YAG Pulsed Laser Welding Research Articles

Correction of Phase Balance on Nd:YAG Pulsed Laser Welded UNS S32750 Using Cobalt Electroplating Technique

Super-duplex stainless steel (SDSS) shows high mechanical and corrosion resistance because of the balanced structure of austenite and ferrite. However, maintaining this phase ratio after welding is a challenge. The use of austenite stabilizing components is recommended to balance the microstructure. The addition of alloying elements presents a challenge because of the characteristics of Nd:YAG pulsed laser welding. An approach, which has proven to be effective, is to use metal electroplating to prepare the surfaces of the mechanical SDSS components that will be welded, therefore promoting the phase balance in the fusion zone. While the effects of metals such as nickel as an austenite stabilizer are well recognized, cobalt’s effects require more research. The present work investigated the influence of the use of cobalt addition in the joining process by preliminary electroplating on UNS S32750 SDSS Nd: YAG pulsed laser welding, specifically regarding microstructure and microhardness. Three conditions were investigated, changing the thickness of the deposited cobalt layer. The addition of cobalt modified the morphology and increased the volume fraction of austenite. An austenite volume fraction of around 48% was obtained using a 35 μm thick cobalt coating. The microhardness was affected by austenite/ferrite proportions. The microhardness dropped from about 375 HV to 345 HV as the cobalt layer’s thickness rose, being similar to that of the base metal. The effect of cobalt as an austenite stabilizer was observed, and the cobalt electroplating technique was effective to correct the phase balance on UNS S32750 laser welding.

Experiments on Nd:YAG pulsed laser welding of thin sheets

Laser welding of thin sheets raises a number of issues arising from the difficulty to maintain a good contact between the components to be joined. A set of experiments were performed using a Nd:YAG PE-W500II laser welding equipment, aiming to identify solutions to avoid local separation of thin sheets due to the induced thermal influence. The welding results were analysed by optical and scanning electron microscopy. The effects of the current intensity and pulse width had a decisive influence on the quality of the resulting welded joint. Furthermore, the use of a device that ensures good localized contact in the area to be welded, also leading to increased local energy dissipation, was essential to achieve high-quality microwelds.

Effect of pulse overlap rate on the microstructure and mechanical properties of laser welds of thin AZ31 magnesium alloy

In order to improve the quality of pulsed laser welding on magnesium alloy, the effect of pulse overlap rate on the microstructure and mechanical properties of the welds is studied. A Nd:YAG pulsed laser welding machine is used to carry the butt welding experiments on AZ31 magnesium alloy (1 mm), and the pulse overlap rate (10%, 30%, 50%, 70%, 90%) is varied by adjusting the welding velocity. The experimental results show that with the increases of pulse overlap rate, the sensitivity of the solidification cracks and pores decreases. The grain size of the columnar grains at the fusion boundary (FB) and the equiaxed grains in the center of the welds increases and then decreases and reach the maximum value of 114.72 ± 3.06 μm and 74.08 ± 5.87 μm, respectively, when the overlap rate is 70%. The percentage of the ductile fracture in the fracture of tensile specimens is proportional to the pulse overlap rate. With the increases of the pulse overlap rate, the tensile strength and maximum force show a trend of decreasing first and then increasing, and both reach the maximum value of 94.583 MPa and 1.135 kN when the overlap rate is 90%. Besides, the microhardnesses of the welds decrease first and then increase, and it’s maximum value is 86 HV0.2. In summary, the welds with better surface morphology can be obtained at the pulse overlap rate of 70%, while the mechanical properties of the welds are better when the overlap rate is 90%.

Optimizing the mechanical properties of weld joint in laser welding of GTD-111 superalloy and AISI 4340 steel

