Showing papers on "Heat-affected zone published in 1998"

Properties of friction-stir-welded 7075 T651 aluminum

Abstract: Friction stir welding (FSW), a new welding technique invented at TWI, was used to weld 7075 T651 aluminum, an alloy considered essentially unweldable by fusion processes. This weld process exposed the alloy to a short time, high-temperature spike, while introducing extensive localized deformation. Studies were performed on these solid-state welds to determine mechanical properties both in the longitudinal direction, i.e., within the weld nugget, and, more conventionally, transverse to the weld direction. Because of the unique weld procedure, a fully recrystallized fine grain weld nugget was developed. In addition, proximate to the nugget, both a thermomechanically affected zone (TMAZ) and heat affected zone (HAZ) were created. During welding, temperatures remained below the melting point and, as such, no cast or resolidification microstructure was developed. However, within the weld nugget, a banded microstructure that influences room-temperature fracture behavior was created. In the as-welded condition, weld nugget strength decreased, while ductility remained high. A low-temperature aging treatment failed to fully restore T651 strength and significantly reduced tensile ductility. Samples tested transverse to the weld direction failed in the HAZ, where coarsened precipitates caused localized softening. Subsequent low-temperature aging further reduced average strain to failure without affecting strength. Although reductions in strength and ductility were observed, in comparison to other weld processes, FSW offers considerable potential for welding 7075 T651 aluminum.

Dynamics of keyhole and molten pool in laser welding

Abstract: In laser and electron-beam welding, a deep cavity called a keyhole or beam hole is formed in the weld pool due to the intense recoil pressure of evaporation. The formation of the keyhole leads to a deep penetration weld with a high aspect ratio and this is the most advantageous feature of welding by high-energy-density beams. However, a hole drilled in a liquid is primarily unstable by its nature and the instability of the keyhole also causes the formation of porosity or cavities in the weld metal. In particular, the porosity formation is one of the serious problems in very high-power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The article will describe the real-time observation of keyhole and plume behaviors in the pulsed and continuous-wave laser welding by high-speed optical and x-ray transmission methods, the cavity formation process and its suppression measures.

Improving the weldability and service performance of nickel-and iron-based superalloys by grain boundary engineering

Abstract: The principal limitation of today’s Ni- and Fe-based superalloys continues to be their susceptibility to intergranular degradation arising from creep, hot corrosion, and fatigue. Many precipitation-strengthened superalloys are also difficult to weld, owing to the formation of heat-affected zone (HAZ) cracks during postweld heat treatments (PWHTs). The present work highlights significant improvements in high-temperature intergranular degradation susceptibility and weldability arising from increasing the relative proportion of crystallographically “special” low-Σ CSL grain boundaries in the microstructure. Susceptibility to intergranular degradation phenomena is reduced by between 30 and 90 pct and is accompanied by decreases in the extent and length of PWHT cracking of up to 50-fold, with virtually no compromise in mechanical (tensile) properties upon which the functionality of these specialty materials depends. Collectively, the data presented suggest that “engineering” the crystallographic structure of grain boundaries offers the possibility to extend superalloy lifetimes and reliability, while minimizing the need for specialized welding techniques which can negatively impact manufacturing costs and throughput.

Numerical analysis of GTA welding process with emphasis on post-solidification phase transformation effects on residual stresses

Abstract: The objective of this work was to analyze the residual stress state in spot welds made in an HY-100 steel disk by an autogenous gas tungsten arc (GTA) welding process An uncoupled thermal-mechanical finite element (FE) model was developed that took into account the effects of liquid-to-solid and solid-state phase transformations Effects of variations in mechanical properties due to solid-state phase transformations on residual stresses in the weld were studied Extensive experimental testing was carried out to determine the mechanical properties of HY-100 steel The residual stresses in the disk with the spot weld were measured by a neutron diffraction (ND) technique The FE results are in good agreement with the ND measurements The results show that the volumetric changes associated with the austenite to martensite phase transformation in HY-100 steel significantly affect residual stresses in the weld fusion zone and the heat affected zone

Friction stir welding with roller stops for controlling weld depth

Abstract: Stir friction welding is accomplished by applying force to plunge a nonconsumable, rotating welding post into the region to be welded. The rotation creates friction which at least partially melts the material to be welded, as the welding post penetrates. The force is continued, causing the post to continue to penetrate. A set of rollers associated with the welding head eventually reaches the surface of the workpiece when the welding post reaches the proper depth, and prevents further penetration of the welding post. In one embodiment of the invention, the rollers have crowned surfaces, and the force is applied by a hydraulic cylinder.

