Showing papers in "Welding in The World in 2009"

A Consideration of Allowable Equivalent Stresses for Fatigue Design of Welded Joints According to the Notch Stress Concept with the Reference Radii r ref = 1.00 and 0.05 mm

Abstract: In the literature, allowable stresses (FAT-values) for the fatigue design of welded joints are established according to the notch stress concept with the reference radii rref = 1.00 mm for thick connections (t ≥ 5 mm) and 0.05 mm for thin connections (t < 5 mm). However, it is not clear for which strength hypothesis they are valid. As local equivalent stresses may be calculated by the principal stress or von Mises hypotheses, it is necessary to distinguish between the applied hypotheses. The FAT-values according to the principal stress and von Mises hypotheses are compiled for steel, aluminium and magnesium for the reference radii rref = 1.00 and 0.05 mm. The allowable stresses are derived from normal as well as from shear stresses. However, the values derived from pure normal loading (axial or bending) and from pure torsion are not compatible when the principle stress or the von Mises hypotheses are applied. Therefore, in case of biaxial loading, the stated incompatibility between the values obtained from different loading modes should be overcome by the Gough-Pollard relationship.

Fatigue Behaviour of Welded High-Strength Steels after High Frequency Mechanical Post-Weld Treatments

Abstract: Investigations have been carried out regarding the fatigue strength of welded details improved by high frequency treatment methods. These methods increase the fatigue strength by cold forming of the surface, so that the weld toe is smoothened, the surface hardness is increased and compressive residual stresses are present up to a depth of 1 to 1.5 mm. In this paper, the surface residual stresses at the weld toe are investigated before and after different loading conditions and for different steel grades. It is shown that only high tensile fatigue loading can lead to a slight residual stress relaxation for low-strength steels. The fatigue crack behaviour is analysed in more detail. The crack propagation rates with and without surface treatment are investigated, using defined lines of rest. The study shows that crack propagation in the edge layers is reduced. Several cracks may start in the UIT-treated zone but will not propagate further, until one final crack, often close to the edge of the specimens, will lead to failure. The results of fatigue tests for butt welds and longitudinal stiffeners improved by high frequency hammer peening are presented. The fatigue strength is seen to be doubled. For high-strength steels, the improvement at different load levels is identical, but for lower-strength steels, high stress ranges lead to reduced improvement. This fact results in flatter SN-curves and can be explained by the lower maximum of residual stresses and residual stress relaxation.

Friction Stir Spot Welds between Aluminium and Steel Automotive Sheets: Influence of Welding Parameters on Mechanical Properties and Microstructure

Abstract: Hybrid configurations between aluminium and steel are needed to meet today’s requirements for lightweight construction in the automotive industry. Different studies showed that Friction Stir Welding (FSW) as well as Friction Stir Spot Welding (FSSW) processes are suitable for joining aluminium to steel. In this work, dissimilar FSSW of aluminium AA5754 and galvanised steel HX 340LAD were examined. In particular the influence of different spindle speeds and dwell times on microstructure and the mechanical properties of the weld were analysed. In doing so, the cross-section microstructure of the weld interface was observed by light optical microscope (LOM) and scanning electron microscope (SEM). The strength of the welds was evaluated both by tensile shear and vibration fatigue tests. The influences of the individual parameters on the weld are presented in detail. The appearance of intermetallic phases (IMPs), a severe problem for conventional fusion welding processes between aluminium and steel, were investigated for the welded samples and a link to the mechanical properties is given.

Development of Advanced 3-Electrode MAG High-Speed Horizontal Fillet Welding Process

Abstract: In the shipbuilding industry, the improvement of welding efficiency and weld quality are desirable in order to cope with the recent growth in shipbuilding orders and the size of ship hulls. Innovation in welding efficiency and weld quality in horizontal fillet welding is an important issue since the amount of horizontal fillet welding work generally accounts for 70 percent or higher of the total welding work for ship hulls. As one of the available high-speed horizontal fillet welding processes, the Twin Tandem One Pool process (hereinafter referred to as the TOP process) has been used in actual fabrication. In the TOP process, a set of two wire electrodes is positioned in tandem on both sides of the web plate of a fillet joint and the wire-to-wire distance is kept short, so as to create one weld pool. However, the maximum welding speed of the TOP process is approximately 1.5 m/min. In order to improve the welding speed of the TOP process, the authors have employed an additional filler wire positioned between the two wires of the TOP process. The additional filler wire carries DC-EN currents, as opposed to DC-EP currents for the main welding wires, thereby reducing the arc interference and arc blow at high currents. With this new technique, the authors have improved the stability of the weld pool formation at high currents. Consequently, horizontal fillet welding at a speed of 2.0 m/min on primer-coated steel plates has been achieved with excellent bead appearance, shape and porosity resistance.

