Reduction in liquid metal embrittlement cracking using weld current ramping

QP980 advanced high-strength steel used in automobile is usually protected from corrosion by Zn-based coating. However, liquid metal embrittlement (LME) is tended to occur in galvanized QP980 steel during the resistance spot welding process and therefore leads to the reduction of load-carrying property of the involved welded joint. In this paper, the effect of welding current on LME of the resistance spot welded galvanized QP980 steel was comparatively studied. The results showed that more and longer LME cracks inside and around the spot weld indentation could be observed in the specimens with increasing the welding current, reflecting that the increased welding current with the added heat input could result in the LME cracks under the same electrode force. Therefore, appropriate welding current was required to control the heat input and thus suppress the LME of the QP980 welded joint.KeywordsAdvanced high strength steelResistance spot weldingLiquid metal embrittlementWelding currentExpulsion

Influence of selected alloying variations on liquid metal embrittlement susceptibility of quenched and partitioned steels

Advanced high strength steels have been widely used in the automobile structures to enhance passengers’ safety [1]. Hot stamping process is recently applied and the strength of press hardening steels is elevated via martensitic phase transformation [2; 3]. Nevertheless, the oxidation and decarburization generally occur during the heat treatment prior to stamping[4], which in turn deteriorate the properties of steels [5]. Hot-dip galvanizing has been applied in steel industries due to its effectiveness to produce a Zn coating with both barrier and cathodic protections again corrosion [6]. Recently, hot-dip galvanized coatings have been studied to evaluate the protection on steels during hot stamping and the corrosion resistance of the resultant coating after hot stamping. The critical issues of the galvanized steels for hot stamping are oxidation and liquid metal embrittlement [7; 8]. The former deteriorates the resistance spot weldability and paintability, whereas the later has a negative effect on formability [9]. Silane, a kind of Si-based coating on light metals, has been used in galvanized steels to offer barrier protection against corrosion [10]. In this study, silane coating was applied on galvanized (GI) and galvannealed (GA) steels to elevate the practicability for the hot stamping application. The GI and GA steel sheets were received from China Steel Co., Taiwan, with coating thickness of approximately 10µm. Silane coating film was formed by a laboratory rolling method. The treating solution was prepared by mixing tetraethyl orthosilicate (TEOS), acidified water (pH= 1.0, adjusted by HNO3), γ-glycidoxypropyltrimethoxysilane (GPTMS), followed by stirring for 2 h at room temperature. The GI and GA steel sheets with and without the silane coating were isothermally held at 900℃ under the atmosphere for 5 min to simulate austenitization prior to hot stamping. XRD, SEM, TEM, and XPS were employed to characterize the surface microstructure of the various steels. Corrosion behaviors were tested by electrochemical stripping and potentiodynamic polarization measurements. The GI steel displayed a gray color after austenitization. In contrast, the GI steel with the silane coating still exhibited a metallic color, suggesting the silane coating is effective in preventing the GI steel from oxidation at 900℃ (Fig(a)). This is consistent with the XRD pattern showing the absence of the peaks resulting from ZnO for the GI steel with the silane coating (Fig.(b)). However, the silane coating offered a less degree of protection on the GA steel. The cross section of the silane-coated GI and (b) GA steels was further characterized by the SEM. Zn oxides were readily observed on the GA steel; conversely, Zn oxide scale was hardly observed on the GI steel. After austenitization, the silane-coated GI coating transformed to a mixture of Γ phase and Zn saturated α-Fe, which retained the cathodic protection over the steel substrate. However, the alloy layer of the silane-coated GA steel was solely composed of Zn saturated α-Fe and the cathodic protection was largely reduced after austenitization. Reference [1] R. Kuziak, R. Kawalla, S. Waengler, Archives of civil and mechanical engineering 8 (2008) 103-117.[2] D.W. Fan, H.S. Kim, B.C. De Cooman, Steel Research International 80 (2009) 241-248.[3] H. Karbasian, A.E. Tekkaya, Journal of Materials Processing Technology 210 (2010) 2103-2118.[4] D.W. Fan, B.C. De Cooman, steel research international 83 (2012) 412-433.[5] L. Dosdat, J. Petitjean, T. Vietoris, O. Clauzeau, steel research international 82 (2011) 726-733.[6] A. Marder, Progress in materials science 45 (2000) 191-271.[7] C.W. Lee, D.W. Fan, I.R. Sohn, S.-J. Lee, B.C. De Cooman, Metallurgical and Materials Transactions A 43 (2012) 5122-5127.[8] R. Autengruber, G. Luckeneder, S. Kolnberger, J. Faderl, A.W. Hassel, steel research international 83 (2012) 1005-1011.[9] C.-W. Ji, I. Jo, H. Lee, I.-D. Choi, Y. Do Kim, Y.-D. Park, Journal of Mechanical Science and Technology 28 (2014) 4761-4769.[10] R. Figueira, C.J. Silva, E. Pereira, Journal of Coatings Technology and Research 12 (2015) 1-35. Figure 1

