Prediction of laser-spot-weld shape by numerical analysis and neural network

The finite element method (FEM) and neural network were applied for predicting the bead shape in laser spot welding of type 304 thin stainless steel sheets. The parameters of pulsed Nd:YAG laser spot welding such as pulse energy, pulse duration, sheet metal thickness, and gap between sheets were varied for various experiments and numerical simulations. The penetration depth and nugget size of spot welds measured for specimens without gap were compared with the calculated results to verify the proposed finite element model. Sheet metal thickness, gap size, and bead shape of the workpiece without gap were selected as the input variables for the back-propagation learning algorithm of the neural network, while the bead shape of the workpiece with and without gap was considered as its output variable. Various combinations of stainless steel sheet metal thickness were considered to calculate the laser-spot-weld bead shape of the workpiece without gap, which was then used as the input variable of neural network to predict the bead shape for various gap sizes. This combined model of finite element analysis and neural network could be effectively applied for the prediction of bead shapes of laser spot welds, because the numerical analysis of laser spot welding for the workpiece with gap between two sheets is highly limited.     

On the Failure Behavior of Highly Cold Worked Low Carbon Steel Resistance Spot Welds

Highly cold worked (HCW) low carbon steel sheets with cellular structure in the range of 200 to 300 nm are fabricated via constrained groove pressing process. Joining of the sheets is carried out by resistance spot welding process at different welding currents and times. Thereafter, failure behavior of these welded samples during tensile-shear test is investigated. Considered concepts include; failure load, fusion zone size, failure mode, ultimate shear stress, failure absorbed energy, and fracture surface. The results show that HCW low carbon steel spot welds have higher failure peak load with respect to the as-received one at different welding currents and times. Also, current limits for failure mode transition from interfacial to pullout or from pullout to tearing are reduced for HCW samples. Fusion zone size is the main geometrical factor which affects the failure load variations. Ultimate shear stress of spot welds is increased with decreasing the heat input and for HCW samples at a specific welding current and time, it is lower than that of the as-received ones. Before pullout mode, failure absorbed energy (FAE) for HCW low carbon steel spot welds is higher than that of the as-received one, but after failure mode transition, trend would be vice versa and FAE of the as-received spot welds is extremely higher (about 3 times). In addition, spot welds fracture surface (in interfacial failure mode) for HCW sample is more dimpled which indicates higher energy absorption than that of the as-received one.     

The effects of process variables on pulsed Nd:YAG laser spot welds: Part I. AISI 409 stainless steel

The weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of Nd:YAG laser welding parameters, including laser pulse time and power intensity, and material-dependent variables, such as absorptivity and thermophysical properties, on laser spot-weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and sur-face cratering, have been studied experimentally. The results of this work are reported in two parts. In Part I, the weldability of AISI 409 stainless steel by the pulse laser welding process is reported. In Part II, the weldability of A A 1100 aluminum under the same operating con-ditions is reported and compared to those of the stainless steel. When welding AISI 409 stainless steel, weld pool shapes were found to be influenced most by the power intensity of the laser beam and to a lesser extent by the pulse duration. Conduction mode welding, keyhole mode welding, and drilling were observed. Conduction mode welds were produced when power in-tensities between 0.7 and 4 GW/m² were used. The initial transient in weld pool development occurred in the first 4 ms of the laser pulse. Following this, steady-state conditions existed and conduction mode welds with aspect ratios (depth/width) of about 0.4 were produced. Keyhole mode welds were observed at power intensities greater than 4 GW/m². Penetration of these keyhole mode welds increased with increases in both power intensity and pulse time. The major weld defects observed in the stainless steel spot welds were cratering and large-occluded gas pores. Significant metal loss due to spatter was measured during the initial 2 ms of keyhole mode welds. With increasing power intensity, there was an increased propensity for occluded gas pores near the bottom of the keyhole mode welds. Formerly Graduate Student.     

