Laser-welded Joints Research Articles

Fatigue Performance of Laser Welds in Heavy-Gage Press Hardening Steels

This work investigates and compares the fatigue performance of laser-welded joints of two press hardening steels: a standard 22MnB5 and a variant modified by a combination of niobium and molybdenum (NbMo) alloying. The results indicate that joint geometry aspects, superposed to an intrusion-generated damage mechanism, were prevalent in causing a poor fatigue life in the case of peak stress values greater than 11% of the base metal's ultimate strength being around 1450 MPa. As identical process procedures were employed, the tests allowed investigating the influence of the alloy design on fatigue performance. The results of geometrical analysis and fatigue tests indicated that the NbMo modified alloy performed better than the standard 22MnB5 steel. The results also suggest that, if extremely tight quality limits are used in the manufacturing procedures, laser-welded joints of press hardened steels could offer a very favorable fatigue performance being considerably better than that of conventional and high strength structural steels.

Microstructure and corrosion performance of Zr–1Nb alloy laser welded joints treated by micro-arc oxidation

The protective coatings on laser-welded joints of Zr–1Nb alloy were prepared by the micro-arc oxidation (MAO) process. The morphology, microstructure and phase composition of MAO coatings at different regions of Zr–1Nb alloy laser-welded joints were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion behaviors of the coatings. The MAO coatings at the fusion zone (FZ) and heat-affected zone (HAZ) were smooth with some small holes, while MAO coatings at base metal (BM) were relatively rough with large holes. The cross-sectional topography at FZ coatings was quite dense with less distribution of pores and cracks, which can effectively improve the protective properties of the coatings. HAZ coatings were divided into outer loose layer and inner dense layer. The BM coating layer possessing many holes was a typical sparse layer structure. The phase compositions of welded joints treated by MAO were diverse at different zones of the joints, but all of them consisted of the monoclinic ZrO2 and tetragonal ZrO2 phases. The coatings of FZ had the best corrosion resistance, compared with HAZ and BM. The results of EIS indicated that the thickness and defects of the inner compact layer played a key role in improving the corrosion resistance of the MAO coatings on the surface of the Zr–1Nb alloy laser-welded joints.

Effect of CoCrFeNiMn high entropy alloy interlayer on microstructure and mechanical properties of laser-welded NiTi/304 SS joint

In order to suppress the formation of Fe-Ti IMCs in NiTi SMA/304 SS laser-welded joint and improve the properties of the joint. In this study, NiTi SMA/304 SS dissimilar alloys joints with and without CoCrFeNiMn high entropy alloy interlayer were obtained by micro laser welding process. The effect of CoCrFeNiMn high entropy alloy interlayer on the microstructure and properties of NiTi SMA/304 SS laser-welded joint was investigated systematically. Results showed that the addition of HEA interlayer significantly reduced the formation of the brittle intermetallic compounds (IMCs) such as FeTi phase. The addition of high entropy alloy interlayer promotes the formation of (Fe,Ni) solid solution phase, therefor, the joint with HEA interlayer is composed of (Fe,Ni) solid solution and small amount of FeTi IMCs and B2 phase. On the other hand, the average microhardness of weld metal was decreased from 732 HV to 221 HV by adding the HEA interlayer. The addition of HEA interlayer significantly improves the tensile shear properties of the joint, and the tensile shear load of the joint can reached 175 N, which is 6 times higher than that of the joint without HEA interlayer.

Mechanical properties of laser-welded and adhesive-bonded ultra-high-strength steel lap joints

The need to manufacture lightweight structures from ultra-high-strength steels is forcing industry to look for new joining technologies. Hybrid joints are one of the new ways to join materials. This paper investigates joining ultra-high strength steel plates using laser welding and adhesive bonding. Steel used in this paper was abrasion resistant steel (AR600) with tensile strength (Rm) ≥ 2 GPa. Three different epoxy adhesives were used in the experiments. The weld morphologies were investigated using optical microscopy. The mechanical properties of the lap joints were evaluated by hardness measurements and tensile shear tests. Examining the profiles of the joints, it became clear that the adhesive causes an air gap between the plates, which affects the morphology of the joint. The air gap caused by the adhesive increased the width of the weld at the interface of the plates. The width of the weld at the interface was approximately 25% larger in the adhesive-bonded joint. Based on shear strength tests, the use of adhesive in addition to the laser weld significantly improved the shear strength of the lap joint. The shear strength of the best laser-welded and adhesive-bonded joint was approximately 84% higher than that of laser-welded joint.

