Showing papers in "Welding Journal in 1997"

Prediction of welding distortion

Abstract: This paper presents a numerical analysis technique for predicting welding-induced distortion. The technique combines two-dimensional welding simulations with three-dimensional structural analyses in a decoupled approach. The numerical technique is particularized on evaluating welding-induced buckling. The numerical predictions can be utilized either as a design evaluation or manufacturing analysis tool. As a design tool, the effect of the welding procedures can be determined and incorporated into the evaluation and optimization of the design configurations. As a manufacturing analysis tool, for a fixed design, different welding processes and procedures can be evaluated to minimize welding distortion. Experimental results obtained from small- and large-scale mock-up panels verify the numerical modeling approach.

Influence of Mn and Ni on the microstructure and toughness of C-Mn-Ni weld metals

Abstract: A systematic investigation has been carried out to study the microstructure and toughness of C-Mn-Ni low-alloy shielded metal arc (SMA) weld metals. The manganese and nickel concentrations were progressively changed to determine their influence on weld microstructure and mechanical properties as well as to identify their interactions. The results obtained showed that manganese and nickel have considerable effect on the weld metal microstructure, and both Mn and Ni affect the microstructure in a similar way, i.e., promoting acicular ferrite at the expense of proeutectoid ferrite (grain boundary ferrite and ferrite sideplates). The results in the top bead also showed that there is an optimum composition range that produces an optimum balance of weld metal microstructures. For optimum toughness, a combination of 0.6-1.4% manganese and 1.0-3.7% nickel is suggested. Additions beyond this limit promotes the formation of martensite and other microstructural features, which may be detrimental to weld metal toughness.

Cold welding - theoretical modeling of the weld formation

Abstract: Based on experimental investigations and an improved understanding of the mechanisms of weld formation in cold welding, a general theoretical model for weld strength in cold welding - earlier developed by Bay - has been extended and modified. The new model presented in this paper simulates the whole cold welding process, including the deformation of base metals and the resulting weld strength in welding similar, as well as dissimilar, metals. To verify the theoretical model, the calculated weld strengths are compared with experimental measurements. Good accordance is generally found, which shows the model is applicable.

An analytical solution to predict the transient temperature distribution in fillet arc welds

Abstract: This paper presents an analytical solution for predicting the transient temperature distribution in fillet arc welds. The analytical solution is obtained by solving a transient, three-dimensional heat conduction equation with convection boundary conditions on the surfaces of an infinite plate with finite thickness and mapping an infinite plate onto the fillet weld geometry with an energy equation. The electric arc heat input on the fillet weld and on the infinite plate is assumed to have a traveling bivariate Gaussian distribution. To check the validity of the solution, gas tungsten arc (GTA) and flux cored arc (FCA) welding experiments were performed under various conditions. The actual isotherms of the weldment cross sections at various distances from the arc start point are compared with those of the simulation result. As the result shows satisfactory accuracy, this analytical solution can be used to predict the transient temperature distribution in fillet welds of finite thickness under a moving bivariate Gaussian distributed heat source. The simplicity and short calculation time of the analytical solution provide rationale to use the analytical solution for modeling welding control systems or for optimizing welding process parameters.

Cold welding - experimental investigation of the surface preparation methods

Abstract: This paper presents a systematic experimental investigation for cold welding of five different metal combinations- Al-Al, Cu-Cu, steel-steel, Al-Cu and Al-steel - applying seven different surface preparation methods, including anodizing of Al, semibright electrolytic Ni plating, electroless Ni plating, heat-treated electroless Ni plating, electrolytic hard Cr plating, scratch brushing and degreasing with acetone. After roll welding with varying reductions, the weld strength was tested by a shear test system developed in the present work. The weld strength curves for alternative methods of surface preparation were compared for each metal combination. It was found that different surface preparation methods resulted in different weld strengths. The semibright Ni plating and electroless Ni plating are optimum for Al-Al, electroless Ni plating for Cu-Cu, heat-treated electroless Ni plating for steel-steel, and scratch brushing for Al-Cu and Al-steel.