Dissimilar Spot Welds of AISI 304/AISI 1008: Metallurgical and Mechanical Characterization

This paper aims at investigating structure‐properties relationships in dissimilar resistance spot welding of AISI 304 austenitic stainless steel (SS) and AISI 1008 low carbon steel (CS). Microstructural characterization, microhardness test and the tensile‐ shear test were conducted. It was shown that the shape of the SS/CS fusion zone (FZ) is unsymmetrical and the final fusion line shifts from sheet/sheet interface into the higher resistivity side (i.e. AISI 304). FZ microstructure was ranged from ferrite‐austenite to full martensite depending on the dilution ratio of the base metals. The variation of SS/CS dissimilar welds failure mode was explained in terms of hardness/microstructure characteristics. It was concluded that to ensure pullout failure mode, welding parameters needed to adjust so that the FZ size is sufficiently large and dilution is sufficiently high to produce a martensite FZ. Fusion zone size at CS side proved to be the most important controlling factor of SS/CS peak load and energy absorption. Finally, the mechanical properties of SS/CS dissimilar welds were compared with SS/SS and CS/CS similar welds.     

The effects of process variables on pulsed Nd:YAG laser spot welds: Part II. AA 1100 aluminum and comparison to AISI 409 stainless steel

In this two-part article, the weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of laser pulse time and power density on laser spot weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and surface cratering, have been studied and explained qualitatively in relation to material-dependent variables such as absorptivity and thermophysical properties. The weldability of AISI 409 stainless steel was reported in Part I of this article. In the present article, the weldability of AA 1100 aluminum is reported and compared to that of AISI 409 stainless steel. Weld pool shapes in aluminum were found to be influenced by the mean power density of the laser beam and the laser pulse time. Both conduction-mode and keyhole-mode welding were observed in aluminum. Unlike stainless steel, however, drilling was not observed. Conduction-mode welds were produced in aluminum at power densities ranging from 3.2 to 10 GW/m². The power density required for melting aluminum was approximately 4.5 times greater than stainless steel. The initial transient in weld pool development in aluminum occurred within 2 ms, and the aspect ratios (depth/width) of the steady-state conduction-mode weld pools were approximately 0.2. These values are about half those observed in stainless steel. The transition from conduction- to keyhole-mode welding occurred in aluminum at a power density of about 10 GW/m², compared to about 4 GW/m² for stainless steel. Weld defects such as porosity and cratering were observed in both aluminum and stainless steel spot welds. In both materials, there was an increased propensity for large occluded vapor pores near the root of keyhole-mode welds with increasing power density. In aluminum, pores were observed close to the fusion boundary. These could be eliminated by surface milling and vacuum annealing the specimens, suggesting that such pores were due to hydrogen. Finally, excellent agreement was obtained between experimental data from both alloys and an existing analytical model for conduction-mode laser spot welding. Two nondimensional parameters, the Fourier number and a nondimensional incident heat flux parameter, were derived and shown to completely characterize weld pool development in conduction-mode welds made in both materials.     

Peak load and energy absorption of DP600 advanced steel resistance spot welds

The paper aims at investigating the microstructure, failure mode transition, peak load and energy absorption of DP600 dual phase steel during the tensile-shear test. It was found that the welding current has profound effect on the load–displacement characteristics. In the low welding current, welds failed in interfacial failure mode. Increasing welding current resulted in sufficient weld nugget growth to promote double-sided pullout failure mode with improved mechanical properties. Further increase in the welding current caused expulsion and failure mode was changed to single-sided pullout with reduced energy absorption capability. It was found that the fusion zone size is the key parameter controlling the mechanical properties of DP600 resistance spot welds in terms of peak load, maximum displacement and failure energy.     