Determination of Residual Stresses in Low Transformation Temperature (LTT -) Weld Metals using X-ray and High Energy Synchrotron Radiation

Abstract: Crack and fatigue resistance are relevant evaluation criteria for welded joints and are decreased by tensile residual stresses resulting from the welding and cooling process, while compressive residual stresses can have a positive influence on the characteristics mentioned. In order to generate compressive residual stresses, a set of post weld treatment procedures is available, like shot peening, hammering, etc. These procedures have the disadvantage that they are time and cost extensive and have to be applied after welding. As another point, such technologies can only produce compressive stresses at the top surface, i.e. can only contribute to the reduction of the risk of cracks initiated at the surface, like fatigue cracks. A chance to generate compressive stresses over the complete weld joint during the welding procedure is offered by the so-called Low Transformation Temperature (LTT -) filler wires. Compared to conventional wires, these materials show lower phase transformation temperatures, which can work against cooling-related tensile stresses, resulting from respective shrinkage restraint. In consequence, distinct compressive residual stresses can be observed within the weld and adjacent areas. The strength of these fillers makes them potentially applicable to high-strength steel welding. Welds produced with different LTT — filler wires have shown different levels and distributions of the resulting residual stresses depending on the specific transformation temperature. The transformation temperatures are determined by temperature measurement. Classical X-ray diffraction as well as diffraction methods using high energy synchrotron radiation have been used for residual stress analysis. By means of high energy synchrotron diffraction in reflection mode residual stress depth gradients can be determined non-destructively. The phase selective nature of the diffraction measurements enables the simultaneous determination of the phase specific residual stresses of all contributing crystalline phases within one experiment. The application of white beam diffraction implies recording of a multitude of diffraction lines within the energy range of the provided energy spectrum of the white beam. By this means phase specific residual stress depth distributions up to distances of 150 μm below the surface can be analysed for steel using the energy dispersive set-up of the HMI-beamline EDDI at the Bessy site, Berlin, providing an energy range between 20–150 keV. As a side effect quantitative phase analysis can be carried out using white energy dispersive diffraction e.g. the determination of the content of retained austenite in the weld.

Magnetic Control of Arc Plasma and its Modelling

Abstract: This paper describes the control of arc plasma by a cusp-type magnetic field. The cusp-type magnetic field, which is produced by four magnetic poles, changes the cross-section of arc plasma from the conventional circular shape to an elliptical shape. Previous studies using solenoid coils to produce a cusp-type magnetic field reported that magnetized arc plasma provides deeper penetration. However, the solenoid device developed for the cusp-type magnetic field was too large, in comparison with the size of the welding torch used for production welding. In the present study, magnetized arc plasma with the use of high-performance permanent magnets is experimentally and theoretically investigated. It is confirmed experimentally by tungsten inert gas (TIG) arc welding that permanent magnets can produce arc plasma with an elliptical cross-section. In a series of experiments, a good bead appearance was obtained in high-speed welding with magnetic control. A three-dimensional numerical model of arc plasma was also constructed to determine the optimum arrangement of the magnets. It was analytically shown that the elliptical shape of the magnetized arc plasma and its effect on welding depended on the magnet height from the base metal and the magnetization direction.

Thick Plate CO2-Laser Based Hybrid Welding of Structural Steels

Abstract: Laser and hybrid laser-arc welding are after two decades of intensive work now well-established processes in the welding of structural steels as well as other materials. Many results have been reported over the years and many advantages in comparison with traditional welding processes have been obtained. Very thick part laser welding using penetration depth in excess of approximately 12 mm in structural steel does however continue to be a serious challenge. This is partly due to limitations in the achievable welding depth because the penetration as a function of laser power shows a very non-linear behaviour, and partly to the large molten volume that must be controlled. Although fibre- and disc-lasers today are being developed to high powers, CO2-lasers sources are still dominating larger penetration welding. In the present investigation the potential and limitations of hybrid CO2-laser/MAG welding of thick plate structural steels have been investigated. Welds in excess of 12 mm thickness of structural steels are presented together with achieved mechanical properties as e.g. hardness, toughness and fatigue. The investigations were made using structural steels of the grades S235, S355 and S690QL.

