IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Ultrasonic consolidation is a novel additive manufacturing process with immense potential for fabrication of complex shaped three-dimensional metallic objects from metal foils. The proportion of bonded area to unbonded area along the layer interface, termed linear weld density (LWD), is perhaps the most important quality attribute of ultrasonically consolidated parts. Part mechanical properties largely depend on LWD and a high level of LWD must be ensured in parts intended for load-bearing structural applications. It is therefore necessary to understand what factors influence LWD or defect formation and devise methods to enhance bond formation during ultrasonic consolidation. The current work examines these issues and proposes strategies to ensure near 100% LWD in ultrasonically consolidated aluminum alloy 3003 parts. The work elucidates the effects of various process parameters on LWD and a qualitative understanding of the effects of process parameters on bond formation during ultrasonic consolidation is presented. The beneficial effects of using elevated substrate temperatures and its implications on overall manufacturing flexibility are discussed. A preliminary understanding of defect morphologies and defect formation is presented, based on which a method (involving surface machining) for minimizing defect incidence during ultrasonic consolidation is proposed and demonstrated. Finally, trade-offs between part quality and build time are discussed.

Department of Metallurgical and Materials Engineering

World University root node

17th Solid Freeform Fabrication Symposium, SFF 2006

Improving linear weld density in ultrasonically consolidated parts

Ultrasonic consolidation (UC) is a novel additive manufacturing process wherein three-dimensional metallic objects are fabricated layer by layer in an automated fashion from thin metal foils. The process has immense potential for fabrication of injection molding tooling with conformai cooling channels, fiber-reinforced composites, multi-material structures, smart structures, and others. The proportion of bonded area in relation to the total interface length, termed linear weld density (LWD), is perhaps the most important quality attribute of UC parts. A high level of LWD is desirable in parts intended for load-bearing structural applications. It is therefore necessary to understand what factors influence LWD and devise methods to enhance bond formation during ultrasonic consolidation. The current work elucidates the effects of process parameters on LWD in AI alloy 3003 UC parts. A set of optimum parameters for AI 3003 part fabrication using UC has been obtained, which may vary, however, for different foil materials and sonotrode/foil fric-tional conditions. The beneficial effects of using elevated substrate temperatures and its implications on overall manufacturing flexibility and the trade-offs between part quality and build time are discussed. The mechanism of ultrasonic welding is discussed based on oxide layer removal and plastic deformation at the weld interface. A preliminary understanding of defect formation during UC is presented, based on which a method (involving surface machining) for obtaining near-100% LWD is demonstrated. The findings of the current work encourage wider utilization of the UC process and could stimulate further research in the areas of UC process development and modeling. © 2007 Society of Manufacturing Engineers.

Journal of Manufacturing Systems

Effect of process parameters on bond formation during ultrasonic consolidation of aluminum alloy 3003

Bead on plate, full penetration electron beam welds were produced in 2 mm thickness sheets of Inconel 718 in the solution treated condition. Welds were subjected to an aging treatment with and without post-weld solution treatment. Weld microstructures, high temperature tensile properties and stress rupture properties were evaluated. The as welded fusion zone showed a considerable amount of interdendritic niobium segregation and brittle intermetallic Laves phase. The tensile and stress rupture properties of the welds after post-weld aging treatment were found to be inferior in relation to the base metal. Post-weld solution treatment at 980°C was found to result in partial dissolution of Laves phase, some reduction in niobium segregation and the formation of δ phase needles around the Laves particles. The use of 980°C solution treatment was found to improve the weld properties to some extent, although not to the level of the base metal. The reasons for this behaviour are discussed, correlating microstructures, fracture features and mechanical properties. © 2005 Institute of Materials, Minerals and Mining.

Materials Science and Technology

Microstructure and mechanical properties of Inconel 718 electron beam welds

The presence of Nb-rich, brittle, intermetallic Laves phase in Inconel 718 weld fusion zones is detrimental to weld mechanical properties. In the current work, autogenous bead-on-plate gas tungsten-arc welds were deposited in 2 mm thick IN 718 sheets. The welds were subjected to the following heat treatments: i) direct aging, ii) solution treatment at 980°C followed by aging, and iii) solution treatment at 1080°C followed by aging. Detailed microstructural characterisation was carried out using optical, scanning electron and transmission electron microscopes and electron probe microanalysis. The microstructural features in as-welded and post-weld heat treated conditions are discussed. The results show that post-weld heat treatments alone cannot provide satisfactory solution to the Laves problem in Inconel 718 gas tungsten-arc welds.

