Laser-welded Joints Research Articles

Ensuring the strength of welded joints in laser welding of EP693 heat-resistant dispersion-hardening nickel alloy

The paper considers the process of creating a permanent connection from the EP693 heat-resistant alloy of the Ni-Cr-W-Co-Mo system used in the manufacture of components and parts of gas turbine engines by welding on the TruLaser Cell 7020 CO2 complex with pulse-periodic radiation. EP367 filler wire of the Ni—Mo—Cr—Mn system was used to obtain the weld. The influence of heat treatment on the structure and properties of the heat-affected zone and the weld was studied. Based on the research results, the weld structure and kinks obtained by laser welding was studied, weld physical and mechanical properties were identified, the maximum endurance limit for welded joints was determined at 2•106 cycles. The expediency of laser welding of the heat-resistant dispersion-hardening nickel alloy in the manufacture of shells for the turbine support and stator of gas turbine engines was determined. It was found that combined heat treatment (quenching and aging) provides optimal values of strength limits at room and elevated temperatures, as well as short-term strength of welded joints. Based on the strength calculation of the turbine support and stator of gas turbine engines and the obtained experimental data on the strength of welded joints made using laser welding with pulse-periodic radiation, the safety factor was 1.35 to 3.0. This technology is proposed to be introduced into production in the manufacture of parts and assemblies such as shells for the turbine support and stator of gas turbine engines in order to improve the quality of welds by reducing the time of high-temperature heating due to lower heat input.

∝-Ferrite Suppression during Fiber Laser Welding of Al-Si Coated 22MnB5 Press-Hardened Steel

During laser welding of an Al-Si coated 22MnB5 steel to produce tailor-welded blanks, the Al-Si coating mixes into the weld and causes the formation of the lower strength ferrite phase dispersed in an otherwise martensitic matrix. It has been shown that the presence of the ferrite phase is the principal reason for premature failure of hot-stamped laser-welded joints. Currently, the Al-Si coating is removed prior to welding, which can be time consuming. This work showed that adding Ni to the fusion zone of laser welded Al-Si coated 22MnB5 steel by welding through a pure Ni coating of a specified thickness, ferrite formation can be suppressed, whereby improving the weld strength and successfully shifting failure from the fusion zone, where it normally occurs, to the base material to achieve 100%joint strength. This work also showed that laser welding Al-Si coated 22MnB5 steel through a Ni coating eliminated the need to mechanically or chemically remove the Al-Si coating prior to welding.

Crash Response of Laser-Welded Energy Absorbers Made of Docol 1000DP and Docol 1200M Steels.

The crushing response of a laser-welded square tube absorber made of two commercial steel grades, Docol 1000DP and Docol 1200M, is presented in the paper. Crush experiments are performed at two different loading conditions, namely, quasi-static loading at 0.5 mm/s deformation speed and impact loading at 25–28 m/s. A new approach has been proposed to study the square tube absorber under impact loading using a direct impact Hopkinson (DIH) method. To characterize the mechanical properties of the tested steels, tensile quasi-static and high strain rate testing are also performed with the use of specimens with a 7 mm gauge length. The applied strain rates are 10−3, 100, and above 103 s−1. The laser-welded joints are also characterized by microhardness test involving the base material, heat-affected zone, and fusion zone. The crashworthiness of model square tube absorbers is estimated based on the following parameters: absorbed energy, mean force, crushing force efficiency factor, and specific energy absorbed. It has been found that the square tube absorbers made of Docol 1200M steel show a higher potential in mechanical energy absorption capacity than Docol 1000DP absorber. Moreover, crushing tests prove that laser-welded joints in 0.6 mm sheets made of Docol 1000DP and Docol 1200M steels reveal high cracking toughness. In turn, strength testing at different strain rates confirms the higher strain rate sensitivity of Docol 1000DP steel than in the case of Docol 1200M steel as well as an increase in the high ductility properties of both steel grades under the high strain rate loading conditions.

Assessment of mechanical and biocompatible performance of ultra-large nitinol endovascular devices fabricated via a low-energy laser joining process.