The present research pertains to the optimization of the mechanical properties of weld joint between GTD-111 superalloy and 4340 steel through Nd:YAG pulsed laser welding process. For this purpose, the response surface methodology in combination with desirability function technique were employed to optimize the laser welding parameters. The parameters of laser power (1500, 2000, 2500 W), welding speed (0.5, 1, 1.5 mm/s) and pulse frequency (5, 10, 15 Hz) were considered to improve the responses of tensile strength, elongation, hardness and weld penetration, simultaneously. The microstructure characteristics of the weld joint were examined using optical and scanning electron microscopy. The results indicated that an increase in the laser power up to 2500 W decreased the tensile strength (from 1017 to 705 MPa) and hardness (from 460 to 375 kgf) due to formation of the Laves phase and nucleation of solidification cracks, arising from segregation of Nb, Ti, Mo, W and Ta elements. The increase of welding speed decreased the amount of Laves particles due to lower heat input, which resulted in an improvement in the tensile strength and hardness up to 911 MPa and 417 kgf, respectively. However, an increase of welding speed reduced the elongation and weld penetration to 12.7 % and 1147 μm, respectively. Concurrent increase of the laser power and pulse frequency associated with the formation of shrinkage voids in HAZ of 4340 steel side and the nucleation of liquation cracks in HAZ of GTD-111 superalloy side. The optimal values of parameters were found to be a laser power of 1812 W, pulse frequency of 15 Hz and welding speed of 1.3 mm/s. Under the optimal conditions, the following values of responses have been obtained: tensile properties = 1013 MPa, elongation = 16.9 %, hardness = 416 kgf and weld penetration = 1165 μm.

Influence of Nickel on the Microstructure, Mechanical Properties, and Corrosion Resistance of Laser-Welded Super-Duplex Stainless Steel

Super-duplex stainless steel (SDSS) exhibits an austenite-ferrite dual-phase structure, which promotes many benefits upon single-phase grades, such as high mechanical strength and corrosion resistance. Welding process results in an unbalanced microstructure, with large amount of ferrite, which compromise SDSS’s properties. This paper investigates the effect of using electrolytic nickel foils as an addition metal on UNS S32750 SDSS Nd:YAG pulsed laser welding, through the evaluation of the microstructure, hardness, tensile strength, and corrosion resistance of the weld bead. Six conditions were investigated: autogenous welding and with addition of nickel, varying the thickness of nickel foil added. Microstructural analysis reveals an increase in volume fraction of austenite for the conditions with addition of nickel. Using a 30 µm thick nickel foil, approximately equal amount of austenite and ferrite was obtained in the weld bead. The higher microhardness was obtained for the autogenous welding, 400 HV and decreased with the addition of nickel. The tensile strength decreased 4% in the experimental conditions with high nickel addition. The corrosion resistances were the same for all the conditions with addition on nickel, regardless of the nickel foil thickness added, but it compared to autogenous welding the CPT’s increased approximately 14 °C.

The effect of heat treatment on creep behavior of GTD-111 superalloy welded by pulsed Nd:YAG laser using small punch test

This work is about the evaluating creep characterizations of GTD-111 superalloy, and its weldment, which has high demand in use in turbine blades of aero-engines and land-based gas turbine industries. For this purpose, plates with a thickness of 3 mm have subjected to Nd:YAG pulsed-laser welding. Test specimens were machined from different parts of weldments and prepared in the shape of disks with a thickness of 0.5 mm, including weld metal, heat-affected zone, and base metal. Small punch creep tests were carried out at 700 °C with loads of 350 N and 450 N to find the weakest location in creep consideration of GTD-111 weldment. Analysis of creep diagrams based on time-displacement showed that the weld metal has the highest creep life due fine γ′ particles and MC carbide. The HAZ of the heat-treated specimen showed a longer creep life of about 17 h than the cast specimen. This was due to the reduction of harmful phases for welding, including γ′, γ-γ′, MC, and Cr-M, which were the leading cause of the defect and reduced creep life in HAZ.

Effect of Nd:YAG pulsed laser welding process on the liquation and strain-age cracking in GTD-111 superalloy

The weldability of GTD-111 nickel-based superalloy by Nd:YAG pulse laser with an average power of 250 W was studied using several pre- and post-weld heat treatment cycle, and the characteristics of liquation, solidification and strain-age cracks were also investigated. The results revealed that liquation cracks in the GTD-111 Nickel-Based Cast Superalloy were associated with the constitutional liquation of γ′ particle, MC carbides, inter-dendritic γ-γ׳ eutectic and melting of Cr-rich boride. Also, cracking during welding occurred at the places that the concentration of Al and Ti is high. Crack-free laser welds were observed in the MT1 conditions (1200 ℃ for 2 h) owing to dissolution of deleterious phases before welding. γ′ phase had little effect on the incidence of cracking in the MT1 condition. An analysis of the microstructures indicated that the cracking was caused primarily by liquation in the as-welded condition and was exacerbated by post-weld heat treatment cracking during the subsequent heat treatment. aging heat treatment of samples which were undergone aging heat treatment before welding, resulted in the formation of strain-age cracking due to γʹ and γ-γ׳ eutectic phases precipitation. The results of micro-hardness indicated that the hardness of fusion zone (FZ) was higher than that of HAZ and base metal zone (BMZ). The liquation cracking in the heat affected zone (HAZ) was observed to be affected by the hardness of base-alloy.