Practical Application of the N-SIF Approach in Fatigue Strength Assessment of Welded Joints

Abstract: In the notch stress intensity approach the weld toe is modelled as a sharp V-notch, ρ = 0, and local stress distributions in plane configurations are given on the basis of the relevant Mode I and Mode II Notch Stress Intensity Factors (NSIFs). Then, the local strain energy density over a circular sector surrounding the point of stress singularity can be easily calculated as soon as the NSIFs are known. These parameters include not only the influence of the main geometrical ratios, but also the size effect. Welded joints simply scaled in geometrical proportion have different NSIFs. Whilst the local stress-based evaluation of the NSIFs needs very fine meshes in the vicinity of the points of singularity, which is a drawback of the approach in the presence of complex geometries, the mean value of the elastic strain energy density (SED) on the control volume can be accurately determined by using relatively coarse meshes. Thanks to the SED use, the degree of refinement of FE models is not so different from that usually used to determine the ‘hot-spot stress’ according to the structural stress approach. The present paper deals with this topic, considering both bi-dimensional and three-dimensional welded details and discussing the degree of accuracy and the limits of applicability of the method. An alternative, simplified application of the NSIF approach valid for fillet-welded joints with fatigue failures from the weld toe (the so-called Peak Stress Method -PSM) is also presented.

Extensive Introduction of Ultra High Strength Steels Sets New Standards for Welding in the Body Shop

Abstract: In order to meet upcoming legislative demands regarding acceptable levels of CO2 emissions and to contribute to the fight against global warming, while also meeting customer expectations of reduced fuel consumption, all automotive OEMs are today focusing on lightweight engineering. Some of them, mainly low volume premium brands, have chosen to introduce fairly expensive lightweight materials such as aluminium and magnesium to meet these targets, whereas the main portion of high volume producers are trying to optimize the classic steel concept by introducing different grades of advanced high strength steels. Volvo Cars has decided upon a unique utilization of hot-formed, press-hardened, ultra high strength steel components featuring tensile strength levels in the order of 1 500 MPa, but on the other hand, producing these parts in very thin gauges for weight saving reasons. The first product launched according to this ultra high strength steel intensive concept was the 2008YM version of the Volvo V70 and its sibling, the cross-country version XC70, both built on the EuCD platform. The body contains several parts manufactured by hot-forming and press-hardening. The extensive use of this type of Boron alloyed steel challenged all welding methods commonly used in car body manufacturing, not least, traditional resistance spot welding. This paper will address the following topics: overview of the V70 body structure and utilization of materials, a brief description of the hot-forming, press-hardening process, the procedure for validating weldability before going into series production, lessons learnt from resistance spot welding trials of material combinations involving one or more boron alloyed steel parts, and recommendations for default welding data, influence on the manufacturing system, introducing electro-servo welding guns, adaptive weld timers and ultrasonic non-destructive weld quality checking, necessary revisions of existing spot weld requirements. The presentation will end with a glimpse at perspectives for future welding challenges in Volvo body shops, as the need for further weight saving will promote a considerable introduction of various new grades of advanced high strength steels.

Observations on Droplet and Arc Behaviour during Pulsed GMAW

Abstract: The influence of the oscillation behaviour of the weld pool and the pendant droplet on the electric arc in pulsed GMA welding has been studied. High speed video images are related to measurements of the voltage. It is found that the weld pool is triggered into motion by the impact of the transferred liquid droplet. The increase of the current in the peak pulse period hardly affects the weld pool. The remaining liquid at the electrode tip may start to oscillate by the action of surface tension and gravity after the droplet is detached. It appears that the oscillation pendant droplet changes the structure of the arc. A conceptual model is derived to explain the observed behaviour: visual observations and voltage measurements. Two different regions can be distinguished in the arc, a bright region and a normal region. The bright arc region surrounds the droplet and shrinks and expands with the up and down movement of the droplet. The behaviour of the bright arc region can be correlated with the measured arc voltage. The shape of the droplet has a negligible effect on the measured voltage, it only initiates the changes occurring in the arc. The periodical release of additional metal vapour in the arc affects the arc voltage considerably. The results of the experimental work presented here cannot yet explain the mechanism behind the alteration of the arc structure and further research is required to elucidate these phenomena. The results presented in this paper focus on bead on plate-pulsed GMA welding in partial penetration conditions.