Nitinol is an excellent candidate material for developing various self-expanding endovascular devices due to its unique properties such as superelasticity, biocompatibility and shape memory effect. A low-energy laser joining technique suggests a high potential to create various large diameter Nitinol endovascular devices that contain complex geometries. The primary purpose of the study is to investigate the effects of laser joining process parameters with regard to the mechanical and biocompatible performance of Nitinol stents. Both the chemical composition and the microstructure of the laser-welded joints were evaluated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). In vitro study results on cytotoxicity demonstrated that the joining condition of 8 Hz frequency and 1 kW laser power showed the highest degree of endothelial cell viability after thermal annealing in 500°C for 30 min. Also, in vitro study results showed the highest oxygen content at 0.9 kW laser power, 8 Hz frequency, and 0.3 mm spot size after the thermal annealing. Mechanical performance test results showed that the optimal condition for the highest disconnecting force was found at 1 Hz frequency and 1 kW power with 0.6 mm spot size. Two new endovascular devices have been fabricated using the optimized laser joining parameters, which have demonstrated successful device delivery and retrieval, as well as acute biocompatibility.

Improving the Properties of Laser-Welded Al-Zn-Mg-Cu Alloy Joints by Aging and Double-Sided Ultrasonic Impact Compound Treatment.

To improve the loose structure and serious porosity of (Al–Zn–Mg–Cu) 7075 aluminum alloy laser-welded joints, aging treatment, double-sided ultrasonic impact treatment (DSUIT), and a combination of aging and DSUIT (A–DSUIT) were used to treat joints. In this experiment, the mechanism of A–DSUIT on the microstructure and properties of welded joints was analyzed. The microstructure of the welded joints was observed using optical microscopy, scanning electron microscopy, and electron backscatter diffraction (EBSD). The hardness and tensile properties of the welded components under the different processes were examined via Vickers hardness test and a universal tensile testing machine. The results showed that, after the aging treatment, the dendritic structure of the welded joints transformed into an equiaxed crystal structure. Moreover, the residual tensile stress generated in the welding process was weakened, and the hardness and tensile strength were significantly improved. After DSUIT, a plastic deformation layer of a certain thickness was generated from the surface downward, and the residual compressive stress was introduced to a certain depth of the joint. However, the weld zone unaffected by DSUIT still exhibited residual tensile stress. The inner microhardness of the joint surface improved; the impact surface hardness was the largest and gradually decreased inward to the weld zone base metal hardness, with a small improvement in the tensile strength. Compared with the single treatment process, the microstructural and mechanical properties of the welded joint after A–DSUIT were comprehensively improved. The microhardness and tensile strength of the welded joint reached 200 HV and 615 MPa, respectively, for an increase of 45.8% and 61.8%, respectively. Observation of the fractures of the tensile specimens under the different treatment processes showed that the fractures before the aging treatment were mainly ductile fractures while those after were mainly brittle fractures. After DSUIT of the welded joints, a clear and dense plastic deformation layer was observed in the fracture of the tensile specimens and effectively improved the tensile properties of the welded joints. Under the EBSD characterization, the larger the residual compressive stress near the ultrasonic impact surface, the smaller the grain diameter and misorientation angle, and the lower the texture strength. Finally, after A–DSUIT, the hardness and tensile properties improved the most.

Effect of parameters on microstructure and mechanical property of dissimilar joints between 316L stainless steel and GH909 alloy by laser welding

Abstract This paper mainly studied the effects of high-power high-welding speed and low-power low-welding speed on 316 L/GH909 dissimilar metals laser welding joints under the same linear energy. The weld morphology, microstructure and mechanical property were compared and analyzed. The different dimensions of the molten pool between the two welds were verified by the finite element temperature field simulation, and the reason was discussed. From the results, the weld with low-power low-welding speed had a smaller temperature gradient and a lower cooling rate at the edge of the molten pool. The condition of small temperature gradient effectively made the element distribution smoother, relieved the microstructure heterogeneity of the interface layer and alleviated the liquefaction of grain boundaries at the HAZ, thus improving the tensile strength of the welded joint. Since both alloys were based on austenite, there was no common intermetallic compound in this type dissimilar joints. The detailed and systematic elemental analysis of the different phases in the 316 L/GH909 dissimilar metal weld was carried out. Especially, we found that a small amount of Cr element was solid solutioned in γ/Laves low melting phases, and the granular phases distributed at the HAZ of GH909 side were analyzed. This study mainly revealed the nuances between the two welds under the same linear energy, which can provide guidance for this type of GH909/316 L dissimilar metal welding.