Effects of Nd:YAG Pulsed Laser Welding Parameters on the Electrochemical Corrosion Properties of Carbon-Coated 316L Foils in a Simulated PEMFC Environment

Recently, PEM fuel cell bipolar plates made of carbon-coated AISI 316L austenitic stainless steel have attracted a lot of attention. Parameters suitable for Nd:YAG pulsed laser welding on the lap joint were evaluated in a simulated fuel cell environment for the first time in this study. To this end, the parameters: peak power, pulse duration, and frequency, were optimized using the response surface methodology, and a second-order polynomial was developed to predict the trend of corrosion current density. The potentiodynamic corrosion evaluation proved that with a decrease in pulse duration from 3.5 to 1.5 ms, a decrease in peak power from 650 to 130 W, and a decrease in frequency from 20 to 12 Hz occurred. Corrosion current density decreased by 76%, 34%, and 18%. According to the potentiostatic test, the optimum conditions, i.e., a peak power of 130 W (at 2.5 ms) and pulse duration of 1.5 ms (at 390 W), were achieved at anodic current densities of 0.77 µA/cm2 and 0.61 µA/cm2, respectively.

Numerical Simulation of the Processes of Formation of a Welded Joint with a Pulsed ND:YAG Laser Welding of ZR–1%NB Alloy

In recent years use of Zr-Nb alloys has increased in nuclear and chemical industry due to its corrosion resistance and enhanced strength compared to tin based ones. Welding of zirconium alloys is one of the most critical manufacturing processes for nuclear assembly production. To select suitable welding parameters to achieve quality weld, understanding of temperature and velocity fields during process in fusion zone and heat affected zone are essential. In the present study the Nd:YAG pulsed laser welding of zirconium alloy E110 was simulated using three-dimensional heat and fluid flow model. The convection mode of heat transfer and Marangoni stresses in fusion zone are two important mechanisms in controlling the heat transfer weld bead size. The calculated heating and cooling rates are of typical in laser welding and useful in microstructure study of fusion and heat affected zones. Experiments were carried with varying peak power, pulse frequency and duration using Nd:YAG pulsed laser on 0.5 mm thick sheets of E110 to form butt joints. The comparison of the results shows that the weld geometry is well matched with the numerical model.

Study on the acoustic emission effect of plasma plume in pulsed laser welding

Abstract The acoustic emission (AE) signals were detected in real-time during the process of pulsed YAG laser welding. Some characteristic analysis methods, including AE count statistic, root mean square (RMS) waveform calculation and power spectrum distribution, were used to analyze the AE signals. According to the AE signals, the characteristics of plasma plume and penetration of weld bead were studied. The results showed that the source mechanism for AE generation was the recoil force and thermal vibration generated by the plasma plume acting on molten pool. The AE signals detected in welding contained a lot of valuable information about plasma plume. AE count, RMS waveform and power spectrum of AE signals showed the energy characteristics of plasma plume in time and frequency domain. Therefore, AE signals detected in pulsed laser welding had clear correlations with plasma plume, which were valuable to evaluate the plasma plume and the penetration of weld bead.

Effects of Nickel Addition on the Microstructure of Laser-Welded UNS S32750 Duplex Stainless Steel

Welding duplex stainless steels with Nd:YAG pulsed laser welding results in an unbalanced microstructure. The present article reports the effects of nickel addition on the microstructure of UNS S32750 duplex stainless steel welded with an Nd:YAG pulsed laser. Two conditions were used: autogenous welding and welding with the addition of nickel. The addition of nickel resulted in equal proportions of austenite and ferrite in the weld bead, affecting the microstructure.