Praktische Metallographie/Practical Metallography

Microstructural characterisation of Inconel 718 gas tungsten arc welds [Charakterisierung des Gefüges von Wolfram-Inertgas-Lichtbogen- schweißnähten in Inconel 718]

Pulsed Nd-YAG laser welding characteristics of Inconel 718 sheet material (2 mm thick) were investigated. Welds were subjected to three different post-weld heat treatments: direct aging, 980 °C solution treatment + aging (980STA), and 1080 °C solution treatment + aging (1080STA). Weld microstructures and room temperature tensile properties were evaluated. Weld tensile properties in direct aged condition were found to be inferior in relation to the base metal due to the presence of Nb-rich brittle intermetallic Laves phase in fusion zone microstructure. Solution treatment at 980 °C was found to result in considerable dissolution of Laves phase, leading to some improvement in weld properties, though not to the level of base metal. The treatment was also found to result in precipitation of needle-like δ-phase around partially dissolved Laves particles. Solution treatment at 1080 °C was found to result in complete dissolution of Laves phase, but also in significant grain coarsening in the base metal, which led to considerable deterioration in base metal properties. © 2004 Elsevier B.V. All rights reserved.

Journal of Materials Processing Technology

Microstructure and tensile properties of Inconel 718 pulsed Nd-YAG laser welds

The effects of improvement in stress rupture properties of Inconel 718 gas tungsten arc welds using current pulsing are described. The use of current pulsing resulted in significant refinement of the fusion zone solidification structure, with improvement in Laves morphology. The current pulsing method also reduced the extent of Nb segregation during weld metal solidification. The results show that presence of needle-like δ-phase in excessive amounts is detrimental for stress rupture properties in wrought alloy 718.

GENERAL

Journal of Materials Science

Improvement in stress rupture properties of inconel 718 gas tungsten arc welds using current pulsing

The presence of Nb rich Laves phase in Inconel 718 weld fusion zones is known to be detrimental to weld mechanical properties. In the present study, an attempt was made to control the formation of Laves phase in alloy 718 gas tungsten arc welds using pulsed current. Welds were produced in 2 mm thick sheets of the alloy with constant current and pulsed current and were subjected to post-weld solution treatment at 980°C followed by aging. Detailed microstructural studies and tensile tests at 650°C were conducted. The results show that the use of current pulsing (i) refines the fusion zone microstructure, (ii) reduces the amount of Laves phase and exerts a favourable influence on its morphology and (iii) improves the response of the fusion zone to post-weld heat treatment and weld tensile properties. © 2004 Institute of Materials, Minerals and Mining. Published by Maney on behalf of the Institute.

Science and Technology of Welding and Joining

Control of laves phase in inconel 718 GTA welds with current pulsing

Weld solidification cracking in age-hardenable aluminium alloys is usually minimised by adjusting weld metal composition. One way of reducing cracking susceptibility is to refine the weld metal microstructure, which offers the additional benefit of improving mechanical properties. In the current work, grain refinement was achieved in weld fusion zones of a medium-strength Al-Zn-Mg alloy through inoculation with Ti, Ti+B and Zr. It was found that the grain structure in the refined welds was equiaxed in all three dimensions and that this led to a reduction in hot cracking tendency and to an improvement in hardness and tensile properties, especially ductility. © 2003 Elsevier Science B.V. All rights reserved.

Microstructural refinement through inoculation of type 7020 Al-Zn-Mg alloy welds and its effect on hot cracking and tensile properties

The refinement of weld fusion zone microstructures helps in reducing hot cracking and improving the weld metal mechanical properties. Pulsing of the welding current is one technique that could provide such grain size reduction. In the current work, gas tungsten-arc welding was used to deposit welds in Al-Li-Cu and Al-Zn-Mg alloys both in the pulsed and unpulsed modes. Current pulsation was found to induce grain refinement in three ways: by reducing the columnar grain size towards the weld centre, widening of the chill zone adjacent to the fusion boundary and producing a number of fine-grained bands across the length of the weld. While the reduction in average grain size improved tensile elongation, the strength was not much affected. The higher ductility of the pulsed welds was maintained in the peak-aged condition also.

