Showing papers on "Heat-affected zone published in 2005"

Review of laser hybrid welding

Abstract: In this article, an overview of the hybrid welding process is given. After a short historic overview, a review of the fundamental phenomenon taking place when a laser (CO2 or Nd:YAG) interacts in the same molten pool as a more conventional source of energy, e.g. tungsten in-active gas, plasma, or metal inactive gas/metal active gas. This is followed by reports of how the many process parameters governing the hybrid welding process can be set and how the choice of secondary energy source, shielding gas, etc. can affect the overall welding process. An overview of the benefits and drawbacks of hybrid welding is presented, including reports on gap bridging ability, changes in welding speed and weld penetration, overall weld quality, and changes in heat input to the material being welded. This overview is followed by a few examples of industrial applications of hybrid welding. Finally, a section is devoted to explain about further work required in order to understand and tackle the hybrid welding process more efficiently in the future.In this article, an overview of the hybrid welding process is given. After a short historic overview, a review of the fundamental phenomenon taking place when a laser (CO2 or Nd:YAG) interacts in the same molten pool as a more conventional source of energy, e.g. tungsten in-active gas, plasma, or metal inactive gas/metal active gas. This is followed by reports of how the many process parameters governing the hybrid welding process can be set and how the choice of secondary energy source, shielding gas, etc. can affect the overall welding process. An overview of the benefits and drawbacks of hybrid welding is presented, including reports on gap bridging ability, changes in welding speed and weld penetration, overall weld quality, and changes in heat input to the material being welded. This overview is followed by a few examples of industrial applications of hybrid welding. Finally, a section is devoted to explain about further work required in order to understand and tackle the hybrid welding process more ...

Characterization of aluminum/steel lap joint by friction stir welding

Abstract: The welding of a lap joint of a commercially pure aluminum plate to a low carbon steel plate (i.e., Al plate top, and steel plate bottom) was produced by friction stir welding using various rotations and traveling speeds of the tool to investigate the effects of the welding parameters on the joint strength. The joint strength depended strongly on the depth of the pin tip relative to the steel surface; when the pin depth did not reach the steel surface, the joint failed under low applied loads. Meanwhile, slight penetration of the pin tip to the steel surface significantly increased the joint strength. The joint strength tended to increase with rotationspeed and slightly decrease with the increase in the traveling speed, although the results were quite scattered. The effects of the welding parameters were discussed metallographically based on observations with optical and scanning electron microscopes.

Laser welding of a NiTi alloy: Mechanical and shape memory behaviour

Abstract: The present paper reports the effect of Nd:YAG laser welding on the mechanical and functional properties of Ni–51 at% Ti alloy The specimens were prepared from thin sheets and tested in martensitic conditions The mechanical and shape memory effects of both welded and unwelded material (reference) were measured using standard tensile tests and thermo-mechanical cycles Differential scanning calorimeter investigations were carried out to determine the phase transformation temperatures Micro-hardness tests and microscopic investigations were performed in order to evaluate the extension of the heat affected zone and the melting zone Finally, a systematic comparison of the results between welded and reference specimens was carried out

Friction Stir Spot Welding of Advanced High-Strength Steels - A Feasibility Study

Abstract: An exploratory study was conducted to investigate the feasibility of friction stir spot welding advanced high-strength steel sheet metals. The fixed pin approach was used to weld 600MPa dual phase steel and 1310MPa martensitic steel. A single tool, made of polycrystalline cubic boron nitride, survived over one hundred welding trials without noticeable degradation and wear. Solid-state metallurgical bonding was produced with welding time in the range of 2 to 3 seconds, although the bonding ligament width was relatively small. The microstructures and hardness variations in the weld regions are discussed. The results from tensile-shear and cross-tensile tests are also presented.

The humping phenomenon during high speed gas metal arc welding

Abstract: A commonly observed welding defect that characteristically occurs at high welding speeds is the periodic undulation of the weld bead profile, also known as humping. The occurrence of humping limits the range of usable welding speeds in most fusion welding processes and prevents further increases in productivity in a welding operation. At the present time, the physical mechanisms responsible for humping are not well understood. Thus, it is difficult to know how to suppress humping in order to achieve higher welding speeds. The objectives of this study were to identify and experimentally validate the physical mechanisms responsible for the humping phenomenon during high speed gas metal arc (GMA) welding of plain carbon steel. A LaserStrobe video imaging system was used to obtain video images of typical sequences of events during the formation of a hump. Based on these recorded video images, the strong momentum of the backward flow of molten metal in the weld pool that typically occurred during high speed welding was identified as the major factor responsible for the initiation of humping. Experiments with different process variables affecting the backward flow of molten weld metal were used to validate this hypothesis. These process variables included welding speed, welding position and shielding gas composition. The use of downhill welding positions and reactive shielding gases was found to suppress humping and to allow higher welding speeds by reducing the momentum of the backward flow of molten metal in the weld pool. This would suggest that any process variables or welding techniques that can dissipate or reduce the momentum of the backward flow of molten metal in the weld pool will facilitate higher welding speeds and productivity.