Role of the internal oxidation layer in the liquid metal embrittlement during the resistance spot welding of the Zn-coated advanced high strength steel

Modern advanced high-strength steel sheets for automotive applications are mostly zinc coated for corrosion resistance. However, the presence of zinc can—besides its positive effects—increase the material’s susceptibility to liquid metal embrittlement (LME) during resistance spot welding (RSW). Zinc and its eutectics are, due to their low melting point, present in liquid state during the welding process. This fact can, in combination with other factors like tensile strains or stresses, lead to the formation of brittle, intergranular cracks in the weld, and heat-affected zone. This phenomenon is commonly called liquid metal embrittlement. In order to understand the process from a practical perspective, one must learn what factors facilitate it. In this study, industry-relevant parameters are investigated regarding their influence on the occurrence of LME, embodied by the formation of surface cracks. It was found that electrode wear has less of an influence on the cracking susceptibility than welding current or tensile stresses. Finite element analysis is believed to provide a powerful tool in order to gain insights on the formation process. Modeling of the process shows promising initial results, revealing the underlying local stress and strain fields, unmeasurable with common techniques.

Modeling of resistance spot welding process on galvanized steel sheet was investigated. Mathematical models developed by nonlinear multiple regression analysis and artificial neural network approach were employed in the prediction of welding quality factors, namely nugget diameter, penetration rate and tensile shear strength, under some welding conditions. According to the prediction models on quality, the prediction systems of welding process parameters were formulated respectively on the basis of Newton-Raphson iterative algorithm and cascade forward back propagation algorithm in order to obtain the desired welding quality. The results showed that the prediction precision of cascade forward back propagation algorithm was higher than Newton-Raphson iterative algorithm. The current duration had the largest prediction error, followed by electrode force and welding current. Therefore, it was concluded that the current duration was the most difficult parameter in prediction system of welding process in order to obtain the desired welding quality.

TIG welding is an important process, which is commonly used in the aircraft industry. A number of elements of airplane bodies or shields of engines are made using this technique. Numerical models of this process are needed to understand better phenomena involved in the TIG welding and to design the optimal process parameters. In the present paper numerical simulations were performed and the possibilities of modeling heat transfer in the investigated process were evaluated. Comparison of results of numerical simulations with the experimental data confirmed good predictive capabilities of the model, as far as realistic description of the phenomena involved in this process are considered. Correctness of the Goldak model, which describes density of the energy in the heat source, was confirmed as well.