A comparison of residual stress in hammer-peened, multi-pass steel welds - a514 (s690q) and s41500

Abstract: As part of a program to assess the in-situ weldability and mechanical performance of the candidate high strength low alloy A514 (S690Q) steel as an alternative to the S41500 martensitic stainless steel for hydro-turbines, three aspects of the welds were studied: residual stress, Charpy toughness and cavitation erosion resistance. The experimental set-up involved robotized gas metal arc welding (GMAW), performed on U-groove and double-V weld preparation cut into 50 and 75 mm-thick steel plates. Half of the welds were robotically hammer-peened after each weld layer, except for the root pass. Strain gauges measured longitudinal and transverse strains during welding and hammer-peening. Once the weld cooled down to room temperature, the strain gauges provided the surface residual stress level at their location. Two-dimensional, sub-surface, longitudinal, residual stress distributions were measured on cut sections with the contour method, using an optical profilometer. The results showed that hammer-peening completely eliminates the near-surface tensile welding residual stress on the A514 steel, whereas on the S41500 steel, the process is less useful due to the already beneficial effect of the low temperature martensitic transformation during weld cooling. Furthermore, hammer-peening the last weld layer confines tensile residual stress inside the weld, while inducing compressive stress at the weld surface. Charpy test results showed that the A514 weld presented better toughness than the S41500 weld and comparable cavitation erosion resistance. Finally, hammer-peening showed a beneficial effect on cavitation resistance of the weld surface.

Computational Tools for Modelling FSW and an Improved Tool for NDT

Abstract: The good quality of Friction Stir Welding (FSW) joints enables the significant development of industrial applications of solid state welding technologies. This high quality standard is even more significant when FSW is compared to the fusion techniques. Nevertheless, in FSW joints some defects may arise which are very sensitive to small variations in some process parameters. Moreover, the results from computational modelling of the FSW are only valid for non-defective welds. Thus, in order for modelling of the process to contribute to the industrial consolidation of the FSW process, the experimental implementation results need to be supported by a reliable, Non-Destructive Testing (NDT) system. This work addresses an integrated scheme of two computational tools which enables the support of a faster establishment of process parameters, addressing the material flow analysis, with numerical coupling between fluid dynamics and solid mechanics; using the analytical iSTIR code; and a new, NDT, eddy currents probe, able to detect the typical FSW root imperfections.

Numerical Modelling of Cold Cracking Initiation and Propagation in S 1100 QL Steel Root Welds

Abstract: Although the phenomenon of hydrogen assisted cold cracking (HACC) and respective avoidance procedures have extensively been investigated in the seventies and eighties, the reasons for recent failures are still a lack of knowledge about the basic hydrogen effects on steel microstructures and, in particular, a lack of welding procedure specifications and standards accounting directly and consistently for cold cracking avoidance in modern high strength structural steels with yield strengths of up to 1 100 MPa. In previous several contributions the consequences of various heat treatment procedures targeted at HACC avoidance have been shown, as for instance their effects on stress-strain build up and on hydrogen diffusion in high strength steel welds. But, a principal interaction of three local influences on hydrogen assisted cold cracking, i.e. local microstructure; local mechanical load and local hydrogen content have not yet been studied in detail for these materials. For this, a numerical model for HACC has been developed, accounting particularly for crack-initiation and crack-propagation criteria, like the hydrogen redistribution during the process of cracking. The numerical model has been used to investigate HACC in such materials, i.e. in the weld microstructures of an S 1100 QL steel, under severe restraint and various hydrogen levels. The results were achieved by in depth thermal and structural finite element simulations combined with numerical hydrogen diffusion modelling. By such procedure, HACC in single-layer welded plates with thickness of 20.0 mm at realistic restraints has been studied. As a particular result, it turned out that the crack-initiation location is typically in the centre of the weld metal (WM), where only a single crack is initiated at hydrogen contents of up to 10.0 Nml/100 g Fe. But, it was evidently shown by such analyses that the crack-initiation location is shifted into the HAZ and that multiple cracking occurs at higher hydrogen contents of up to 15.0 Nml/100 g Fe.

