Cheolhee Kim

Influence of driving forces on weld pool dynamics in GTA and laser welding

IntroductionThere are several driving forces such as surface tension, buoyancy force, arc pressure, electromagnetic force, recoil pressure, drag force, etc. in gas tungsten arc (GTA) welding and laser welding, which have different influences on flow dynamics in the weld pool.Mathematical modelsThis paper investigates the influence of respective driving forces on weld pool dynamics by using mathematical models and numerical simulations. In numerical simulations, the flow patterns in the weld pool and the maximum fluid velocity caused by respective driving forces can be observed, since driving forces can be applied separately.Results and discussionAs all driving forces are applied under experimental conditions, the results of experiments and numerical simulations were compared to validate the numerical simulations and mathematical models used in this paper. In GTA welding, Marangoni flow can be considered as the most dominant force in the radial direction, while the velocity magnitudes of the z-axis direction of all respective forces except buoyancy and drag force are almost the same. The influence of buoyancy force is negligible. In keyhole laser welding, however, recoil pressure can be considered the dominant force, while the other driving forces have only a negligible influence on fluid dynamics in deep keyhole welding. In laser–GTA hybrid welding, recoil pressure can be considered the dominant force.

CO2 laser-micro plasma arc hybrid welding for galvanized steel sheets

A laser lap welding process for zinc-coated steel has a well-known unsolved problem-porosity formation. The boiling temperature of coated zinc is lower than the melting temperature of the base metal, which is steel. In the autogenous laser welding, the zinc vapor generates from the lapped surfaces expels the molten pool and the expulsion causes numerous weld defects, such as spatters and blow holes on the weld surface and porosity inside the welds. The laser-arc hybrid welding was suggested as an alternative method for the laser lap welding because the arc can preheat or post-heat the weldment according to the arrangement of the laser beam and the arc. CO2 laser-micro plasma hybrid welding was applied to the lap welding of zinc-coated steel with zero-gap. The relationships among the weld quality and process parameters of the laser-arc arrangement, and the laser-arc interspacing distance and arc current were investigated using a full-factorial experimental design. The effect of laser-arc arrangement is dominant because the leading plasma arc partially melts the upper steel sheets and vaporizes or oxidizes the coated zinc on the lapped surfaces. Compared with the result from the laser-TIG hybrid welding, the heat input from arc can be reduced by 40%.

Weld strength of laser-welded hot-press-forming steel

There is an increasing demand for ultrahigh-strength steel in automotive industries to facilitate the manufacture of lightweight vehicles without compromising their security features. Although transformation-induced plasticity and dual phase steels have strengths of under 1 GPa, boron-alloyed steel obtained after the hot-press process has a strength of over 1500 MPa. Al–Si-coated steel has been developed to prevent excessive oxidation during high-temperature processes, but the Al–Si coating is known to lower weld properties. In this research, the laser weldability of hot-press-forming steels with and without Al–Si coating was examined. The specimens were laser-welded in butt and overlap joints by using disk and fiber lasers. The influence of the process parameters on the weld strength was investigated.

Porosity formation mechanisms in cold metal transfer (CMT) gas metal arc welding (GMAW) of zinc coated steels

The porosity formation in cold metal transfer (CMT) gas metal arc welding (GMAW) of zinc coated steel is studied over a wide range of the heat inputs (160–250 J mm− 1), which shows low porosity in weld bead ( < 2% of bead area) in low ( < 250 J mm− 1) and high (>350 J mm− 1) heat inputs and maximum at medium (250–350 J mm− 1) heat inputs. The high speed imaging of weld pool shows that the highest frequency of zinc vapour escapes at high heat inputs compared to other conditions. Numerous experiments show that size and location of pores along with escaping of zinc vapour are the results of competition of viscosity of weld pool against buoyancy and vapour pressure within the time required to reach solidification temperature. Based on this concept, mechanisms involved in porosity formation, growth and escape phenomena are disclosed, which can help select the optimised welding conditions to obtain porosity free welds in CMT-GMAW of zinc coated steels.

Solidification crack and morphology for laser weave welding of Al 5J32 alloy

Abstract Linear and weaving laser welding were performed on a self-restraint tapered specimen of an Al 5J32 alloy. Linear welding produced columnar grains along the fusion line and equiaxed grains along the centreline. Solidification crack developed along the centreline having equiaxed grains. For laser weave weld, the solidification crack disappeared at a weaving frequency of 5 Hz. However, as the weaving frequency increased further, the length of the columnar grains inside the weaving trajectory curve became smaller than that outside the curve, and the equiaxed grains did not necessarily grow along the centreline of the weld. Therefore, the wide equiaxed grains deviated from the transverse weaving profile and a near linear solidification crack developed. The simulated morphology using solidification rate and temperature gradient correlated well with the solidification morphology obtained from the experiments. The limiting boundary condition for differentiating between the columnar and the equiaxed microstructures in the alloy was G = 3·5R.