Finite element modeling of residual stresses in austenitic stainless steel pipe girth welds

Abstract: Three-dimensional finite element modeling of residual stresses in a girth-welded pipe is presented in this paper, with an emphasis on modeling procedures for the global residual stress characteristics. The shell element model with a heat source moving along the circumferential joint is demonstrated to be cost effective and capable of predicting the global residual stress features. Transient residual stress behaviors near the weld start and stop locations are discussed. The effects of pipe wall thickness and welding speed on residual stresses also are presented in this paper.

Stainless steel cladding deposited by automatic gas metal arc welding

Abstract: Weld surfacing is increasingly employed to enhance the life of and to reduce the cost of engineering components. Gas metal arc (GMA) cladding is extensively applied in its automatic mode to obtain good quality stainless steel claddings. In stainless steel cladding, the amount of dilution and the mode of solidification of claddings are vital factors affecting the quality of claddings. Developed mathematical models relating GMAW process control parameters to cladding dimensions were used to deposit 316L and 309L stainless steel on structural steel IS:2062 and obtained 12% dilution in single layer claddings. The metallurgical features, such as cladding chemistry, microstructures, modes of solidifications, ferrite content, transition zone chemistry, etc., of single and multilayer claddings were analyzed. Controlled dilution level (12%) facilitated the achievement of the required levels of alloy content meeting corrosion resistance requirements and producing crack-free claddings. The hardness of the transition zone was found to be below 400 VHN due to low-carbon levels used in stainless steel filler metals and lower dilution achieved in cladding. Cladding solidified initially with planar or cellular structure and then gradually changed to cellular-dentritic structure depending upon the heat input condition and the dilution involved. Color metallography revealed three modes of solidification of stainless steel claddings and observed modes of solidification were in good agreement with the predicted modes. Estimated ferrite contents were also in close agreement with their corresponding measured values. Two types of ferrite morphology such as vermicular and lathy were found, and, at higher ferrite content levels, lathy morphology was predominant.

Self-learning fuzzy neural networks and computer vision for control of pulsed GTAW

Abstract: The objective of this research is to apply intelligent control methodology to improve weld quality. Based on fuzzy logic and artificial neural network theory, a self-learning fuzzy and neural network control scheme has been developed for real-time control of pulsed gas tungsten arc welding (GTAW). Using an industrial TV camera as the sensor, the weld face width of the weld pool, i.e., the feedback signal in the closed loop system, is obtained by computer image processing techniques. The computer vision providing process status information in real-time is an integral part of self-learning fuzzy neural control system. Such a system enables adaptive altering of welding parameters to compensate for changing environments. The experiments on the control of the pulsed GTAW process show that the scheme presented in this paper can be used to control complicated variables such as encountered in welding processes.

Soft zone formation in dissimilar welds between two Cr-Mo steels

Abstract: Two dissimilar weldments between 9Cr-1Mo and 2.25Cr-1Mo ferritic steels have been characterized for their microstructural stability during various postweld heat treatments (PWHTs). The samples for the investigation were extracted from bead-on-plate weldments made by depositing 2.25Cr-1Mo weld metal on 9Cr-1Mo base plate and vice versa. Subsequent application of PWHT resulted in the formation of a soft zone in the low Cr ferritic steel weld or base plate. A carbide-rich hard zone, adjoining the soft zone, was also detected in the high Cr side of the weldment. Unmixed zones in the weld metal provided additional soft and hard zones in the weld metals. The migration of carbon from low-Cr steel to high-Cr steel, driven by the carbon activity gradient, has been shown to be responsible for the formation of soft and hard zones. A carbon activity diagram for 2.25Cr-1Mo/9Cr-1Mo weldments has been proposed to aid in the selection of welding consumables for reducing or preventing the soft zone formation.

Universal quality assurance method for resistance spot welding based on dynamic resistance

Abstract: Welding of mild steel is one of the most common processes in spot welding. Numerous methods for ensuring high-quality welds are based on dynamic resistance. However, in general, these methods are only useful over a relatively narrow range of welding parameters. This article presents a universal quality assurance method based on dynamic resistance that is valid over a wide range of welding time, welding current, electrode force and electrode tip geometry (up to and including complete tip mushrooming). This method was constructed on the basis of models describing current density redistribution at the faying surface during the welding process. Validation of the method has been confirmed experimentally. The comparison of results shows that the method provides the maximum nugget size.