Experimental Investigation on Mechanical and Distortion Characteristics of Titanium/Aluminium Dissimilar Metal Joint Using Laser Beam Welding

Laser beam welding (LBW) is based on interaction between the laser source and base metals. Different methods have been developed recently to produce weld joints of light metals. This produces good weld bead to simplify the structure and reduce the weight and cost to meet the important concerns of aerospace industry. To achieve these, Ti (Ti6Al4V) and Al (AA2024) dissimilar alloy sheets are welded with butt joint configuration using Nd: YAG pulsed laser welding unit. The weldment is subjected to tests to evaluate mechanical and distortion characteristics. From the test results, it is found that LBW is very much suitable for joining Titanium/Aluminium (Ti/Al) alloy sheets.

Studies on weldment performance of Ti/Al dissimilar sheet metal joints using laser beam welding

Laser beam welding is more advantageous compared to conventional methods. Titanium/Aluminium dissimilar alloy thin sheet metals are difficult to weld due to large difference in melting point. The performance of the weldment depends upon interlayer formation and distribution of intermetallics. During welding, aluminium gets lost at the temperature below the melting point of titanium. Therefore, it is needed to improve a new metal joining techniques between these two alloys. The present work is carried for welding TI6AL4V and AA2024 alloy by using Nd:YAG Pulsed laser welding unit. The performance of the butt welded interlayer structures are discussed in detail using hardness test and SEM. Test results reveal that interlayer fracture is caused near aluminium side due to low strength at the weld joint.

Magnesium Loss in Nd:YAG Pulsed Laser Welding of Aluminum Alloys

In the current investigation, the loss of magnesium (Mg) in Nd:YAG pulsed laser welding of a number of aluminum (Al) alloys was studied both experimentally and numerically. The experimental results showed that at a fixed average laser power, with the increasing pulse frequency, Mg concentration of the weld metals decreased. A numerical model was developed for the calculation of Mg evaporation from the weld pool. The model predicted a trend in the changes of Mg concentration of the weld pool, working best for alloys containing the higher levels of Mg. The calculations showed that with the increasing pulse frequency from 15 to 40 Hz, due to a decrease in the average temperature of the weld pool, Mg loss in terms of mass per unit area of the weld spot actually decreased from 230 × 10−5 to 120 × 10−5 kg/m2. However, due to a decrease in the weld penetration/volume, the effective Mg loss of the weld metal in terms of concentration increased from 110 × 10−1 to 260 × 10−1 kg/m3. It was shown that due to interactions between Mg and other main alloying elements, the activity coefficient of Mg had to be modified for the model to accurately predict Mg evaporation of the weld pool.

Effect of heat treatment on mechanical properties and microstructure of Ti/Al sheet metal joint using laser beam welding

Industry demands new materials to meet its requirements for that advanced technique are adopted to find solution through research. One such attempt is use of laser beam welding to join Ti/Al dissimilar alloy for the aerospace and other applications. It is still a challenge to achieve the satisfactory joint between these dissimilar alloys. In the present work, Ti6Al4V and AA2024 alloys thin sheet combinations are welded using the Nd: YAG pulsed laser welding unit. Subsequently the welded joints are subjected to age hardening and the results are compared with weld joints prior to aging. Test results reveal that aging treatment brings strength for the structure more than 60% and laser beam welding is suitable for dissimilar metal joining.

Nd:YAG pulsed laser welding of TC4 Ti alloy to 301L stainless steel using Ta/V/Fe composite interlayer

The present article reports the feasibility of Nd:YAG laser welding of Ti alloy to stainless steel through composite layer Ta/V/Fe. The laser beam was focused on the centerline of Ta and Fe sheets respectively to ensure that V interlayer was not completely melted. Two pass welding was employed involving creation of a joint with two weld zones separated by remaining unmelted V. Composite interlayer Ta/V/Fe was used to prevent the formation of Ti-Fe intermetallics and improve microstructure and properties of the Ti alloy-SS joint. The joint fractured at the unmelted V with the tensile strength of 627 MPa.

