paper investigates the essential conditions to improve the accuracy of a resistance spot welding computational study of advanced zinc coated steel sheets using rounded tip electrode. An experimental analysis is performed to highlight the required considerations for a suitable simulation. A sequential Electrical-Thermal-Metallurgical and Mechanical (ETMM) finite element analysis with appropriate precautions of the contact conditions enables to accurately simulate the nugget development during the welding. A critical smooth evolution of the contact radius is required. A fine meshing with an interfacial mesh size of at least 0.05×10−3m combined with a coupling time step of 0.0025s between the electrical-thermal-metallurgical and the mechanical analysis allows a regular incrementation of the contact radius, without burdening the time computing. Accurate values of the contact resistance depending on the interfacial pressure and temperature are essential for a good simulation of the nugget size. The ETMM calculation is successfully extended to the simulation of the welding of a typical two sheets assembly.