Galvanized advanced high strength steels (AHSS) will be the most competitive structural material for automotive applications in the next decade. Oxidation of AHSS during the recrystalization annealing process in a continuous galvanizing line to a large extent influences the quality of zinc coating on the final galvanized steel product. For example, formation of oxides of alloying elements (e.g. Mn, Cr, Si) at the steel surface during annealing prior to galvanizing leads to poor adhesion of a zinc coating. Yet, knowledge on the high temperature oxidation behaviour of AHSS is rather limited. The primary aim of this thesis is to provide fundamental understanding on the kinetics of internal oxidation of AHSS during annealing. The classical Wagner internal oxidation theory for binary alloys was extended to account for multi-component alloys. To this end, a generic coupled thermodynamic-kinetic internal oxidation model based on Fick’s 1st law was developed in order to predict the kinetics of internal oxidation, as well as the concentration depth profiles of internal oxides and solute elements in alloy matrix, considering the finite solubility product of oxide precipitates, the non-ideal behaviour of solid solution and the formation of multiple type of oxide species. The internal oxidation behaviour of Fe-Mn and Fe-Mn-Cr steel alloys were experimentally studied to validate the model. It has been found that for Fe-Mn and Fe-Mn-Cr steel alloys, the effect of non-ideal behaviour of solution on internal oxidation is negligible, and local thermodynamic equilibrium is established within internal oxidation zone. Besides, the kinetics of Wustite formation on pure iron and Mn alloyed steels annealed in CO2 + CO or H2O + H2 gas mixtures as well as the reduction kinetics of the Wustite scale in Ar + H2 gas mixtures were investigated. The growth of Wustite scale on iron and Mn alloyed steels follows the linear rate law. However, adding Mn to iron, even at a relatively low concentration (say 1.7 wt%), dramatically lowers the growth rate of Wustite scale. Nevertheless, reduction kinetics of the Wustite scale on iron and Mn alloyed steels are almost the same. During the reduction process a dense iron layer is formed which separates the remaining Wustite scale from the reduction atmosphere. The rate of Wustite reduction by H2 is controlled by the diffusion of solute oxygen dissolved in the formed iron layer.

This study focuses on the element effects of Al and Si on the liquid metal embrittlement (LME) phenomenon. Four series of chemical compositions have been chosen for this research, which are 20% higher Al DH780, 20% higher Si DH780, 20% higher Al DH980, and 20% higher Si DH980. The total LME crack length of the welding spot of each state sample has been statistically analysed. The high Al DH780 sample shows the lowest total crack length, whereas the high Si DH980 sample shows the highest total crack length. Then the semi-in situ tensile tests have also been employed to evaluate the mechanical properties of these four different chemical composition samples. The high Al sample shows less LME cracks and higher elongation compared with the high Si sample. The shoulder position transmission electron microscopy (TEM) sample has been cut by the focused ion beam. It could be seen from the TEM observation that the distribution of Si element is close to that of Zn. The Si and Al solute atoms would diffuse towards grain boundaries to form solute-rich clusters in the manner of solute-vacancies loops. The stronger Zn-Si adhesion bonding attracts Zn infiltrating deeper into substrate. The Zn-rich intermetallic phase nucleation on grain boundaries would further worsen the LME sensitivity. The enrichment of Al content would repel Zn on grain boundaries and also stabilise the Fe-Zn intermetallic phase. High Al could also reduce the austenite amount in steel. Thus, it mitigates the LME response of steel. The LME phenomenon of Zn would be severe for the higher Si sample.

Current friction stir welding (FSW) process modeling research is mainly concerned with the detailed analysis of local effects such as material flow, heat generation, etc. These detailed thermomechanical models are typically solved using finite element or finite difference schemes and require substantial computational effort to determine temperature, forces, etc., at a single point in time, or for a very short time range. Dynamic models describing the total forces acting on the tool throughout the entire welding process are required for the design of feedback control strategies and improved process planning and analysis. In this paper, empirical models relating the process parameters (i.e., plunge depth, travel speed, and rotation speed) to the process variables (i.e., axial, path, and normal forces) are developed to understand their dynamic relationships. First, the steady-state relationships between the process parameters and the process variables are constructed, and the relative importance of each process parameter on each process variable is determined. Next, the dynamic characteristics of the process variables are determined using recursive least-squares. The results indicate the steady-state relationship between the process parameters and the process variables is well characterized by a nonlinear power relationship, and the dynamic responses are well characterized by low-order linear equations. Experiments are conducted that validate the developed FSW dynamic models.