Follow-up ultrasonic vibration assisted laser welding dissimilar metals for nuclear reactor pump can end sealing

A novel follow-up ultrasonic vibration assisted welding process was proposed to reduce the common defects in laser welding of dissimilar metals for nuclear reactor pump can end sealing, such as unmixed zone, precipitated phase, and uneven element distribution. The influences of ultrasonic vibration on morphology, microstructure and property of C-276/304 laser-welded joint was analyzed. The results indicated that, the most significant ultrasonic vibration at welding position was achieved at parameters of 25 N pressing force and 13 mm distance from ultrasonic action point to welding position. By the application of follow-up ultrasonic vibration, the width of unmixed zone was reduced and the macro element distribution was homogenized due to the enhancement of molten convection. The average grain radius was refined from 1.03 μm to 0.55 μm because acoustic pressure in the molten pool reduced critical nucleus size rk and caused grain fragmentation. The higher diffusion coefficient and cooling rate caused by ultrasonic vibration suppressed element segregation, and thus the amount of precipitated phase decreased from 2.15% to 0.62%, which finally improved the intergranular corrosion resistance of weld metal. The load capacity of weld joint was improved by 12.6%, and the micro-hardness of weld metal was also increased, attributing to the grain refinement strengthening. The follow-up ultrasonic vibration process proposed in this paper provided an efficient, convenient and widely applicable process for laser seal welding of dissimilar metals for nuclear reactor pump can end sealing with higher qualities.

The laser welded joints properties of cryogenic austenitic nitrogen-containing steel*

ABSTRACT Formation, structure and properties of welded joints in cryogenic austenitic nitrogen containing steel of chromium – nickel – manganese – nitrogen alloying system with reduced nickel content are investigated. Under optimal laser welding conditions, a weld of small width with almost parallel walls is formed. This helps to reduce residual stresses and strains. There is a significant reduction in the structure size parameters of the weld compared to arc welding. The mechanical properties of laser welds are within acceptable limits, so the tensile strength is 0.84–0.96 of the base metal. Fractographic analysis of the fracture surface of welded samples after testing showed the presence of developed zones of plastic deformation, which characterizes the laser-welded joint as highly plastic.

A Study on Lap Joint Welding of Thin Plate ASTM F1684 Using Fiber Laser Welding

The International Maritime Organization (IMO) has developed stricter regulations on emission standards for sulfur oxides, etc., and the demand for Liquefied Natural Gas (LNG) is increasing as an alternative to satisfy these standards. This study relates to fiber laser welding, an approach which offers high-speed welding and low welding deformation for ASTM F1684, which has a low coefficient of thermal expansion (CTE) even in a cryogenic environment. In this study, through three preliminary experiments using 0.25 mm thick Invar, the conditions required to secure sufficient penetration depth and back bead were identified. Through the cross-sectional observation analysis, the welding conditions without defects were identified and the trend of penetration shape according to increasing welding speed was identified. Following a lap joint laser welding experiment under the secured conditions, the mechanical properties were evaluated through the shear strength test and the heat influence range of a fiber laser was identified through the temperature measurement of a welding part. As a result, it was confirmed that the shear strength of the lap joint laser welding part was 86.8% that of the base metal.

Low-Cycle Fatigue Properties of Ti6Al4V Laser-Welded Joints Based on a Local Strain Approach

The low-cycle fatigue (LCF) properties of Ti6Al4V and laser-welded joints with different welding speeds are studied using smooth and notched flat specimens. A local strain approach is proposed to estimate the LCF properties of the notched flat specimen based on elastic–plastic finite element analysis (EPFEA). The local strain used in the approach is defined as the average strain in the plastic zone at the notch root and critical section of the notched flat specimen. The results indicate that the local strain approach is suitable for estimating the LCF properties of the notched flat Ti6Al4V titanium alloy specimen. The stress–strain relationship derived from the microhardness can be used in the EPFEA. The LCF properties of the Ti6Al4V laser-welded joints are worse in the heat-affected zone (HAZ) than in the base metal (BM) due to the martensite structure in the HAZ and the mechanical heterogeneity in the welded joints. The LCF properties of the Ti6Al4V laser-welded joints with different welding speeds are close under low strain levels and worse with higher welding speeds under high strain levels.

An Investigation on Laser Welding Parameters on the Strength of TRIP Steel

The energy required for joining steel segments by using laser welding is relatively very low compared with arc welding, gas welding, or any other conventional welding techniques. Moreover, the rapid cooling may create a significant effect on different regions, such as the fusion zone (FZ), heat affected zone (HAZ), and base metal (BM), and in turn affect different parameters. In this study, the characteristics of the laser-welded joint were investigated by varying laser power, welding velocity and incident angle, and tensile strength. In our, experiments. the microhardness was increased by varying the power of laser welding. The strength of the joint was increased to 549 MPa with 2200 W high power, 30 mm/s velocity, and 80º laser incident angle. By increasing the power and velocity of the laser, the welding gun strength was improved; conversely, the angle of laser incident on the welding location decreased while its strength was increased.