Controlling intermetallics formation at V-4Cr-4Ti/316SS laser-welded joint

In a fusion reactor, the connection between the blanket and peripheral components is closely related to heat transfer. Therefore, the sound joining of the structural materials of the blanket and peripheral components, V-4Cr-4Ti and 316 stainless steel (316SS), is of great importance. However, the properties of these joints are restricted by brittle intermetallics. The effect of a Cu interlayer on the formation of brittle intermetallics was studied. After welding, no significant defects (e.g., pores, macrocracks or microcracks) were observed in the welded joints, which showed good weldability. According to the microstructural analysis, it was verified that there were a large number of Cu-rich phases, Fe-rich phases, and a small amount of the σ phase which comprised Fe and Cr in the weld. In the hardness test, the welded joint was divided into hardened and softened zones. The average hardness of the softened zone was approximately 160 HV, while the average hardness of the hardened zone was approximately 292 HV, which was due to the hardening effect caused by a small amount of the σ phase. According to the tensile test results, the introduction of the Cu interlayer improved the strength and elongation of the weld. The ultimate tensile strength and maximum elongation of the weld were 469 ± 3 Mpa and 9.5 ± 0.2%, respectively. The tensile fracture morphology showed that the welded joint was ductile. The correlation between the microstructure and mechanical properties is discussed.

Optimization of beam oscillation on macro/micro formation and mechanical property in low-expansion superalloy laser-welded joint

Low-expansion superalloys have been widely used in aerospace, ships, nuclear power fields due to the high strength and low expansion coefficient. Generally, laser beam oscillation was adopted to solve the defects including poor bead formation, inner pores, and solidification cracks in the weldment of GH909 low-expansion superalloys medium-thick plates. Therefore, the molten pool flow, joint formation, microstructure, and mechanical properties were systematically studied in this paper. Results showed that beam oscillation effectively stabilized the molten pool and keyhole inside, thus forming a well-formed joint without obvious pores. In tensile tests, the strength and elongation of the joint at 300 Hz were measured as 859.5 MPa and 38.6 %, significantly higher than those of laser joint (731.1 MPa and 17.9 %) with plenty of pores and cracks. The average microhardness of the fusion zone (FZ) using laser oscillating welding (LOW) was about 16 HV larger than that of single laser welding (SLW). In FZ, γ/Laves eutectic precipitated phase was distributed discretely without obvious solidification cracks. There existed coherent and incoherent interfaces between eutectic γ and Laves phases. Moreover, coherent interfaces were detected to account for 73.9 %, presenting a major fraction. The improvement of the mechanical properties of LOW joint was mainly caused by solid solution strengthening and precipitation strengthening. This research has a guiding reference for well-formed and high-quality low-expansion superalloy welding and relevant industrial application.

Effect of nickel interlayer thickness on lap joint laser welding for aluminium-steel dissimilar materials

Laser lap welding of steel and aluminium with different thickness nickel foil was carried out. The effects of nickel interlayer thickness (TNi) on penetration depth, phase transition, mechanical properties were investigated. As the TNi increased from 0 to 50 µm, the weld penetration depth was first increased and then decreased, reaching a maximum of 382 µm when TNi was 20 µm. There was a γ-phase transformed from δ-ferrite in welded nugget and the transformation of high-temperature δ-ferrite into austenite was promoted as the TNi increased. The shear resistance of the steel/Al joint reached the maximum value of 103 N mm−1 when TNi was 20 µm. Which was almost 92% higher than the strength of the joint without nickel foil added.