In this investigation, the regression analysis together with the desirability function approach was involved to optimize the resistance welding parameters for the high-strength steel HSLA 420. The Box-Behnken experimental design (BBD) was employed for the control factors of welding time, welding current, and electrode pressure. The values of nugget diameter, ultimate peak load, maximum displacement, and absorption energy were identified and processed using the approach of multiple-objective optimization based on ratio analysis (MOORA). The weight loss of the base metal before and after welding was measured and computed to estimate the welding expulsion magnitude. The calculated statistical parameters displayed that the welding current had the most momentous influence on the welding quality and welding expulsion. The welding joints free of expulsion with sufficient mechanical properties can be achieved at the welding current of 8.4 kA, electrode pressure of 0.5 MPa, and welding time of 13 cycles.

Abstract. Automotive structural components from Advanced High Strength Steels (AHSS) can be manufactured with Flexible Roll Forming (FRF). Flange wrinkling is a common shape defect in FRF, this restricts the application of FRF in the automotive industry. The new Incremental Shape Rolling process (ISR) showed that a high tensile transverse strain is developed in the flange and that assists the plastic deformation in the flange. Hence, wrinkle severity can be significantly reduced when weight-optimized components are formed. In this study, the ISR process is applied to a variable width profile from DP600. This investigated profile is a modified automotive component. The forming strains and wrinkling severity in ISR are compared with those obtained from the FRF case. The ISR and the FRF experimental trials are performed on a prototype FRF facility and then used to validate the numerical models which are applied to analyse the deformation behaviour in both processes. The ISR results show a significant reduction in wrinkling. This is due to the high tensile transverse strain that is developed in the ISR flange which facilitates the plastic deformation and hence the flange is stably compressed to the required shape.

Advanced High Strength Steels or AHSS (Advanced High Strength Steels) have superior properties to conventional steel are widely used in the automotive industry, they provide weight reduction and at the same time, increase vehicle safety in the event of collisions.Flangeability is an essential element for forming automotive components such as chassis and suspension components.Research is ongoing into the development of forming techniques that can improve the performance of AHSS during industrial operations, particularly flange drawing.Therefore, there is great interest in investigating the extensibility of the pre-cut cutting edge and determining the factors that significantly influence the formability of the product.The test used to evaluate flangeability is the Hole Expansion Ratio (HER).In this research, the objective was to evaluate the influence of the conditions of four edge cutting methods on the hole expansion test of advanced multiphase high-strength steel class 780 MPa through four methods: Machining, LASER and punching with expansion of the hole in the direction of the punch (burr downwards) and in the opposite direction (burr upwards).The results of this study corroborate the literature that indicates that the state of the edge after cutting directly influences the expansion capacity of the hole.

Resistance spot welding (RSW) is a common joining technique in the production of car bodies in white for example, because of its high degree of automation, its short process time, and its reliability. While different steel grades and even dissimilar metals can be joined with this method, the current paper focuses on similar joints of galvanized advanced high strength steel (AHSS), namely dual phase steel with a yield strength of 1200 MPa and high ductility (DP1200HD). This material offers potential for light-weight design. The current work presents a multi-physical finite element (FE) model of the RSW process which gives insights into the local loading and material state, and which forms the basis for future investigations of the local risk of liquid metal assisted cracking and the effect of different process parameters on this risk. The model covers the evolution of the electrical, thermal, mechanical, and metallurgical fields during the complete spot welding process. Phase transformations like base material to austenite and further to steel melt during heating and all relevant transformations while cooling are considered. The model was fully parametrized based on lab scale material testing, accompanying model-based parameter determination, and literature data, and was validated against a large variety of optically inspected burst opened spot welds and micrographs of the welds.

The prediction of stress level during Resistance Spot Welding at high temperature is very useful to reduce and/or avoid Liquid Metal Embrittlement phenomenon. However, the experimental estimation of stress level into metal sheets during a Resistance Spot Welding process is impractical. Therefore, a numerical methodology to predict mechanical stresses, using more accessible experiments, is proposed in this work.