Improvement of weldability of TRIP steels by use of in‐situ pre‐ and post‐heat treatments

Low alloy TRIP‐aided steels are very interesting for the automotive industry as they combine both a high strength and an excellent formability. Though the actually developed TRIP steels can be considered as low alloyed when compared to the first generations of steels exhibiting TRIP effect, due to their chemical composition, they still exhibit a quite high carbon equivalent. This is particularly detrimental for the weldability of those materials. After solidification, welds are very hard and can show a brittle behaviour. The hardness of the heat affected zone of the welds can even exceed 500HV and cold cracking phenomena is prone to occur. In the automotive industry, spot welding is the main joining process. During spot welding of TRIP steels, the interface between the plates can act like a notch and promote fracture of the weld. This is particularly dangerous when brittle welds are submitted to peel stresses. The aim of the paper is to demonstrate that a careful choice of the process parameters can significantly improve the resistance of the welds. The selection of the welding cycle parameters is far from being an easy task as many different parameters are involved. Therefore, a design of experiment methodology (DOE) was chosen to optimise the welding cycle for a cold‐rolled TRIP steel with a tensile strength above 700 MPa. Mechanical properties of the welds were significantly improved by use of pre‐ and post‐heat treatments. Those improved welding cycles were realised without excessive extension of the total weld cycle on a conventional spot welding machine. This means that the optimised welds can be obtained in the existing production lines without any additional investment or significant decrease in productivity.     

A Study of Spot Welding of Advanced High Strength Steels for Automotive Applications

The application of advanced high strength steels in automotive industry has highlighted the need for research into spot weldability of these steels.Using weld lobe diagrams,the spot weldability of DP600 steel was found to be poor with conventional weld schedules.An enhanced weld schedule consisting of two pulses with reduced current on the second pulse gave a substantial increase in the lobe width;the first pulse removed the zinc coating and the second pulse controlled the nugget growth.A data acquisition system was designed to monitor weld expulsion during the weld operation.Of the three possible control strategies proposed,especially with AC welding equipment,the dynamic resistance signal is easily obtained and the least expensive.Expulsion phenomena,microstructural characterization and mechanical properties of spot-welded hot dipped galvanized DP600 steel and interstitial free steel were investigated.Further work on dissimilar welds in DP 600 and HSLA 350 was also conducted with emphasis on tensile and fatigue properties and fracture characteristics.The performance of dissimilar spot welds was different from that of the similar spot welds in each of the HSLA350 and DP600 steels.The DP600 weld properties played a dominating role in the hardness and tensile properties of the dissimilar spot welds.However,the fatigue performance of the dissimilar welds was similar to that of the HSLA welds.Details will be presented at the conference.     

A Study on Friction Stir Multi Spot Welding Techniques to Join Commercial Pure Aluminum and Mild Steel Sheets

To achieve significant improvement in the shear strength of dissimilar joints between aluminum and mild steel sheets, four methods of friction stir multi-spot welding processes, were investigated. Initially, in all these methods, plasticized aluminum layer was deposited on the steel side by friction surfacing. Subsequently, the deposited aluminum was compacted by friction forming. After dressing, spot welding with different tool configurations was performed. Tool rotational speeds of 900, 1120, 1400 and 1800 rpm were used to analyze their effects on the weld nugget. Different mechanical and metallurgical characterizations were done on the welds thus made. The process with aluminum layer on grooved mild steel followed by friction stir multi-spot welding using concave tipped welding tool resulted in welds. These welds had better metallurgical bonding characteristics and higher shear strength, which at a rotational speed of 1120 rpm was more than twice that of the welds made with conventional friction stir spot welding.     

Mechanical Behavior and Failure Mechanism of Q&P980 Steel During In Situ Post-Weld Heat Treatment (PWHT) Resistance Spot Welding

Metallurgical and Materials Transactions A - The microstructure evolution of Q&P 980 steel resistance spot welds under two different welding conditions, i.e., single pulse and dual pulse,...     

低碳相变诱发塑性钢的综合性能

对抗拉强度600 MPa级的低碳相变诱发塑性钢(TRIP钢)进行了拉伸、成形、高速冲击拉伸和激光焊接试验.对焊接前后的综合性能进行了对比,并对其金相组织、显微硬度进行了观察和测试分析.与原始板材相比,激光焊接后,低碳TRIP钢的强度和硬度增加,而伸长率略有下降;焊接热影响区的硬化较明显,但均未发现明显的焊接裂纹及其它缺陷,表明其焊接性能良好.发现低碳TRIP钢成形性能、抗凹陷性能、激光焊接性能良好.     