Effect of pre-heating and post-weld heat treatment on structure and mechanical properties of laser beam-welded Ti2AlNb-based joints

The effect of pre-heating and post-weld heat treatment on microstructure and mechanical properties of laser-welded joints in a Ti–23Al–23Nb-1.4V-0.8Zr-0.4Mo-0.4Si (at.%) alloy was studied. Laser beam-welding was carried out at room temperature as well as after pre-heating up to 800°С. The post-weld heat treatment comprised either air quenching from 920°С followed by aging at 800°С or only aging at 800°С. The microstructure of the fusion zone consisted of columnar β-grains after welding at room temperature and 400 °C or both the columnar and large equiaxed crystals at 600 and 800 °C. An increase in the pre-heating temperature caused the columnar β-crystals growth as well as an increase in the fusion zone and heat-affected zone widths. Meanwhile, a decrease in the Al and Ti content, as well as an increase in both the porosity and gaseous elements content (O and N) after welding at 600–800 °C were found. The microhardness of each joint obtained after welding with pre-heating temperatures up to 600 °C was lower than that of the base material. All the welded joints showed the yield strength and ultimate tensile strength levels between 1070 and 1110 MPa, which correspond to approximately 80% of the base metal level. Reasonable total elongation of the joint was achieved after welding at 400 °C (4.3%). The post-weld heat treatment involving air quenching from 920 °C with subsequent aging at 800 °C for 6 h demonstrated the best results. The heat treatment resulted in the precipitation of the O- and α2-phases and an increase in total elongation to 6.5%.

The Simulation of Temperature Field and Residual Stress on the High Power Laser Welded Joint of Clf-1 Steel for Nuclear Fusion

The Simulation of Temperature Field and Residual Stress on the High Power Laser Welded Joint of Clf-1 Steel for Nuclear Fusion

Microstructure and Mechanical Properties of Laser Welding Joint of Ti-6Al-2Zr-3Nb-Mo Alloy

Microstructure and Mechanical Properties of Laser Welding Joint of Ti-6Al-2Zr-3Nb-Mo Alloy

Investigations on laser welding of dissimilar joints of stainless steel and copper for hot crack prevention

Fusion joining of stainless steel to copper without a filler material for pipe processing proves itself to be challenging due to the very different material properties of the joining partners and very heterogeneous characteristics in the weld metal due to different mixing ratios. One of the most common weld defects in this material combination is solidification cracking in the weld metal due to liquid copper accumulation between the stainless steel grain boundaries, which cannot withstand the tensile tensions while cooling down. Consequently, these cracks can reduce the mechanical properties, lead to leakages or cause faster corrosion. To prevent cracking, a precise control of the melting and mixing ratio of stainless steel and copper is needed. Laser beam welding offers many capabilities to influence the resulting weld metal shape and size as well as the mixing ratios with different approaches such as parameter optimization, inline process control, or the use of different beam shapes. This work shows different weld configurations and applications using sheets with thicknesses in the area of 1 mm and pipe samples. The main focus is on different solutions to influence the copper dilution and the weld metal geometry for solidification crack prevention. Therefore, a design of experiment approach and inline weld depth control using optical coherence tomography (OCT) data is used for the lap weld configuration to limit the copper dilution below 10 wt. % in steel dominated weld metals. Moreover, the benefits of adjustable intensity profiles for the butt weld configuration to control the shape and dimensions of the weld metal and mixing behavior with different power distributions in the laser beam welding spot are shown. The overall results indicate that solidification cracking in steel-copper joints can be influenced by different process approaches.

Structural performance of laser welded edge joints made of high-strength steel

Laser welding is a widely-used fusion welding process in industry. However, laser welding is not a common welding process in the manufacture of industrial crane structures. There has been a remarkable increase in the available strength classes of steel grades over the last 10 years, such that strengths of up to 1200 MPa are now commercially available. This enables the use of thinner materials in welded products and at the same time, has opened up new possibilities for using laser welding more widely in the manufacture of steel structures. This study focuses on the static and fatigue strength of the laser-welded joints. Details investigated are edge joint with flange preparation between two rectangular tubes and edge joint between flat bar and rectangular tube. A novel fatigue strength assessment concept, the FATmod method, is applied to assess the theoretical fatigue performance of the joint in comparison with the effective notch stress method with a FAT630 design curve. The FATmod method is based on the local stress ratio at a fatigue-critical point of the joint and the analysis considers the strength of the material, surface quality and applied stress ratio in the assessment of fatigue. The study shows that the samples failed from the base material side in static tests and the FATmod method developed was found to agree well with the test results.