Arc light sensing of droplet transfer and its analysis in pulsed GMAW process

Abstract: A new method and its principle for sensing droplet transfer in GMAW of steel and aluminum alloy have been researched in this paper. A practical arc light sensing and controlling system has been developed. The reliability of the arc light characteristic signal that indicates droplet detachment and the control accuracy of the system have been verified using high-speed photography. Based on the mathematical model established in the paper, the mechanism of the signal and relative phenomena were analyzed. The parameters that influence arc light radiation were given and the limitation of this sensing method was discussed.

Ultrasonic energy welds copper to aluminium

Abstract: Attempting to weld copper to aluminum by conventional means through the application of thermal energy to melt and fuse the two metals (fusion welding) can result in an unreliable weld. The oxide layer on aluminum is difficult to remove, the melt temperatures of the two metals are not close, the two metals exhibit high thermal conductivity and alloying of the two metals creates a brittle intermetallic that is mechanically and electrically unreliable. An alternative for design engineers is to weld copper to aluminum by applying ultrasonic energy, which joins the metals without melting. The ultrasonic process creates a high quality weld both mechanically and electrically without forming a brittle intermetallic and without intermediate steps. Ultrasonic welding of copper to aluminum has been shown to be efficient and effective as demonstrated by a number of practical production applications. The problems of tough oxides, high thermal conductivity, high electrical conductivity, intermetallics and brittle alloys are not significant with the ultrasonic welding process. And similarly, the problems associated with pre and postweld cleaning, fluxes, hot metal and high energy costs are eliminated. Modern ultrasonic welding equipment is capable of monitoring energy and controlling the critical welding process variables.

Laser beam welding of austenitic-ferritic transition joints

Abstract: Transition joints between austenitic stainless steels and ferritic low alloy steels are extensively utilized in many high-temperature applications in energy conversion systems. Problems related to the use of such dissimilar metal welds (DMWs) have long been recognized, because of premature failures often occurring during service, connected to thermal stresses generated at the weld interface and to metallurgical changes (carbon migration, carbide precipitation) observed after prolonged exposure to high temperature. This paper reports the results of an investigation on DMWs carried out on plates and tubes by a deep penetration laser beam welding (LBW) process, within the framework of researching innovative welding procedures to allow for a better control of metallurgical changes and a minimization of thermal stresses. The experimental work included metallographic observations, hardness tests, x-ray diffractometry and estimation of phases on melt zones. These data are compared with the evaluation given by the Schaeffler diagram. The results show the possibility of obtaining chemical compositions and phases according to predictions. In most instances, the melt zone constitution was close to the desired one. Further trials with filler metal of more proper composition are in progress, to improve the soundness of the joint and to optimize structure of the melt zones.

CO2 laser beam welding of aluminum 5754-O and 6111-T4 alloys

Abstract: Legislative and market pressures have caused the automotive industry to consider more fuel efficient designs of vehicles in recent years. Many lightweight material options are being explored to reduce vehicle weight. Aluminum alloys are receiving a great deal of attention. As a result, development of rapid joining techniques for aluminum alloys has become an important research issue. The objective of this investigation was to develop welding procedures for autogenous CO 2 laser beam welding of aluminum 5754-O and 6111-T4 alloys for application in tailor welded blanks. The mechanical and microstructural characteristics of the welded joints were evaluated using tensile tests, microhardness tests, optical microscopy, and energy dispersive X-ray (EDAX) for local chemical analysis. Results indicate that both the alloys can be autogenously laser welded with full penetration, minimum surface discontinuities and little, if any, loss of magnesium through vaporization from the fusion zone. It was found that welds made on 5754-O with a 3-kW laser beam and a travel speed ranging between 100 and 400 in./min had total longitudinal elongation (17.3-23.6%), close to the base metal value (22%). Similar welds on Alloy 6111-T4 welds had lower longitudinal elongation (8.6-18.7%), compared to the base material (26%). The reduced ductility observed in 6111 laser welds is probably due to weld solidification cracking in the fusion zone. Based on the results, laser welded aluminum alloys possess potential for use in automotive fabrication applications.