A study on weldability of zinc-coated steel sheets in lap joint configuration

The weldability of Zn-coated steel sheets 0.7 mm thick was investigated using resistance spot welding process. The effect of welding current, welding time and holding time on weld nugget characteristics, microstructure, and mechanical properties was discussed. Then, the possibility of replacing this welding process with laser beam welding was outlined. In this respect, quality of weld joints as a function of zinc removal by grinding prior to welding was evaluated. It is found that resistance spot welding current and time are the most significant parameters in affecting both expulsion and Zn-induced porosity. Expulsion was avoided and Zn-induced porosity was reduced with the decrease in welding current and/or welding time. Zn-induced porosity was completely eliminated by zinc-removal by grinding prior to welding. The best weld joint concerning nugget characteristics, soundness and tensile shear strength was obtained using welding current of 10 kA, weld cycle of 20, holding cycle of 18. Unlike resistance spot welds, high quality of CO₂ laser welds free from Zn-induced porosity could be made without zinc removal by grinding before welding.     

Bake Hardening of Laser Welded Dual Phase Steel

The influence of bake hardening on the mechanical properties of laser welded dual phase steel was investigated. A remarkalbe increase of the hardness in the zone influenced by laser welding was observed. The fusion zone had a low carbon lath martensite microstructure. The laser weld region had a higher interstitial carbon content than the base material. The dual phase steel exhibited a clear bake hardening effect in both the as‐received and the laser‐welded conditions. The bake hardening effect is more pronounced in the prestrained laser welded condition. A pronounced decrease of the ductility was observed for prestrained laser welded DP steel.     

Q235A碳钢活性A—TIG点焊的焊接性研究

对Q235A碳钢A—TIG点焊进行了工艺性的研究,讨论了活性剂对焊点成形。结果表明,使用活性剂后能显著增加焊点熔深。焊点表面成形良好,提高了焊接接头力学性能。焊接电流、点焊时间和弧长均对焊接熔深的增加产生影响。     

A Novel Shim-Assisted Resistance Spot Welding Process to Improve Weldability of Medium-Mn Transformation-Induced Plasticity Steel

Medium-Mn transformation-induced plasticity steels have great potential to significantly reduce vehicle weight and improve fuel economy due to their outstanding combination of high strength and excellent ductility. One bottleneck to the application is their poor weldability resulting from their high Mn contents. In this paper, three resistance spot welding set-ups, including no shim, an interstitial-free steel shim at the faying interface (shim-in) and shims against the electrodes (shim-out), were incorporated to investigate the weldability of Fe-7Mn-0.14C medium-Mn steel. Tensile-shear, cross-tension, and microhardness tests were used to evaluate the mechanical properties of the welds. Experimental results demonstrated that the failure mode of the welds transitioned from the interfacial fracture in the case of no shim to the desired nugget pull-out fracture in the shim-out set-up, resulting in dramatical improvements in both peak loads and their corresponding extensions during the tensile-shear and cross-tension tests. In contrast, the shim-in set-up made no improvement. What can contribute to such improvement was then discussed on the basis of observed morphologies and microstructures of welds.