Numerical study on the effect of residual stress on mechanical properties of laser welds of aluminum alloy 2024

The fusion zone is the weak area of weld joint which will experience complex thermal mechanical process during laser welding aluminum alloy 2024-T4. The resulting residual stress in the weld joint may exceed the yield stress, which will significantly affect the hardness and tensile strength. The purpose of this study is to predict the residual stress, and analyze its relationship with hardness and yield stress of the aluminum alloy 2024-T4 weld joint. Herein, three sequential models are established: (i) a thermal finite element model using double cone combined heat source that allows the prediction of time-dependent temperature distribution during laser welding of aluminum alloy 2024; (ii) a mechanical model using sequential coupling method and loading temperature in each increment to predict strain and residual stress; (iii) a theoretical empirical model to link the residual stress, hardness and yield stress. The simulated temperature field from the thermal model is in good agreement with the experimental result. The predicted stress field shows that the average von Mises stress at the center of the Al 2024-T4 and Al 2024-O weld joints are 303.60 MPa and 62.85 MPa, respectively, which are close to their yield stress. Moreover, it is found that the hardness decreases by 6 HV0.1 under the influence of residual stress in Al 2024-T4 fusion zone. The results indicate that the yield stress of 2024-T4 weld joint decreases by 16.5 MPa as a result of the residual stress. Above results indicate that residual stress has non-negligible influences on the mechanical properties of laser weld joint.

Fatigue Performance of Thin Laser Butt Welds in HSLA Steel

This work is focused on understanding the significant factors affecting the fatigue strength of laser-welded butt joints in thin high-strength low-alloy (HSLA) steel. The effects of the weld profile, imperfections, hardness, and residual stresses were considered to explain the results found in the S-N curves of four welded series. The results showed acceptable fatigue strength although the welded series presented multiple-imperfections. The analysis of fatigue behavior at low stress levels through the stress-concentrating effect explained the influence of each factor on the S-N curves of the welded series. The fatigue limits of the welded series predicted through the stress-concentrating effect and by the relationship proposed by Murakami showed good agreement with the experimental results.

Estimation of Al/Cu laser weld penetration in photodiode signals using deep neural network classification

In Al/Cu overlap laser welding joints for electric vehicle batteries, power and load are transmitted through the connection between the upper and lower sheets. As a thin sheet is placed on the upper part of the overlap joint, the laser processing parameters should be precisely adjusted to avoid weld defects such as incomplete penetration into the lower part or excessive upper sheet melting. In this work, a support vector machine and two deep neural networks (i.e., a fully connected neural network and a convolutional neural network) were used to classify penetration as unsatisfactory, transient, or good in photodiode signals. A bandpass filter was applied to the photodiode signals to transmit only the Cu emission wavelength. For modeling, 405 datapoints were collected, and 283, 61, and 61 datapoints were used for training, validation, and testing of the models, respectively. The machine learning models predicted the penetration mode every 50 ms, and the test results showed a high classification performance, exceeding 90% accuracy. The convolutional neural network was verified experimentally by gradually increasing the laser output power, thus demonstrating the feasibility and applicability of neural network classification to estimate Al/Cu laser weld penetration.