Ferrite determination in stainless steel welds -- Advances since 1974

Abstract: Since the Comfort A. Adams Lecture of W. T. DeLong in 1974, significant advances in the determination of ferrite content in stainless steel welds have taken place. These are reviewed in the present Comfort A. Adams Lecture. Examination of MagneGage Number 3 Magnet strengths led to a concept for extending, by extrapolation, the calibration range of AWS A4.2-74 to ferrite levels above 28 FN. Ferrite Numbers could then be assigned to thinner coating thickness standards for primary calibration of MagneGages over the extended range. Round robin studies were conducted to establish the reproducibility of measurement of Ferrite Numbers in duplex stainless steel welds. The method was incorporated into ANSI/AWS A4.2-91. Calibration using primary standards (coating thickness standards) is limited to a very few instruments, due to the difference in distribution of ferromagnetic material in coating thickness standards vs. that in stainless steel weld metal. Secondary standards, covering the range from near zero to about 100 FN, became available for calibrating additional instruments at the beginning of 1995. A round robin of tests established that the interlaboratory reproducibility of measurement after calibration by the secondary standards is similar to that observed with MagneGages calibrated by use of primary standards. Excessive ferrite in duplex stainless steel weld metals has adverse effects on weld properties. The utility of the Ferrite Number measurement system for duplex stainless steels is thus established. Development of a solid link between Ferrite Number and ferrite percent, determination of ferrite in the heat-affected zone (HAZ) of duplex stainless steel weldments, and further development of predicting diagrams remain for the future.

Thermal stress development in a nickel based superalloy during weldability test

Abstract: A finite element model has been developed to quantitatively evaluate the local thermomechanical conditions for weld metal solidification cracking in a laboratory weldability test (the Sigmajig test). The loading mechanism in the Sigmajig test was simulated by means of nonlinear spring elements. The effects of weld pool solidification on the thermal and mechanical behaviors of the specimen were considered. An efficient algorithm was developed to include the solidification effects in the material constitutive relations. Stress/temperature/location diagrams were constructed to reveal the local stress development behind the traveling weld pool where solidification cracking occurs. Based on the concept of the material resistance to cracking and the mechanical driving force for cracking, the calculated local stress in the solidification temperature range was used to explain the experimentally observed cracking initiation behaviors of a nickel-based superalloy single crystal under different welding and loading conditions.

Analysis of welding shrinkage

Abstract: The problems of distortion, residual stresses and reduced strength of structure in and around a welded joint are of major concern in the shipbuilding industry and in other similar manufacturing industries. The present paper deals with the theoretical analysis of transverse shrinkage in a welded butt joint. The mathematical model used in this analysis is based on the assumption that the plate undergoing welding is made up of a thermoelastoplastic zone and a fully elastic zone. The analysis provides a simplified approach for estimating welding shrinkage.

Prediction of axis shift distortion during circumferential welding of thin pipes using the finite element method

Abstract: Axis shift distortion is one type of distortion encountered during the circumferential welding of large, thin pipes. the result of this is the loss of coaxiality of the pipes. This type of distortion is caused by the time lag in the solidification of various segments of weld metal around the circumference. The development of shrinkage forces in the weld metal especially in the axial direction is nonuniform due to the time lag, and this causes axis shift for the pipes. the development of a mathematical model using the finite element method for prediction of axis shift distortion in thin pipes is described in this work. Thermal analysis and subsequent elastic-plastic stress analysis for the pipe are performed using four-noded, bilinear degenerated shell elements. In addition to axis shift distortion, axial and circumferential stress distributions in the pipe are also determined by the model. The model is validated by conducting partial welding on the circumference of a thin pipe and measuring the transient thermal history and root opening at the bottom. Details of thermal and elastic-plastic analyses, welding trials, thermal and displacement results and axial and circumferential stress results are presented in this paper.