Influence of Assembly Gap Size on the Structure and Properties of SUS301L Stainless Steel Laser Welded Lap Joint.

The microstructure and properties of laser welding lap joints with different assembly gap sizes are experimentally investigated. The laser weld joint is composed of γ-austenite and δ-ferrite, and the strip ferrite phase is mainly distributed at the austenite grain boundary. The weld metal presents the austenitic-ferritic (AF) solidification mode. When there is a gap between the two plates, a triangular region composed of similar equiaxed crystals can be found, and the size of the cellular crystals in this region decreases significantly. When the assembly gap size increases from 0.1 mm to 0.4 mm, the weld penetration state of the joint changes from full penetration to semi-penetration, and the surface collapse increases. The excessive size of the gap leads to a decrease in the tensile-shear force and fatigue strength of laser welded joints. In order to ensure that the surface morphology and properties of the welded joint meet the quality standard and requirement, the assembly gap should be less than 0.1mm.

Optimizing weld morphology and mechanical properties of laser welded Al-Si coated 22MnB5 by surface application of colloidal graphite

In the automotive industry, the demand for reduced vehicle weight, improved safety and enhanced crashworthiness qualities continues to rise which introduces the need for parts with tailored properties. This demand can be met by employing tailor-welded blanks which are made of two or more types of press-hardened steels that are laser welded together and then hot-stamped to produce highly optimized parts for use in vehicle body-in-white applications. The most commonly used press-hardened steel for high strength applications is the Al-Si coated 22MnB5 grade steel that has a strength of around 1500 MPa in the hot-stamped condition. Although the Al-Si coating offers excellent barrier protection against oxidation and decarburization during hot-stamping, the Al-Si coating proves to be problematic during laser welding as it melts and mixes into the weld pool increasing the Al-content in the fusion zone, which promotes the formation of ferrite during the heat-treatment stage of hot-stamping, resulting in a dual-phase fusion zone microstructure that consists of ferrite grains embedded in a martensitic matrix. The presence of the softer ferrite phase has been shown to be the principal reason for premature failure of hot-stamped laser-welded joints. Currently, this issue is resolved by an additional, and relatively costly, manufacturing step that involves the laser ablation of the Al-Si coating prior to welding. The present work shows that this step can be eliminated from the manufacturing process by welding Al-Si coated 22MnB5 steel through a colloidal graphite coating which results in the in-situ ablation of the Al-Si coating during welding. This changes the alloying chemistry of the fusion zone which affects the resulting morphology by preventing ferrite formation which improves the mechanical properties of the weld. Failure in these welded samples was successfully shifted from the fusion zone, where it normally occurs, to the base material.

Influence of Thermal Modification on Al-Si Coating of Hot-Stamped 22MnB5 Steel: Microstructure, Phase Transformation, and Mechanical Properties

The hot-stamped steel with ultrahigh strength is a promising material for the fabrication of automotive components. However, the coating on the sheet surface leads to a softening problem in the welded joint. Instead of the costly coating removal process, heat treatment is an economical and effective method for the diffusion process, which can decrease the Al concentration in the coating. In this study, a preheating treatment was carried out on Al-Si-coated 22MnB5 hot-stamped steels for the homogeneity of Al, followed by laser welding and hot stamping. The effects of the preheating on the microstructure and mechanical properties of the laser-welded joints were investigated. With the preheating treatment, the Al-Si coating transformed into an Fe-Al intermetallic compound and the difference in Al content between the coating and substrate was reduced. The Al content in the weld of the specimen with the preheating treatment was reduced, compared with that without the preheating treatment. The amount of δ-ferrite in the weld after laser welding was reduced largely. The distribution of long-bland-like segregation was changed to a fine and uniform distribution. With the preheating treatment, the tensile strength of the welded joint was significantly improved and comparable to that of the decoated joint. In conclusion, the preheating treatment before the welding is an effective method to suppress the formation of δ-ferrite and improve the mechanical properties of the welded joint.