Fatigue Tests and Numerical Analyses of a Connection of Steel Sandwich Plates

Abstract: In shipbuilding, steel sandwich plates are applied in several areas. Particularly the connections between the sandwich plates may be prone to fatigue in the case of cyclic loads. Therefore, the fatigue strength of a fillet-welded connection of a steel sandwich plate under in-plane loading has been investigated experimentally and numerically. Numerical calculations are performed using the crack propagation and the effective notch stress approach. In the notch stress approach, the modelling of the weld root may create problems as the relatively thin face plates are affected by the notch rounding of 1 mm. Two different shapes are considered. Furthermore, the small size notch approach with a reference radius of 0.05 mm is applied. From the comparison of the results with the fatigue strength found in the tests, conclusions are drawn for appropriate modelling.

Significance of Strength Mismatch in Fracture Performance Evaluation of Welded Joints — Characterization of Mismatch Effect by the Weibull Stress Criterion

Abstract: The effect of strength mismatch in welds on the fracture driving force of the heat-affected zone (HAZ) is investigated by 3-dimensional FE-analysis The Weibull stress is used for the evaluation of the fracture driving force The CTOD ratio, γ = δMismatch /δMatch, is proposed to quantify the mismatch effect, where δMismatch and δMatch are CTODs of the mismatched joint and the matched joint, respectively, at the same level of the Weibull stress The CTOD ratio γ is decreased by strength overmatching, which is due to the constraint effect of the overmatch WM on the stress fields in the HAZ The equivalent CTOD ratio, β = δ/δWP, is employed to correct the constraint loss in the structural component, where δ and δWP are CTODs of the fracture toughness specimen and the wide plate component, respectively, at the same level of the Weibull stress The analysis using β and γ indicates that strength overmatching leads to a severe requirement of the HAZ toughness, which cannot be directly derived from the CTOD — remote strain relationship for the wide plate joint By contrast, the toughness requirement of the WM is relaxed by strength overmatching, as a result of the shielding effect of the overmatch WM

Fracture Toughness of Welded Commercial Lean Duplex Stainless Steels

Abstract: Duplex stainless steels are successfully used in pressure vessel applications due to high mechanical strength combined with superior corrosion resistance. There has been a debate concerning the tou ...

Experimental Studies and Mathematical Modelling of Penetration in TIG and A-TIG Stationary Arc Welding of Stainless Steel

Abstract: Experimental investigations of the kinetics of penetration of stainless steel in TIG and A-TIG welding, and theoretical analysis of thermal, electromagnetic and hydrodynamic processes occurring in the weld pool metal were conducted. The stationary arc welding process was chosen as the focus of investigation, in order to study physical-metallurgical peculiarities of metal penetration in TIG and A-TIG welding. This makes it possible to describe processes occurring in the weld pool within the framework of axisymmetric approximation. The conjugate model of heat exchange in the base metal and hydrodynamics of the weld pool, with the melt affected by the electromagnetic (Lorentz force), buoyancy (Archimedean force) and thermal-capillary (Marangoni effect) forces, was formulated. Comparative analysis of the impact of the electric characteristics of the arc (current, voltage, anode spot diameter) and different force factors causing motion of liquid metal on hydrodynamics of the weld pool and shape of penetration of stainless steel in TIG and A-TIG welding was carried out by the method of mathematical modelling. The relationship between the deformation of the free surface of the molten metal (due to changes in its density depending on the temperature) and energy characteristics of the arc was analysed. It is shown that in contraction of the anode region of the arc (with decrease in diameter of the anode spot, which is characteristic of A-TIG welding), the magnetic-hydrodynamic processes become the dominating factor, determining the penetration depth and formation of the weld as a whole.

Advanced welding analysis methods applied to heavy section welding with a 15 kW fibre laser

Abstract: For butt joint welding of 8 mm and of 16 mm thick stainless steel a 15 kW fibre laser was applied, achieving full penetration for a welding speed of 7 m/min and 3 m/min, respectively. Optics with a ...

Effects of Carbon Content on Intermetallic Compound Layer and Joint Strength in Friction Welding of Al Alloy to Steel