Evaluation of the circular patch test for assessing weld solidification cracking. Part 1: Development of a test method

Abstract: A new test methodology for assessing weld metal solidification cracking using the circular patch test is proposed. The proposed test methodology utilizes (1) a new test sample and fixture design, (2) current ramping to determine an effective current range specific to each material, and (3) the best index for comparative cracking susceptibility evaluation based on continuous surface-breaking centerline cracking. The development of anew test sample and fixture enables improved weld bead control and increased restraint resulting in a high degree of reproducibility. With the preliminary current ramping method, an effective current range can be determined. By comparing these effective current ranges for various alloys, a single constant current can be selected for further cracking susceptibility comparison among the materials. Quantitative cracking susceptibility is based upon the degree of continuous surface-breaking centerline cracking. This was the easiest and most reproducible means of evaluating and comparing cracking susceptibility. This paper discusses the background and experimental approach involved in the development of the procedure proposed.

Scandium places aluminium welding on a new plateau

Abstract: An interesting new development in nonferrous metallurgy seems to be taking the world by storm. The topic is scandium, a rare metal that is now being mined in the Ukraine. Known for its toughness, it was formerly a top secret material used in fighter planes, including the MIG 29, during the existence of the Soviet Union. This metal is now being introduced into a variety of aluminum alloys in this country, both in base metal and filler metal forms. The results are increases in strength, fatigue life and weldability. US interest in scandium-containing products is burgeoning, examples include aircraft, components for automobiles and trucks, high-speed trains, and boats. The technology is being developed and implemented at two laboratories, one in Baltimore, MD, and the other in Kiev, Ukraine. Both laboratories belong to Ashurst Technology Ltd., Hamilton, Bermuda. The author recently visited the Baltimore laboratory where he learned more about scandium and its current and potential effects on industry. This report documents what he learned.

Weld metal microstructure calculations from fundamentals of transport phenomena in the arc welding of low-alloy steels

Abstract: In recent years, significant progress has been made toward understanding the development of the weld pool shape and size from the numerical calculations of heat transfer and fluid flow in the weld pool. Although such calculations have provided detailed information about the welding processes, no efforts have been made to understand the development of fusion zone microstructures from the fundamentals of transport phenomena. The aim of this work is to address this. Heat transfer and fluid flow during manual metal arc welding of low-alloy steels containing different concentrations of vanadium and manganese were investigated by solving the equations of conservation of mass, momentum and energy in three-dimensional transient form. The computed microstructures are found to be in good agreement with the experimentally observed microstructures. The agreement indicates significant promise for predicting weld metal microstructure from the fundamentals of transport phenomena.

Effect of welding conditions on microstructure and properties of type 316L stainless steel submerged arc cladding

Abstract: Mathematical models were developed using response surface methodology for studying the direct and interaction effects of submerged arc welding parameters on stainless steel cladding geometry. The process parameters obtained from those models were employed to clad IS:2062 structural steel plate of 20-mm thickness using 316L stainless steel wire of 3.15-mm diameter. A low dilution of 22.57% was achieved in the cladding. Dilution was low when both voltage and welding speed were either high or low. Requirements of carbon and ferrite contents in the cladding were met by achieving low dilution in a single layer as well as multilayer cladding. Color metallographic techniques revealed that in the as-welded condition of cladding, the microstructural constituents of the HAZ, fusion boundary zone, and cladding surfaced with a low dilution condition were bainite and ferrite, martensite, and austenite plus ferrite, respectively. The hardness of the existing martensitic structures at the intermediate mixed zones in overlays was below 400 VHN, which was attributed to the lower carbon content in the cladding. The solidification modes were found to be austenitic, ferritic, and austenitic and ferritic. The microstructure was found to be mainly cellular or cellular dendritic. The measured ferrite contents of cladding were well within their corresponding predicted values. The cladding possessed good ductility and resistance to intergranular corrosion.