Effect of pulsed welding current on the solidification structures in Al-Li-Cu and Al-Zn-Mg alloy welds [Der Einfluss des gepulsten Schweissstromes auf die Erstarrungsgefuege in Schweissungen aus Al-Li-Cu- und Al-Zn-Mg-Legierungen]

The refinement of weld solidification structures will be useful not only because it is known to reduce the tendency to hot cracking, but also because the mechanical properties of the fusion zone are likely to be improved. In the current work, welds were produced in a type 2090 Al-Li alloy using two filler materials based on types 2319 and 4043. Grain refinement was induced in the weld fusion zones through inoculation with Ti, Ti+B and Zr. Microstructural characterization showed that all three reduced grain size considerably, with Zr being the most effective. The structural refinement was found to reduce hot cracking susceptibility (measured by Varestraint testing), enhance post-weld age-hardening response and also improve tensile properties, especially ductility. The results showed that in the peak-aged condition the tensile elongations in the inoculated welds equalled or exceeded that in the base material. © 2000 Elsevier Science S.A.

Materials Science and Engineering A

Use of inoculants to refine weld solidification structure and improve weldability in type 2090 Al-Li alloy

In the development of Al-Li alloys for aerospace structures, their behaviour during welding plays an important role. One way of improving weldability is to refine weld solidification structures, which can be achieved by a variety of means. In this work, a type 2090 Al-Li alloy was gas tungsten arc welded with two different filler materials corresponding to types 2319 (Al-6·3Cu) and 4043 (Al-5·2Si). Inoculation with titanium together with arc oscillation through an imposed alternating magnetic field was used to refine the weld fusion zone microstructures. Post-weld aging and tensile testing were employed to assess possible improvements in performance. It was found that the combined treatment of inoculation and magnetic oscillation resulted in fully equiaxed, fine grained structures and that this led to a noticeable increase in aging response and tensile properties, especially ductility.

Combined effect of inoculation and magnetic arc oscillation on microstructure and tensile behaviour of type 2090 Al - Li alloy weld fusion zones

In an effort to refine the weld metal grain structure in α-β titanium alloys, gas tungsten arc welding was carried out, during which transverse oscillations of the arc were induced through the use of an alternating external magnetic field. At optimum values of oscillation amplitude and frequency in both the alloys investigated, considerable refinement of the fusion zone grain structure was achieved. This could be attributed to factors that include enhanced fluid flow, reduced temperature gradients, and a continually changing weld pool size and shape owing to the action of the imposed magnetic field. The reduction in the prior β grain size was shown to result in a notable increase in fusion zone tensile ductility. Post-weld annealing increased ductility in all cases, but the magnetically treated material continued to show a higher elongation than that of the untreated material even after post-weld heat treatment.

Use of magnetic arc oscillation for grain refinement of gas tungsten arc welds in α-β titanium alloys

Pulsing of the welding current is one approach for refining the fusion zone grain structure in α-β titanium alloy welds. This paper reports work in which gas tungsten-arc welds were produced in two α-β titanium alloys under a variety of conditions including direct current (d.c.) pulsing and alternating current (a.c.) pulsing. The results show that, while d.c. pulsing did also refine the weld metal β grain structure, the effect of a.c. pulsing was much greater. Current pulsing enhances fluid flow, reduces temperature gradients and causes a continual change in the weld pool size and shape. These effects, which are believed to be responsible for refining the solidification structure, are much stronger in a.c. pulsing than in d.c. pulsing. The observed grain refinement was shown to result in an appreciable increase in fusion zone tensile ductility. Post-weld heat treatment improved ductility both in pulsed and unpulsed welds, but pulsed welds showed greater tensile elongation even in the heat treated condition. © 1999 Elsevier Science S.A. All rights reserved.

Microstructural refinement of weld fusion zones in α-β titanium alloys using pulsed current welding

Grain size reduction in weld fusion zones confers the advantages of an increased resistance to solidification cracking and an improvement in mechanical properties. Oscillation of the welding arc through an imposed alternating magnetic field is one of several approaches to modify weld solidification structures. In this study, gas tungsten arc welds were produced in two high strength, age hardenable aluminum alloys with and without an external magnetic field. Metallographic characterization revealed the degree of structural refinement produced by magnetic arc oscillation. The decrease in grain size was found to increase tensile elongation, while the effect on strength and age hardening response was only meager. The improvement in ductility was partially maintained in the peak aged condition also.