High deposition rate pulse gas metal arc welding for Al 5083 thick plate

The recent rapid advances in digital inverter-based welding power sources have allowed the realization of pulse gas metal arc welding, which can achieve stable metal transfer even at low welding current and thus reduce heat input to the base material. Moreover, a tandem-wire process using two electrodes can dramatically increase welding productivity. In such a process, two pulse power sources are synchronized and the interference between the adjacent welding arcs is minimized. In this study, high deposition rate pulse welding for Al 5083 alloy was implemented by using Al 5183 wire of three different diameters—1.6, 2.4, and 3.2 mm. Unlike the single-wire process, tandem-wire welding was only implemented using 1.6-mm-diameter wire. The bead shape, deposition rate, process windows, and stability were evaluated for each process. The results indicate that large wire diameters and the tandem process are preferable for high deposition rates, and the penetration depth is dependent on the welding current and speed. The tandem process achieves a remarkably high deposition rate, but spatter generation at high-current levels needs to be addressed.

An Evaluation of Global and Local Tensile Properties of Friction-Stir Welded DP980 Dual-Phase Steel Joints Using a Digital Image Correlation Method

The effect of the microstructure heterogeneity on the tensile plastic deformation characteristic of friction-stir-welded (FSW) dual-phase (DP) steel was investigated for the potential applications on the lightweight design of vehicles. Friction-stir-welded specimens with a butt joint configuration were prepared, and quasi-static tensile tests were conducted, to evaluate the tensile properties of DP980 dual-phase steels. The friction-stir welding led to the formation of martensite and a significant hardness rise in the stir zone (SZ), but the presence of a soft zone in the heat-affected zone (HAZ) was caused by tempering of the pre-existing martensite. Owing to the appearance of severe soft zone, DP980 FSW joint showed almost 93% joint efficiency with the view-point of ultimate tensile strength and relatively low ductility than the base metal (BM). The local tensile deformation characteristic of the FSW joints was also examined using the digital image correlation (DIC) methodology by mapping the global and local strain distribution, and was subsequently analyzed by mechanics calculation. It is found that the tensile deformation of the FSW joints is highly heterogeneous, leading to a significant decrease in global ductility. The HAZ of the joints is the weakest region where the strain localizes early, and this localization extends until fracture with a strain near 30%, while the strain in the SZ and BM is only 1% and 4%, respectively. Local constitutive properties in different heterogeneous regions through the friction-stir-welded joint was also briefly evaluated by assuming iso-stress conditions. The local stress-strain curves of individual weld zones provide a clear indication of the heterogeneity of the local mechanical properties.

Analysis of Laser and Resistance Spot Weldments on Press-Hardened Steel

For the manufacture of safe, lightweight vehicles, the demand for ultra-high-strength steel in the automotive industry is increasing. Although transformation-induced plasticity (TRIP) and dual-phase (DP) steels have a strength of under 1 GPa, boron-alloyed steel produced using the hot press forming process has a strength of more than 1500 MPa. Laser and resistance spot welding processes are used to join press-hardened steel, but the characteristics of the resulting weldments are not yet fully understood. In this study, the thermal cycles for both welding processes were investigated using finite element (FE) analysis. Resistance spot welding was analyzed using a combination of thermal, electric, and mechanical models, whereas the thermal behavior of laser welding was predicted using only a thermal model. The calculated bead shapes were compared with experimentally measured ones to validate the simulation models. The mechanical and metallurgical characteristics of the weldments were explained using the thermal history of each welding process.

Effects of Weaving Laser on Weld Microstructure and Crack for Al 6k21-T4 Alloy

Abstract For Al 6k21-T4 overlap weld joint, the shear-tensile strength by using the weaving laser was improved as compared to the case of linear laser. For the specimen of low strength, the porosity was distributed continuously along the intersection between the plates and fusion line. However, for the optimized welding condition, large oval-shaped porosities were located only in the advancing track of the concave part. Therefore, the continuity of cracks and porosities played a key role to determine the strength. And, the weaving width was also the important parameter to control the strength. Furthermore, the concave part had more significant hot and cold cracking in the weld and heat-affected zone (HAZ), respectively, than the convex part.

Back Bead Characteristics during Butt Welding of a Thick Plate for Various Backing Conditions

In a welded structure, thick plates are joined by multi-pass welding in the butt joint. During the first pass of multi-pass welding, burn-through, lack of fusion, and incomplete penetration were readily found as weld defects. Accordingly, the backing condition should be carefully selected in welding of a thick plate, because improper backing conditions lead to weld defects. In the job site, a steel backing strip is usually adopted, although it reduces the fatigue strength. No backing conditions or removable backing is recommended to increase the fatigue strength, but selection of the proper backing and welding conditions is complicated. In this study, several backing methods, such as ceramic backing, water-cooled copper backing, and even the use of no backing, were investigated during GMA (Gas Metal Arc) welding of a thick steel plate. The gas metal arc welding conditions were established for each backing method, and the bead shape and mechanical properties were examined after welding.