Abstract: The microstructure and bond strength of the friction-welded interface of Al-Mg alloy A5052 to carbon steel S45C have been investigated, to establish their dependence on C content of steel by comparison with those observed in the joint of A5052 alloy to low C steel S10C. TEM observations revealed that an intermetallic compound layer 100–1 000 nm thick was formed at the interface, consisting of Fe2Al5 and Fe4Al13, similar to that observed at the A5052/S10C interface. The thickness of the intermetallic compound layer was increased almost in proportion to friction time in both joints, while that of the A5052/S45C joint grew at a lower rate than that of the A5052/S10C joint. In the intermetallic compound layer, granular Fe2Al5 and Fe4Al13 were distributed almost randomly, in contrast to those observed at the interface of the diffusion couple or diffusion-bonded joint, where intermetallic compounds formed as layers distributed in the order of their chemical compositions. This suggests that the formation of the intermetallic compound layer was significantly influenced by a factor other than the diffusion of Al and Fe. In the steel adjacent to the intermetallic compound layer, a very fine grain zone was observed, suggesting that the steel surface underwent heavy plastic deformation during the friction process. The thickness of the fine grain zone was also increased in proportion to friction time. It was found that the thickness of the intermetallic compound layer increased with that of the fine grain zone, obeying a relation almost independent of the C content of the steel and chemical composition of the Al alloy. These results suggest a significant contribution of mechanical intermingling of Fe with Al in the formation of the intermetallic compound layer. On tensile tests using specimens with a circumferential notch at the interface, the A5052/S45C joint was fractured at the interface region, showing higher fracture strengths than the A5052/S10C joint, probably because of the lesser thickness of the intermetallic compound layer.

Numerical Simulation of Diffusion of Multiple Metal Vapours in a TIG Arc Plasma for Welding of Stainless Steel

Abstract: In the present paper, a TIG arc in helium or argon is modelled, taking into account the contamination of the plasma by the metal vapour from a stainless steel weld pool. Iron, chromium and manganese are considered the metal vapour species in this model. A viscosity approximation is used to express the diffusion coefficient in terms of the viscosities of the shielding gas and the metal vapour. The time-dependent, two-dimensional distributions of temperature, velocity and metal vapour concentrations of iron, chromium and manganese are predicted, together with the weld penetration, as a function of time for a 150 A arc at atmospheric pressure, for both helium and argon welding gases. The distribution of the metal vapours depends on the diffusion term and the convection term. Due to the cathode jet, the convection term has a strong effect. Consequently, it is found that the metal vapours expand in the radial direction and are concentrated around the weld pool surface. The concentration of manganese vapour is larger than those of iron and chromium vapours, despite the fact that the proportion of manganese in stainless steel is significantly smaller.

Ultrasonic Evaluation of Spot Welding Nugget Diameter with a Line-Focused Probe

Abstract: Resistance spot welding has been widely used for the construction of thin sheet metal structures. The structures’ reliability depends upon the weld quality as well as the weld size. Therefore, various non-destructive inspections have been applied to the spot welds. Among them, the ultrasonic evaluation is widely used for the detection of defects and for the measurement of weld size. However, this method is too difficult for the detection of the fusion zone or nugget non-destructively, in spite of the high demand for their evaluation. Due in part to the recent application of high-strength steel sheets to automobiles, this problem has become a crucial subject in spot welding. The discrimination of the nugget from the solid state-welded zone or the corona bond is now strongly demanded. This study focused on the non-destructive discrimination between the nugget and the corona bond; a new, ultrasonic measurement with a line-focused probe was proposed. In the experiment, spot welded specimens both with and without surface indentations were prepared, in order to determine the affecting factors on the ultrasonic measurement of spot welds. Moreover, two types of special, thin sheet specimens were prepared to determine the acoustic phenomena in the fusion zone. The first type was the resistance spot-welded specimen and the other was the gas tungsten arc-welded specimen. The measurement of these two special specimens showed that the sound velocity in the nugget differed from the base material and was dependent upon the inhomogeneous microstructures. The different sound velocity caused a different amplitude of reflected waves in the nugget, the change of which also depended upon the measuring conditions of the water path. From these results, it was shown that the measurement based on the different amplitude and the dependence on the water path enabled the non-destructive discrimination between the nugget and the corona bond.

Fracture Toughness Evaluation of Laser Beam-Welded Joints of 780 MPa-Strength Class Steel

Abstract: For fracture assessment of steel structures with laser beam-welded joints, it is significantly important to evaluate the fracture toughness of the weld metal. However, the fracture toughness of weld metal is often impossible to measure by standard Charpy impact testing in the brittle-to-ductile transition or upper-shelf temperature region, because of fracture path deviation (FPD) from the weld metal due to narrow weld bead and a high degree of overmatching in strength. In this work, evaluated fracture toughness of laser beam-welded joints of 780 MPa-strength class (HT780) steel is examined by Charpy impact testing and CTOD fracture toughness testing. The improved Charpy impact tests, using the specimen with three parallel welds or the side-groove specimen, are conducted in order to prevent FPD. The applicability and significance of the improved impact test methods to the laser beam-welded joints of high-strength steel are investigated by means of Weibull stress analysis. The Weibull stress analysis indicates that the 3-weld method could not necessarily prevent FPD in laser beam-welded joints of HT780 steel, since side beads are not as hard as main weld beads, due to reheating at the main weld. On the other hand, a side-groove specimen is effective for avoiding FPD, but intrinsic VE of the weld metal could not necessarily be obtained by using this specimen.