Microstructure and Mechanical Properties of Laser-Welded DP Steels Used in the Automotive Industry

The aim of this work was to investigate the microstructure and the mechanical properties of laser-welded joints combined of Dual Phase DP800 and DP1000 high strength thin steel sheets. Microstructural and hardness measurements as well as tensile and fatigue tests have been carried out. The welded joints (WJ) comprised of similar/dissimilar steels with similar/dissimilar thickness were consisted of different zones and exhibited similar microstructural characteristics. The trend of microhardness for all WJs was consistent, characterized by the highest value at hardening zone (HZ) and lowest at softening zone (SZ). The degree of softening was 20 and 8% for the DP1000 and DP800 WJ, respectively, and the size of SZ was wider in the WJ combinations of DP1000 than DP800. The tensile test fractures were located at the base material (BM) for all DP800 weldments, while the fractures occurred at the fusion zone (FZ) for the weldments with DP1000 and those with dissimilar sheet thicknesses. The DP800-DP1000 weldment presented similar yield strength (YS, 747 MPa) and ultimate tensile strength (UTS, 858 MPa) values but lower elongation (EI, 5.1%) in comparison with the DP800-DP800 weldment (YS 701 MPa, UTS 868 MPa, EI 7.9%), which showed similar strength properties as the BM of DP800. However, the EI of DP1000-DP1000 weldment was 1.9%, much lower in comparison with the BM of DP1000. The DP800-DP1000 weldment with dissimilar thicknesses showed the highest YS (955 MPa) and UTS (1075 MPa) values compared with the other weldments, but with the lowest EI (1.2%). The fatigue fractures occurred at the WJ for all types of weldments. The DP800-DP800 weldment had the highest fatigue limit (348 MPa) and DP800-DP1000 with dissimilar thicknesses had the lowest fatigue limit (<200 MPa). The fatigue crack initiated from the weld surface.

The multi-objective optimization modelling for properties of 301 stainless steel welding joints in ultra-narrow gap laser welding

Stainless steel has been widely used in the manufacture of metro car-body because of superior characteristics. But the traditional welding joints are prone to poor properties. Ultra-narrow gap laser welding (UNGLW) with low heat input can address property issues. However, due to the complicated parameter interaction in UNGLW, it is difficult to acquire the welding joint with excellent comprehensive properties and improve the welding efficiency simultaneously. For getting the glorious welding joint and improving efficiency, we combined response surface methodology (RSM) with multi-objective optimization to study the UNGLW process. The regressive mathematical models of four welding targets were established successfully. The influence rules of welding parameter interaction on welding targets were determined by RSM analysis. We obtained the optimal welding parameters and the optimized welding joint with excellent comprehensive properties by multi-objective optimization. The relationship between comprehensive property improvement and microstructure characteristics of the optimized welding joint was revealed. It is of great significance to improve the welding quality and efficiency of stainless steel welding joint in UNGLW.

Research of Technology for Repair of Heat Exchangers of Nuclear Power Plants by Laser Welding

Extending the lifetime of energy facilities is extremely important today. This is especially true of nuclear power plants, the closure (or modernization) of which poses enormous financial and environmental problems. High-quality repair of reactors can significantly extend their service life. One of the critical parts is heat exchangers, the tubes of which quite often fail. Sealing, as a type of repair of heat exchanger tubes by the plugs, is promising provided that the joint quality is high. Practical experience in the use of welding to solve this problem has shown the need to search technological solutions associated with increasing the depth of penetration and reducing the area of thermal effect. The aim of the work was to develop a highly efficient technology for repair and extension of service life of heat exchangers of nuclear power plants based on the results of studying the technological features of laser welding of joints of dissimilar austenitic steels AISI 321 and AISI 316Ti in the vertical spatial position. Based on the results of the analysis of mechanical test data, visual and radiographic control, impermeability tests and metallographic studies of welded joints, the appropriate modes of laser welding of plugs have been determined. The principal causes of defects during laser welding of annular welded joints of dissimilar stainless steels are determined and techniques for their elimination and prevention of their formation are proposed. Based on the results of the research, technological recommendations for laser welding of plugs in the heat exchange tube of the collector are formulated, which significantly improves the technology of repair of steam generators of nuclear power plants and extends the service life of reactors.

Effects of Laser Offset on Microstructure and Properties of NiTi/Copper Laser-Welded Joint

Effects of Laser Offset on Microstructure and Properties of NiTi/Copper Laser-Welded Joint

Evaluation of Fatigue Strength for Laser Welding Joint of SPCC Based on Dissipated Energy

Evaluation of Fatigue Strength for Laser Welding Joint of SPCC Based on Dissipated Energy

Influence of Heat Treatment of Laser-Welded Joints of Aluminum-Lithium Alloys on the Instability of Plastic Flow

Influence of Heat Treatment of Laser-Welded Joints of Aluminum-Lithium Alloys on the Instability of Plastic Flow