Transient liquid-phase bonding using coated metal powders

Abstract: Powder particles coated with a small amount of melting point depressant (MPD) reveal different sintering behavior in comparison to an uncoated powder mixture of the same composition. Interlayers consisting of the coated powder particles were used in the transient liquid-phase (TLP) bonding process. The coating material and the thickness of the deposit are important parameters that influence shrinkage. The amount of MPD was controlled such that the volume fraction of the liquid was very small but existed at all contacts, thus improving densification of the interlayer. Ni-20Cr and 304L stainless steel powders coated with Ni-10P were applied to join 304 stainless steels. Fully dense joints with mechanical properties comparable to those of the base metals were obtained with Ni-20Cr powder interlayers, whereas joints with 304L stainless steel powder interlayers showed inferior mechanical properties due to residual porosity in the joints.

An evaluation of rapid methods for diffusible weld hydrogen

Abstract: An evaluation has been carried out of rapid methods for measuring diffusible weld hydrogen, and the results have been compared with those of the primary room temperature mercury method. Different analytical techniques have been studied looking at extraction temperatures of up to 400°C (752°F), while a range of consumable types for shielded metal arc (SMA), flux cored arc (FCA) and submerged arc welding (SAW) deposits was included. With the exception of welds containing porosity, satisfactory agreement with the primary method was obtained at temperatures up to 400°C, provided appropriate care was taken. Analysis times were markedly reduced from 14 to 21 days at 20°C (68°F) to 30 min at 400°C.

Fusion welding of a modern borated stainless steel

Abstract: Experiments designed to assess the fabrication and service weldability of 304B4A borated stainless steel were conducted. Welding procedures and parameters for manual gas tungsten arc (GTA) welding, autogenous electron beam (EB) welding and filler-added EB welding were developed and found to be similar to those for austenitic stainless steels. Following the procedure development, four test welds were produced and evaluated by microstructural analysis and Charpy impact testing. Further samples were used for determination of the postweld heat treatment (PWHT) response of the welds. The fusion zone structure of welds in this alloy consists of primary austenite dendrites with an interdendritic eutectic-like austenite/ boride constituent. Welds also show an appreciable partially molten zone that consists of the austenite/boride eutectic surrounding unmelted austenite islands. The microstructure of the EB welds was substantially finer than that of the GTA welds, and boride coarsening was not observed in the solid state heat-affected zone (HAZ) of either weld type. The impact toughness of as-welded samples was found to be relatively poor, averaging less than 10 J (7.38 ft-lb) for both GTA and EB welds. For fusion zone notched GTA and EB samples and centerline notched EB samples, fracture generally occurred along the boundary between the partially molten and solid-state regions of the HAZ. The results of the PWHT study were very encouraging, with typical values of the impact energy for HAZ notched samples approaching 40 J (29.5 ft-lb), or twice the minimum code-acceptable value. The PWHT results in spheroidization of the boride such that the heat-treated welds have microstructures and failure modes similar to the as-received material. A weld process/PWHT combination that results in acceptable properties was identified, and the feasibility of joining these alloys was demonstrated.

A numerical analysis of molten pool convection considering geometric parameters of cathode and anode

Abstract: The heat transfer and fluid flow in the molten pool during stationary gas tungsten arc welding (GTAW) using Ar shielding gas have been studied with an emphasis on the impact of geometric parameters on the welding arc, such as electrode bevel angle, arc length and surface depression due to arc pressure acting on the molten pool surface. Driving forces responsible for weld pool convection, i.e., self-induced electromagnetic force, surface tension due to the temperature gradient at the surface of the molten pool and shear stress acting on the molten pool surface by the arc plasma flow, were considered. The numerical model was applied to type 304L stainless steel plate with 30 ppm sulfur. As the welding current increased, the discrepancy of penetration at the weld center between the calculation and experiment increased because of the strong flow along the molten pool surface. As the electrode bevel angle decreased, the shear stress acting on the molten pool surface increased. Therefore, the fusion width increased and the fusion depth decreased. Changing the arc length did not influence the fusion shape because variation in shear stress, which determined the surface flow along the molten pool surface, was minor. From the simulation, considering the depression of the molten pool surface, the effects of the increased arc length and elevated anode temperature resulted in a more accurate fusion width.