Effects of torch configuration and welding current on weld bead formation in high speed tandem pulsed gas metal arc welding of steel sheets

Abstract: Undercut and humping bead are the common defects that limit the maximum welding speed of tandem pulsed gas metal arc (GMA) welding. In order to increase the maximum welding speed, effects of the inclination angle, interwire distance and welding current ratio between the leading wire and trailing wire on bead formation in high speed welding are investigated. The undercut and humping bead is attributed to the irregular flow of molten metal towards the rear part of the weld pool. This irregular flow can be prevented by the trailing wire with a push angle from 5° to 13° , which provides an appropriate component of arc force in the welding direction. The irregular flow is also related to the distance between the leading wire and the trailing wire, and the flow becomes regular when the distance is in the range 9–12 mm. Moreover, the stabilisation of the bulge of the weld pool between the two wires, the presence of enough molten metal below the trailing arc, and the reduced velocity of molten metal flow ...

Hot cracking in Al–Mg–Si alloy laser welding – operating parameters and their effects

Abstract: Hot cracking is a phenomenon that frequently occurs in the laser welding of some “special” alloys, such as the aluminium–magnesium–silicon type. Each occurrence of this phenomenon needs to be studied in itself, taking into account not only the individual, but also the interactive, influences of the various parameters. The advantage of using laser beams in welding processes lies in the speeds that can be reached. The disadvantage, however, is that, owing to the high cooling rates characteristic of the interaction between the laser beam and the material, the welding speed itself becomes a cause of hot cracking. The aim of this paper is to see how this disadvantage may be eliminated. We consider what the most important parameters may be, relating to tensile strength and the quantity of cracks produced, that might influence the presence or absence of hot cracking. The most influential factors in avoiding hot cracking are the welding speed and wire parameters. Also important is welding stability, as instability generates cracks. We can then determine a technological window, useful for industrial applications, which takes into account the values of these influential factors and stability.

On the role of liquated γ′ precipitates in weld heat affected zone microfissuring of a nickel-based superalloy

Abstract: The role of γ′ precipitate particles, which is the principal strengthening phase of most of the precipitation hardened nickel base superalloys, on weld heat affected zone (HAZ) cracking was investigated. The HAZ microstructures around welds in a commercial nickel-based superalloy IN 738LC were simulated using Gleeble thermomechanical simulation system. Microstructural examination of the simulated HAZs and those present in actual tungsten inert gas (TIG) welded specimens showed that γ′ particles persisted during heating to the welding simulation temperatures, where they reacted with the surrounding γ matrix producing liquid film by a eutectic-type reaction, which subsequently infiltrated the grain boundary regions. The on-cooling ductility of the alloy was significantly reduced as shown by the comparatively low ductility recovery temperature. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that HAZ cracking resistance was damaged by liquation reaction involving the γ′ precipitate particles. The results support previously reported observations which indicated that γ′ precipitate particles could contribute to HAZ microfissuring in superalloys through liquation reaction besides the generally reported rapid solid-state re-precipitation of γ′ phase.

Application of Magnetic Pulse Welding for Aluminum Alloys and SPCC Steel Sheets Joint

Abstract: The magnetic pulse welding (MPW) is a cold weld process of conductive metals to the similar or dissimilar material. MPW uses magnetic pressure to drive the primary metal against the target metal sweeping away surface contaminants while forcing intimate metal-tometal contact, thereby producing a solid-state weld. In this paper the MPW method and its application for several aluminium alloy (A1050, A2017, A3004, A5182, A5052, A6016, and A7075) and steel (SPCC) sheets joint were investigated and the process parameters and welding characteristics are reported.