3D Finite Element Modelling of Contour Laser Transmission Welding of Polycarbonate

Abstract: A three-dimensional finite element model was developed for the simulation of heat transfer in contour laser transmission welding of amorphous polymer — polycarbonate. By introducing mass flow in the model, a time-dependent, contour welding process was solved as a time-independent heat transfer problem (Quasi-static model). The short solution convergence times (under three minutes for a set of process and material conditions) and model output agreement with the experimental data indicate that the Quasi-static approach is convenient for use in solving heat transfer problems in laser contour welding. Combined with experimental data, the simulation results indicate that, in contour welding of PC, welding initiates when temperature at the weld interface reaches the maximum of 200 °C. The simulation results also provide further insight into the effect of proportionally increasing laser scan speed and power (i.e., while maintaining constant line energy).

Improvement of Extremely Low Cycle Fatigue Strength of Welded Joints by Toe Finishing

Abstract: The improvement of extremely low cycle fatigue strength of welded joints by TIG dressing and burr grinding was investigated. The weld toes of T-shape welded joints were improved by TIG dressing or burr grinding and tested under displacement control conditions. The crack initiation life of finished specimens was at least threefold longer than that of as-welded specimens. There was no clear difference in improvement between TIG dressing and burr grinding. We also analysed the local strain around the cracking point by finite element analysis and performed local strain-based fatigue assessment. The results indicated that extremely low cycle fatigue strength of the joints improved by TIG dressing and burr grinding can be assessed by evaluating the local strain and referring to the extremely low cycle fatigue strength curve.

Influence of Welding Conditions on Nugget Formation in Single-Sided Resistance Spot Welding Process

Abstract: This paper describes the single-sided resistance spot welding (SSSW) process which is expected to be a productive welding technology for joining stamped sheet panels to hollow parts for auto bodies. To obtain guidelines for making a good weld with SSSW process, the influence of welding parameters and set-up conditions of the specimen upon nugget growth were investigated experimentally. In addition, a numerical study was carried out to establish the mechanism of nugget growth in the SSSW process. Since deflection of parts during welding is one of the features of the SSSW process, this report focuses on the influence of electrode force and the stiffness of the specimen upon nugget formation. In addition, the effect of electrode geometry was examined.

Improvement of Weld Metal Toughness in High Heat Input Electro-Slag Welding of Low Carbon Steel

Abstract: In recent years, important steel structures of high-rise buildings such as the box columns required high toughness of welded joints considering the shockproof against the large-scale earthquake. In Japan, there are some specifications that demand Charpy impact energy more than 70 J at 0 °C in any welded joints. However in high heat input electro-slag welding, the microstructure of the weld metal becomes very coarse and weld metal toughness is remarkably deteriorated. In this paper, authors tried to form fully acicular ferrite microstructure in the weld metal of high heat input electro-slag welding to improve the weld metal toughness. High heat input (approximately 100 kJ/mm) electro-slag welding was carried out in the 60 mm thickness steel plate joint. Applying high Ti bearing welding wire and low basicity welding flux provided the weld metal with acicular ferrite microstructure, because large amounts of Ti containing oxide inclusions which were known as effective nucleation sites for acicular ferrite formations were dispersed in the weld metal. B addition to the weld metal suppressed the formation of coarse grain boundary ferrite when B/N ratio in the weld metal ranged between 0.5 and 0.8. When B/N ratio in the weld metal exceeded 0.8, the weld metal toughness was deteriorated due to an increase of M-A constituent. Consequently it was possible that fully acicular ferrite microstructure was formed in the weld metal of high heat input electro-slag welding and achieved the excellent weld metal toughness (vE0 > 100 J).