Friction-stir welding: microstructural characterization

Abstract: The microstructures of friction-stir welded aluminum alloys (1100 and 6061) are dominated by dynamic recrystallization. Friction-stir welding of 6061 aluminum to copper produced a complex, intercalated microstructure which was also dominated by dynamic recrystallization. The friction-stir welding process is characterized by extreme plastic deformation in the solid state; there is no associated melting.

Method and system for gas metal arc welding

Abstract: An improved method of gas metal arc welding (GMAW) is disclosed. The method includes utilizing a pulsed current having a variable waveform to ensure the detachment of one-droplet-per-pulse of current. During the welding process, the current is sufficient to produce a droplet at the end of a consumable electrode wire. After the droplet reaches a desired size, the current is lowered to induce an oscillation in the droplet. The current is then increased which, in combination with the momentum created by the oscillation, effects droplet detachment. The oscillation may be monitored by observing the arc voltage to determine a preferred detachment instant. A computer implemented method allows for the adaptive control of the current waveform to accommodate for anticipatable variations in the welding conditions, while maintaining ODPP transfer and a constant pulse period. An accompanying system is disclosed for implementing the method of adaptive welding.

Investigation into properties of laser welded similar and dissimilar steel joints

Abstract: Laser beam welding is currently used in the welding of steels, aluminium alloys, thin sheets, and dissimilar materials. This high power density welding process has unique advantages of cost effectiveness, deep penetration, narrow bead and heat affected zone (HAZ) widths, and low distortion compared to other conventional welding processes. However, the metallurgical and mechanical properties of laser welds and the response of conventional materials to this new process are not yet fully established. The welding process may lead to drastic changes in the microstructure with accompanying effects on the mechanical properties and, hence, on the performance of the joint. The thermal cycles associated with laser beam welding are generally much faster than those involved in the conventional arc welding processes. This leads to the formation of a rather small weld zone that exhibits locally a high hardness in the case of C–Mn structural steels owing to the formation of martensite. It is currently difficult ...

Spatially resolved X-ray diffraction phase mapping and α → β → α transformation kinetics in the heat-affected zone of commercially pure titanium arc welds

Abstract: Spatially resolved X-ray diffraction (SRXRD) is used to map the α → β → α phase transformation in the heat-affected zone (HAZ) of commercially pure titanium gas tungsten arc welds. In situ SRXRD experiments were conducted using a 180-µm-diameter X-ray beam at the Stanford Synchrotron Radiation Laboratory (SSRL) (Stanford, CA) to probe the phases present in the HAZ of a 1.9 kW weld moving at 1.1 mm/s. Results of sequential linear X-ray diffraction scans made perpendicular to the weld direction were combined to construct a phase transformation map around the liquid weld pool. This map identifies six HAZ microstructural regions between the liquid weld pool and the base metal: (1) α-Ti that is undergoing annealing and recrystallization; (2) completely recrystallized α-Ti; (3) partially transformed α-Ti, where α-Ti and β-Ti coexist; (4) single-phase β-Ti; (5) back-transformed α-Ti; and (6) recrystallized α-Ti plus back-transformed α-Ti. Although the microstructure consisted predominantly of α-Ti, both prior to and after the weld, the crystallographically textured starting material was altered during welding to produce different α-Ti textures within the resulting HAZ. Based on the travel speed of the weld, the α → β transformation was measured to take 1.83 seconds during heating, while the β → α transformation was measured to take 0.91 seconds during cooling. The α → β transformation was characterized to be dominated by long-range diffusional growth on the leading (heating) side of the weld, while the β → α transformation was characterized to be predominantly massive on the trailing (cooling) side of the weld, with a massive growth rate on the order of 100 µm/s.