Improving the low-cycle fatigue properties of laser-welded Al–Zn–Mg–Cu alloy joints using double-sided ultrasonic impact treatment

To improve the low-cycle fatigue properties of Al–Zn–Mg–Cu 7075 aluminum alloy laser-welded joints, a post-weld treatment method for double-sided ultrasonic impact treatment (DSUIT) was used to treat the joints. The mechanism of different DSUITs on the microstructure and fatigue properties of welded joints was analyzed. The results showed that DSUIT reduced the welding defects and roughness of the joint surface, and the dendrite structure at the upper and lower surfaces of the joints was broken to form a plastic deformation layer (PDL) with an approximate thickness of 100 μm. Furthermore, the texture strength and grain size at the PDL were reduced. A beneficial residual compressive stress was introduced into the upper and lower surfaces of the joints after the DSUIT. When the number of cycles was 2 × 106, the maximum fatigue strength of the joints was 103.02 MPa after DSUIT, indicating an increase of 111.8% of the untreated joints of 48.62 MPa. Moreover, observance of the fatigue fracture of the joints revealed that the PDL produced by the DSUIT inhibited the initiation and propagation of cracks at the surface. Therefore, the DSUIT effectively improved the low-cycle fatigue properties of the joints.

Effects of Heat Input on Microstructure and Mechanical Properties of Dissimilar Laser-Welded Dual-Phase and Boron Steel Joints

Effects of Heat Input on Microstructure and Mechanical Properties of Dissimilar Laser-Welded Dual-Phase and Boron Steel Joints

Effect of filler wire and post weld heat treatment on the mechanical properties of laser beam-welded AA2198

The mechanical behavior of autogenously and non-autogenously laser beam-welded joints of Al-Cu-Li alloy AA2198 and the effect of post weld heat treatment are examined in this contribution. The deformation texture of the base material does not present any significant change in the macrotexture with applying different artificial ageing times. Autogenously and non-autogenously laser beam-welded joints present a decrease in yield stress and ultimate strength in the as-welded condition, approximately 45% and 36–38%, respectively, when compared with AA2198-Τ3 base material. The addition of the AA4047 filler wire increases the Si and Cu content in the grain interior and in the grain boundaries of the fusion zone of the welded joint. Micro-hardness measurements for autogenously laser-welded joints showed a decrease in hardness by 27% for the fusion zone and 42% for the heat-affected zone, when compared with the non-autogenously laser beam-welded joints. Α quality index was exploited to evaluate the tensile mechanical performance of the welded joints. It is observed that the non-autogenously welded joints always show a higher ‘quality’ than the respective autogenously welded joints and the highest quality index in terms of mechanical performance is achieved for the as-welded and the peak-aged conditions, respectively. Regardless of the post weld heat treatment condition, both autogenously and non-autogenously laser-welded specimens fractured in between the equiaxed and fusion zone during tensile loading.

Residual stress and simulation of 304–430 stainless steel dissimilar laser-welded joints incorporating materials heterogeneity

AbstractAn Nd:YAG laser device is used to conduct laser welding for a 1 mm austenitic stainless steel plate and a ferritic stainless steel plate. A finite element model of the shear punching test is constructed to generate the maximum shear strength in the weld, and the finite element model of laser welding is created using the welding temperature field. The hole drilling test result and the residual stress generated by two algorithms (Nonuniform-material and uniform-material) are compared. Results show that a drop-off of residual stress in the central area of the welded joint is observed when materials heterogeneity is disregarded. When materials heterogeneity is considered, the residual stress curve indicates smooth transition. The value of the latter appears reasonably similar to the experimental value. Therefore, this solution is advantageous in terms of residual stress simulation in dissimilar welded joints and does not critically affect residual deformation.

Distortion Control for Dissimilar Welding of SS321 to Hastelloy C-276 with CO2 Laser Beam Butt Joints Using Taguchi Methods

The laser beam welding process is a promising technology because of its reliability and ability to automate the process easily. This study aims to analyze distortion for dissimilar laser weld joints. Hastelloy C276 and SS321 plates are joined by using the CO2 Laser beam welding. Welding current, welding speed and shielding gas flow are chosen as process parameters for preparing butt joints. Each of the factors has two levels to control the parameters of the output. Experimentation was conducted with four trails by using an L4 orthogonal array. The quality of the welds and bead geometry are verified through macrostructure examination. The Vernier height gauge was used for the measurement of distortion in the weldments. Lower the better-quality characteristic is chosen for the response. ANOVA studies identified laser power at 81%, the weld speed at 16% parameters is a contribution with a statistical of about 95%. Full penetration was observed for all the experimental trails.