Stress corrosion cracking susceptibility of friction stir welded AA7075–AA6056 dissimilar joint

Abstract: Two aluminium alloys, AA7075 and AA6056, were friction stir welded, with the AA7075 alloy placed on the advancing side of the welding tool. Microstructural observations revealed the development of a recrystallised fine-grained weld nugget, with two different grain sizes, resulting from the two different base materials. Slow strain rate tensile (SSRT) tests in air have shown that the weld nugget is marginally overmatched in the weldment, and the fracture occurred in the relatively weaker thermo-mechanically affected zone/heat affected zone (TMAZ/HAZ) of the AA6056 alloy. SSRT tests in 3.5% NaCl solution at a nominal strain rate of 10−6 s−1 have shown that this dissimilar weldment is not susceptible to stress corrosion cracking (SCC) under these test conditions. Also in this case fracture was observed in the TMAZ/HAZ of the AA6056 alloy, with a behaviour very similar to that observed in the tests in air. However, at a still lower nominal strain rate, 10−7 s−1, the TMAZ/HAZ region of AA7075 alloy was found to be susceptible to SCC, exhibiting intergranular fracture. As a whole it is concluded that though the weld nugget is resistant to SCC, the TMAZ/HAZ region of AA7075 in the weldment is prone to SCC in 3.5% chloride solutions at nominal strain rate levels in the order of 10−7 s−1.

Pore formation during hybrid laser-tungsten inert gas arc welding of magnesium alloy AZ31B—mechanism and remedy

Abstract: One of the major concerns during high speed welding of magnesium alloys is the presence of porosity in the weld metal that can deteriorate mechanical properties. This study seeks to analyze the presence method and quantity of pore during hybrid laser-tungsten inert gas arc (TIG) welding of magnesium alloy AZ31B by radiography, optical microscopy and electron probe microanalysis (EMPA). At the same time, it identifies both the mechanism of pore formation and a remedy for this problem. The experimental results indicate that lacking of shielding gas for laser beam is the dominant cause of macroporosity formation during the hybrid of laser-TIG welding of magnesium Alloys AZ31B plate, and hydrogen is not main cause to form large pores. A favorable weld without porosity can be obtained by appending lateral shielding gas for laser beam.

Laser induced stabilisation of the welding arc

Abstract: This paper deals with the influence of laser radiation on the stability of the welding arc. Experiments were conducted using a low power (500 W) Nd:YAG laser in combination with a gas tungsten welding arc. The laser induced arc stabilising effect was measured under various experimental conditions. It was found that the stabilising effect can be explained in terms of two phenomena: the absorption of laser energy by the arc plasma and the change of the arc plasma composition caused by strong evaporation of workpiece material. Both phenomena lead to a reduction of the effective ionisation potential of the plasma and thus provide a more conductive, stable plasma channel for arc root and column that overcomes disturbance by external forces. The proposed stabilisation mechanism was validated by measuring the absorption of laser energy by the arc plasma using a laser energy meter and the changes of arc plasma composition caused by the laser radiation by means of emission spectroscopy.

A study of the heat-affected zone (HAZ) of an Inconel 718 sheet welded with electron-beam welding (EBW)

Abstract: In this paper the heat-affected zones (HAZs) of the Inconel 718 sheets welded with electron-beam welding (EBW) were studied by using hardness measurement, metallographic etch and electrochemically potentiostatic etch methods. Before EBW, the Inconel 718 sheets were pretreated in three different conditions: as-received, solution- and precipitation-treatments. The results show that there is an obvious difference in the dimension of the widths of HAZs evaluated by the methods. The width, ca. 1.5 mm, of the HAZ in solution-pretreated weld can be estimated based on the variation of its hardness distribution curve. But the HAZs in as-received and precipitation-pretreated welds were difficult to detect from their hardness distribution curves. After metallographic etching, the HAZs in the welds were very hard to observe with optical microscope, but those with a width within 100 μm in solution- and precipitation-pretreated welds, except in as-received weld, can be observed with scanning electron microscope (SEM). The HAZ with a width ca. 300 μm of each weld can be revealed by using anodically potentiostatic etching in 3.5 wt.% NaCl solution due to different corrosion rates among the HAZ, fusion zone and base metal in the weld. The above-mentioned behavior of HAZs evaluated by different methods was discussed in detail in this work.

Synergetic effects of hybrid laser/arc welding

Abstract: The present study reports the results of a study examining the synergetic effects of hybrid laser/arc welding. Experiments were carried out with a 500 W Nd:YAG laser in combination with standard gas tungsten arc welding equipment and attention was focused on two aspects: the heat transfer efficiency and the melting efficiency. The heat transfer efficiency was determined by calorimetric measurements, whereas the melting efficiency was obtained from the transverse cross-sections of welds produced under various conditions. In addition, analytic calculations of the melting efficiency were performed on the basis of a modified form of the Rosenthal equation. The results show that the interaction of the laser and the arc does not lead to a noticeable change in the heat transfer efficiency, but results in a significant increase in the melting efficiency. The observed synergic melting effect is caused by addition of the two heat sources (laser and arc) and the contraction of the arc by the laser beam.