Friction plug welding

Abstract: Friction plug welding (FPW) usage is advantageous for friction stir welding (FSW) hole close-outs and weld repairs in 2195 Al—Cu—Li fusion or friction stir welds. Current fusion welding methods of Al—Cu—Li have produced welds containing varied defects. These areas are found by non-destructive examination both after welding and after proof testing. Current techniques for repairing typically small (<0.25″) defects weaken the weldment, rely heavily on welders' skill, and are costly. Friction plug welding repairs increase strength, ductility and resistance to cracking over initial weld quality, without requiring much time or operator skill. Friction plug welding while pulling the plug is advantageous because all hardware for performing the weld can be placed on one side of the workpiece.

Predictions of metal droplet formation in gas metal arc welding. II.

Abstract: Predictions of metal droplet formation in gas metal arc welding (GMAW) are made for mild steel wires with argon as a shielding gas, using a generalized theory for predictions of arc and electrode properties in arc welding The theory is a unified treatment of the arc and the electrodes and includes a detailed analysis of the anode sheath region, together with a free surface treatment for the molten metal at the tip of the welding wire Results of calculations made for a mild steel wire of 012 and 016 cm diameters are in good agreement with experimental measurements of droplet diameter and droplet detachment frequency at currents between 150 and 330 A The results indicate that the anode sheath region and the effects due to variation of surface tension with temperature have an important influence on the dynamics of droplet growth at the tip of the wire in GMAW Predictions are made for the transition from the globular to the spray mode of metal transfer, in agreement with experimental observations in arc welding

Rotor repair system and technique

Abstract: A system for repairing worn, distorted, cracked, or degraded portions of high temperature rotors such as those used in high-pressure and reheat steam turbines is disclosed. The repairs are applicable to low alloy steels generally described in ASTM Specification A-470 classes 3, 7, and 8. Explicit controls on the welding process, the welding consumables, and the placement of the weld fusion line are disclosed. For the welding process, a novel staging of the "relative heat input" for applying the initial cold wire gas tungsten arc weld (GTAW) buttering layer is disclosed. Significantly, the optimum weldment properties are achieved in the cold wire GTAW by utilizing a lower heat input for the crucial second layer relative to the first layer. Faster deposition or weld build-up is achieved over the buttering layer by applying the balance of welding through utilization of the hot-wire GTAW process. Hot-wire weld integrity is assured by control of a helium-argon cover gas mixture, application of a trailing gas shroud, weld-head oscillation, and control of the wire insertion point into the molten puddle. For the weld deposit, a specially modified 9Cr-1Mo filler metal based on the "Grade 91" alloy developed by the Oak Ridge National Laboratory is selected. Additional stringent controls are placed on the chemical composition of the weld wire. Finally, judicious placement of the weld fusion line to insure long service is achieved by a detailed finite-element stress analysis. Near the fusion line, the stresses are limited to values below the minimum stress-rupture strength of the base metal as described by a correlation using the Larson-Miller time-temperature parameter.

Hot plate welding of polypropylene. Part I: Crystallization kinetics

Abstract: The crystalline structure formation in the heat affected zone during hot plate welding has a great influence on the performance of the welded semi-crystalline polymers. The quiescent and flow induced crystallization of polypropylene was investigated experimentally. A simplified, phenomenological crystallization model was developed, which can describe the crystal formation from completely melted and partially melted polymer. Predictions obtained from the model were compared with experimental results.

A proposed mechanism for equiaxed grain formation along the fusion boundary in aluminum-copper-lithium alloys

Abstract: The effects of welding conditions, composition and solidification substrate have been systematically studied in an effort to determine the nature of nondendritic equiaxed grain formation in Al-Cu-Li welds. The equiaxed zone (EQZ) in these alloys forms preferentially at the fusion boundary of arc welds and may have important implications with respect to both weld cracking susceptibility and structural integrity. Over a wide range of weld heat input, achieved by varying weld travel speed and current during autogenous gas tungsten arc welding, it was not possible to eliminate this zone, although its width and the grain size varied considerably. The solidification substrate had a profound effect on EQZ formation. The EQZ did not form when solidification occurred from an as-cast weld metal substrate. Under other substrate conditions, including the as-cast/solution heat treated condition, an EQZ was always present. Both lithium and zirconium influence EQZ formation, with zirconium exhibiting a more dominant effect. At low levels of lithium and zirconium (0.5 Li, 0.03 Zr), an EQZ was not observed. A unified heterogeneous nucleation mechanism is proposed to describe EQZ formation in Al-Cu-Li welds.