     

Post-weld Tempered Microstructure and Mechanical Properties of Hybrid Laser-Arc Welded Cast Martensitic Stainless Steel CA6NM

Manufacturing of hydroelectric turbine components involves the assembly of thick-walled stainless steels using conventional multi-pass arc welding processes. By contrast, hybrid laser-arc welding may be an attractive process for assembly of such materials to realize deeper penetration depths, higher production rates, narrower fusion, and heat-affected zones, and lower distortion. In the present work, single-pass hybrid laser-arc welding of 10-mm thick CA6NM, a low carbon martensitic stainless steel, was carried out in the butt joint configuration using a continuous wave fiber laser at its maximum power of 5.2 kW over welding speeds ranging from 0.75 to 1.2 m/minute. The microstructures across the weldment were characterized after post-weld tempering at 873 K (600 °C) for 1 hour. From microscopic examinations, the fusion zone was observed to mainly consist of tempered lath martensite and some residual delta-ferrite. The mechanical properties were evaluated in the post-weld tempered condition and correlated to the microstructures and defects. The ultimate tensile strength and Charpy impact energy values of the fully penetrated welds in the tempered condition were acceptable according to ASTM, ASME, and industrial specifications, which bodes well for the introduction of hybrid laser-arc welding technology for the manufacturing of next generation hydroelectric turbine components.     

Properties of Laser Welded SAF 2205 Duplex Stainless Steel Sheet

High rate welding methods for sheet material can offer significant cost reduction for mass production application in comparison with more conventional arc processes. Therefore, in this research, laser welds in SAF 2205 duplex stainless steel (DSS) sheets welded using different welding speeds were investigated. Metallography, texture measurements and mechanical testing were carried out on the weld joints. The corrosion properties were not evaluated. The base material was characterised by a bamboo‐like morphology and a ferrite volume fraction of 53 %. For all welding speeds, the ferrite level in the weld zone was higher than 85 % and the austenite showed an acicular morphology. Whereas in the base material a clear element partitioning existed between ferrite and austenite, no partitioning was observed in the welded zone. This is due to the very high cooling rates, which limit the amount of diffusion that can take place. Electron backscattering diffraction revealed that the texture of the cold rolled material was destroyed by the welding process. While the hardness of the base material was about 265 HV, the maximal hardness in the fusion zone exceeded 310 HV and increased with an increase of the welding speed. Yield and tensile stength were however not dramatically influenced. On the other hand, the formability properties were deteriorated by an increase of the welding speed. This behaviour can also be observed on the fracture surfaces of tensile specimens. The tensile tests on the welded sheet resulted in ductile fracture surfaces, but an easier void formation was observed in the laser welds. However, it has to be pointed out that formability of the laser welded DSS sheets is acceptable when a lower welding speed is used. This is also confirmed by the crack propagation observed during the Erichsen test. Therefore, the laser welding can be used as a joining operation for DSS sheet materials providing the corrosion requirements are fulfilled.     

热镀锌钢板点焊工艺研究

热镀锌钢板与普通低碳冷轧钢板相比,其点焊焊接性能显著恶化。研究了三个主要焊接参数（焊接电流、点焊时间及电极压力）对热镀锌钢板点焊质量和电极寿命的影响。试验表明：热镀锌钢板点焊时,焊接电流、焊接时间和电极压力较同等厚度的低碳冷轧钢板都有不同程度的提高;热镀锌钢板的点焊焊接规范调节区较窄;焊点质量对焊接规范特别是电流变化敏感。     

Influence of Yb:YAG Laser Beam Parameters on Haynes 188 Weld Fusion Zone Microstructure and Mechanical Properties

The weldability of 1.2 mm thick Haynes 188 alloy sheets by a disk Yb:YAG laser welding was examined. Butt joints were made, and the influence of parameters such as power, size, and shape of the spot, welding speed, and gas flow has been investigated. Based on an iconographic correlation approach, optimum process parameters were determined. Depending on the distribution of the power density (circular or annular), acceptable welds were obtained. Powers greater than 1700 W, welding speeds higher than 3.8 m mm^?1, and spot sizes between 160 and 320 μm were needed in the circular (small fiber) configuration. By comparison, the annular (large fiber) configuration required a power as high as 2500 W, and a welding speed less than 3.8 m min^?1. The mechanical properties of the welds depended on their shape and microstructure, which in turn depended on the welding conditions. The content of carbides, the proportion of areas consisting of cellular and dendritic substructures, and the size of these substructures were used to explain the welded joint mechanical properties.