Numerical Simulation of MIG Weld Pool in Switchback Welding

Abstract: The authors proposed a switchback welding method to make stable back beads in the first layer weld during one-side multi-layer welding. In the proposed welding method, a welding torch is moved backwards and forwards like a switchback. The torch motion is important in the control of the weld pool. In this paper, the influence of the torch motion on the weld pool shape is investigated in order to find the optimum welding parameters. Firstly, fundamental welding experiments were carried out in thin plate one-side butt welding to determine the parameters of numerical simulation. Secondly, a simple numerical model for switchback welding was constructed and a heat conduction analysis was carried out, to investigate the effect of the travel speed and backward stroke distance of the torch on the weld pool. When the travel speed of the forward stroke was faster than that of the backward stroke, the weld pool length became shorter than a constant travel speed. On the other hand, if the backward stroke distance became longer than half of the forward stroke distance, continuous back beads could be obtained, regardless of such a disturbance as fluctuation of the arc length. The validity of the switchback welding method was verified by numerical simulation and fundamental experiments.

Possibilities and Limitations to Improve the Weldability of Low Carbon 12Cr Ferritic Stainless Steel for Expanded Industrial Applications

Abstract: Ferritic stainless steel X2CrNi12 (EN 10088) is generally appreciated for its relatively low cost and good resistance to wet abrasion and mild environments, but unfortunately its weldability is restricted. Typical applications up to now include railway wagons for coal and iron ore, mining and mineral process and transport equipment, bus frames and chassis, silos, etc. This steel grade nowadays can be fabricated cost effectively with low carbon and impurity levels appreciably improving both the weldability and mechanical properties. In this case, long-term maintenance costs of assemblies produced using this ‘clean’ X2CrNi12 stainless steel will be low, with a suitable coating providing sufficient protection for several decades. For other applications, the use of weldable X2CrNi12 is also more economical than higher alloyed stainless steels. Moreover, joining the steel by laser welding without filler metal should be considered. The present paper provides an overview of results of a research project initiated by the Belgian Welding Institute investigating the possibilities of this modified low carbon ferritic stainless steel. The main objective of this work was to demonstrate that it offers great application ranges for constructing purposes which should expand its application field substantially. In the near future broadening this steel family with even higher mechanical properties allowing further reduction of plate or wall thickness and of production costs will be explored.

Study of Hot Cracking Behaviour of 14Cr-15Ni-2.5Mo Ti-Modified Fully Austenitic Stainless Steels using Varestraint and Hot Ductility Tests

Abstract: Studying the hot cracking behaviour of 14Cr-15Ni-2.5Mo Ti-modified austenitic stainless steel is an important weld-ability consideration because this material solidifies in the prior austenitic mode without any residual delta ferrite and also contains titanium. Hot cracking in this material is attributed to the formation of low melting phases in the solidifying weld metal and in the heat-affected zone that lead to cracking due to shrinkage stresses and restraint imposed on the weld joint. Different heats of this fully austenitic alloy, containing 0.022–0.025% P, 0.75–1.00% Si and 0.21–0.42% Ti, as well as 316L stainless steel (SS) were investigated. Both Varestraint and hot ductility tests were used to evaluate the hot cracking susceptibility of these alloys. Longitudinal Varestraint tests were carried out at four strain levels of 0.5, 1.0, 2.0 and 4.0%, and the brittleness temperature range (BTR) was evaluated during testing. The Varestraint test results indicated that this material has a very high hot cracking susceptibility during autogenous welding, as the total crack length and maximum crack length in both the weld and heat-affected zone as also the BTR values were very high compared to that of SS 316L. Hot ductility tests were also conducted on using a Gleeble thermo-mechanical simulator to determine the nil ductility temperature (NDT). The NDT of this material is lower than SS316L, with the material containing higher titanium and silicon. This paper discusses the results of the hot cracking behaviour of this fully austenitic SS material using the longitudinal Varestraint and hot ductility tests.

New Friction Surfacing Application for Stainless Steel Pipe

Abstract: Friction surfacing, a solid state joining process similar to Friction Stir Welding (FSW), has not received much attention in the repair and modification of pipe and/or roll surfaces. It is well-known that both friction surfacing and FSW are two of several friction variants and that the operating parameters and deposited metal characteristics of the two processes are similar. This experimental work describes the fundamental parameters for friction surfacing AISI 316L austenitic stainless steel plates with AISI 440C quench-hardened, martensitic stainless steel and shows that lower speeds of rotation form more uniform overlays. Based on results of a series of plate experiments, friction surfacing of an 89.1 mm-diameter, AISI 316L pipe along its full circumference with an AISI 440C steel rod was successfully carried out. Since the overlaid metal obtained by friction surfacing has a very fine grain size and is not diluted by the base metal, improvements in toughness, corrosion resistance and wear resistance can be expected.