CO{sub 2} laser beam welding of magnesium-based alloys

Abstract: Magnesium has gained increased attention in recent years as a structural metal--especially in the automotive industry--necessitating the development of welding techniques qualified for this new application. Lasers are known to be an excellent tool for joining metals. This paper presents results of recent investigations on the weldability of several cast and wrought magnesium-based alloys. Plates with a thickness of 2.5--8 mm were butt joint welded with and without filler metal using a 2.5-kW CO{sub 2} laser. The investigations showed that magnesium alloys can be easily laser welded in similar and dissimilar joints. The beam characteristics of the laser leads to small welds and a deep penetration depth. Crackfree welds exhibiting low porosity and good surface finish can be achieved with appropriate process parameters. Generally, the laser welding leads to either no change or a small increase in hardness in the fusion zone (FZ) and in the heat-affected zone (HAZ) relative to the base metal. Less promising results were obtained for the cast alloy QE22, in which cracking in the age-hardened condition and a significant decrease in hardness occurred. Laser welded die cast alloys showed an extremely high level of porosity in the weld.

Elevated-temperature, plasma-transferred arc welding of nickel-base superalloy articles

Abstract: A nickel-base superalloy article which is susceptible to strain-age cracking and has a directionally oriented, single crystal, or equiaxed grain structure is repaired with minimal welding heat input into the article. The article is first heated to a welding temperature of from about 1650° F. to about 2000° F. in an inert atmosphere. A damaged area of the article is weld repaired using a plasma-transferred arc welder which vaporizes a filler metal in a plasma arc and deposited the vaporized metal onto the article to form a weld overlay. Minimal additional heat is added to the article during welding, as the weldment metal is vaporized remotely from the article.

Microstructure of friction weld interface of 1050 aluminium to austenitic stainless steel

Abstract: Type 1050 aluminium was bonded to type 304 stainless steel by a friction welding procedure. The aluminium was greatly deformed, and the grains were elongated and refined near the weld interface. The stainless steel was slightly deformed and partly transformed at the faying surface from austenite to martensite owing to hard friction. As a result, the hardness of both materials in the vicinity of the weld interface was higher than that of the base metals. Constituent elements of both materials had interdiffused through the weld interface, and some intermetal1ic compounds, Fe2Al5, FeAl, and Fe3Al, were formed at the weld interface. It was presumed by estimating with metallographic observation that the welding temperature was lower than the lowest eutectic temperature in the metal system. So the friction welding mechanism of this system was based on interdiffusion of each constituent in the solid state.

Weld pool oscillation as a tool for penetration sensing during pulsed GTA welding

Abstract: This paper deals with weld pool oscillation during pulsed gas tungsten arc (GTA) welding of unalloyed steel. Experiments were carried out in which the weld pool was triggered into oscillation during both the base time and the pulse time by short current peaks, superimposed on the pulsed welding current. The arc voltage was continuously measured during welding and the oscillation frequency spectrum was calculated on-line from the voltage variations by means of Fast Fourier Transform analysis, using a computer program. It was found that the obtained frequency distribution gives direct information about the penetration of the weld pool and can serve as a basis for sensing and in-process control of weld penetration.

Effects of welding parameters on hard zone formation at dissimilar metal welds

Abstract: An experimental study was conducted to determine effects of welding parameters and to optimize those parameters that have the most influence on eliminating or reducing the extent of hard zone formation at dissimilar metal welds (DMWs). Preheat, base metal thickness and welding electrode composition were found to have the most influence. Maintaining an optimum preheat for a given base metal thickness and controlling the maximum interpass temperature throughout welding resulted in drastic reduction and often complete elimination of hard zones at DMWs fabricated with ENiCrFe-3 electrodes, but not those welds fabricated with E309 stainless steel electrodes. This finding indicates that depending on the cooling rate and composition of the welding electrode, hard zones in DMWs can be eliminated. The cooling rate must be slow enough to avert formation of hard allotropic structures (i.e., martensite) and fast enough to avoid precipitation of hard intermetallic phases. The optimum welding electrode composition is one that will retard formation and precipitation of intermetallic phases during welding while the preheat needed to prevent the formation of allotropics is being maintained. Unfortunately, this unique characteristic is not available in most, if not all, austenitic stainless steel electrodes; nickel-based welding electrodes have been demonstrated to be more receptive.