Nd:YAG pulsed laser welding of dissimilar metals of titanium alloy to stainless steel

Pulsed laser welding of TC4 titanium (Ti) alloy to SUS301L stainless steel (SS) has been applied using pure Nb as an interlayer. Microstructures of the joints were analyzed by means of scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Mechanical properties of the joints were evaluated by tensile tests. Based on avoidance of the formation of Ti-Fe intermetallics in the joint, two welding processes for the Ti alloy-Nb-SS joint were introduced. One process was one pass welding involving creation of a joint with one weld zone and a reaction layer separated by remaining unmelted Nb. The weld was formed while the laser was acted on the Ti alloy-Nb interface, and simultaneously, the reaction layer was formed based on interdiffusion and eutectic reaction of Nb and Fe at the Nb-SS interface. The mechanical performance of the joint was determined by the reaction layer at the Nb-SS interface with a tensile strength of 370 MPa. The other was two pass welding involving creation of a joint with two weld zones separated by remaining unmelted Nb, while the laser beam was focused on the Ti alloy-Nb interface and the Nb-SS interface, respectively. At this point, the mechanical performance of the joint was determined by the weld zone formed at the Nb-SS interface with a tensile strength of 171 MPa.

Laser micro-welding technology for Cu–Al dissimilar metals and mechanisms of weld defect formation

To study laser micro-welding technology for copper–aluminum (Cu–Al) dissimilar metals and the intrinsic mechanisms of weld defect formation, we used a 300-W energy negative-feedback Nd:YAG pulsed laser welding system for laser micro-welding of two dissimilar metals, i.e., a Φ0.25-mm thin Cu wire and an Al pin. The former has a high melting point and good fluidity, whereas the latter has poor fluidity, is hard to weld, is easily oxidizable, and easily forms weld defects such as blowholes. Under laser irradiation, the Cu metal was melted to coat the Al material, which then melts, followed by solution-diffusion to accomplish the welding. We selected a laser beam with a rapid ascending and slow descending waveform, a pulse width of 3.0 ms, a peak power of 0.7 kW, and a single pulse energy of 3.3 J to enable successful welding at a negative defocus of −0.5 mm. Finally, the microscopic mechanisms of weld formation during laser micro-welding of Cu–Al dissimilar metals were analyzed; the causes of the formation of hot cracks and blowholes were analyzed in detail from aspects of Cu–Al dissimilar metals such as physical properties, lattice structure, and latent heats of solidification, and accordingly, solutions are proposed to provide a reliable theoretical basis for laser welding of Cu–Al dissimilar metals.

Effects of Cr2O3 Activating Flux on the Plasma Plume in Pulsed Laser Welding

The effects of Cr2O3 activating flux on pulsed YAG laser welding of stainless steel and, particularly, on the behavior of the plasma plume in the welding process were investigated. According to the acoustic emission (AE) signals detected in the welding process, the possible mechanism for the improvement in penetration depth was discussed. The results indicated that the AE signals detected in the welding process reflected the behavior of the plasma plume as pulsed laser energy affecting the molten pool. The root-mean-square (RMS) waveform, AE count, and power spectrum of AE signals were three effective means to characterize the behavior of the plasma plume, which indicated the characteristics of energy released by the plasma plume. The activating flux affected by the laser beam helped to increase the duration and intensity of energy released by the plasma plume, which improved the recoil force and thermal effect transferred from the plasma plume to the molten pool. These results were the main mechanism for Cr2O3 activating flux addition improving the penetration depth in pulsed YAG laser welding.

Hot cracking in laser welding of Hastelloy X with pulsed Nd:YAG and continuous wave fiber lasers

Hastelloy X is a corrosion and heat resistance solution hardened nickel base super alloy. This alloy having an austenitic microstructure and in this regard is expected to be prone to hot cracking in fusion welding. In this research a 400W Nd:YAG pulsed laser and 600W continuous wave fiber laser welding are employed to weld a 2mm thick Hastelloy X plates. Comparison is made with regard to weld penetration and tendency of occurrence of solidification cracking in the weld metal and liquation cracks in the heat affected zone. It is found that with pulsed laser hot cracks are formed, however, with continuous wave laser hot cracking can be avoided. Increasing the laser pulse frequency and pulse duration resulted in a significant reduction in hot cracking tendency in pulsed laser welding. The reduction in cooling rate and thermal stresses during solidification and backfilling of liquated grain boundaries are proposed to be the factors contributing to the prevention or reduction of hot cracks in laser welding of Hastelloy X. With a 0.2mm beam diameter fiber laser at 500W power and 5mm/s travel speed full penetration weld was achieve with no observation of hot cracks in the weld cross sections.