Meeting weld quality criteria when laser welding Ni-based alloy 718
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Significant numbers of aero-engine parts are manufactured with Ni-based superalloys because of their excellent heat and corrosion resistance properties. Weld imperfections, such as those associated with bead geometry, sub-surface porosity and liquation cracking are of concern, given the high-performance service requirements of fabricated Ni-based components in the aerospace sector. Weld quality criteria are stipulated in AWS D17.1M:2010, a standard for the fusion welding of aerospace components although it is known that more stringent criteria, usually company-specific to various aero-engine manufacturers, also exist.In this paper, a high beam quality 1 µm laser source was used to determine the effects of different process parameters on the quality of melt runs made in 3.35 and 5mm thickness Alloy 718. The principal focus of the work was to compare the results obtained for weld geometry, sub-surface porosity and liquation cracking to the Class A weld quality requirements of AWS D17.1M:2010 and a generic, more stringent company-specific standard.The results show that AWS D17.1M:2010 geometry and porosity criteria can be achieved over a fairly wide process window of speed and power with the laser and optical configuration used. This window is greatly reduced in order to produce liquation crack free welds. If, in addition, it is also required to meet the more stringent company-specific criteria for porosity and weld geometry, the window is even further reduced.Keywords:
Liquation
Fusion welding
Liquation
Fusion welding
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As the lightest structural alloys, magnesium alloys offer significant potential for improving the energy efficiency of various transportation systems. The lack of sufficient weldability of Mg alloys is a crucial barrier to their potential use in safety-critical applications. Porosity formation, grain structure engineering, solidification cracking, liquation, and liquation cracking are the key metallurgical challenges to obtain reliable and robust fusion welds in Mg alloys. This critical review highlights the current understating regarding controlling the metallurgical phenomena during fusion welding of Mg alloys and discusses the unresolved metallurgical challenges to shed light on the path forward to enhance fusion weldability of Mg alloys.
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Weldability
Fusion welding
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Friction stir welding (FSW) has become an important application in modern industries. Friction stir welding is a widely used solid state joining process for soft materials such as aluminum alloys as it avoids/minimizes common problems of fusion welding processes, i.e. distortion, porosity, solidification and liquation cracking etc. Improper selection of parameters such as welding speed, rotational speed, forge force, back plate material etc. affects the weld quality. Thermal boundary condition at the bottom of the work pieces to be joined is important in determining the result of weld quality and its properties, for a given alloy type, tool geometry and selected process parameters (welding speed, rotational speed etc), These thermal boundary conditions are governed by the back plate material used. By using backing plates made out of materials with widely varying thermal diffusivity this work seeks to elucidate the effects of the root side thermal boundary condition on weld process variables and resulting joint properties. Welds were made in 5-mm-thick AMг5 (AA 5056) using siliceous coating, stainless steel, mild steel, and aluminum as backing plate (BP) material. Effects of backing plate material on the tensile strength and elongation were obtained for a particular case.
Liquation
Fusion welding
Friction Stir Welding
Friction Welding
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Friction stir welding is a widely used solid state joining process for soft materials such as aluminium alloys as it avoids/minimizes common problems of fusion welding processes such as distortion, porosity, solidification and liquation cracking etc. Improper selection of parameters such as welding speed, rotational speed, forge force, back plate material etc. affects the weld quality. Thermal boundary condition at the bottom of the work pieces to be joined is important in determining the resulting weld quality and properties. For a given alloy type, tool geometry and selected process parameters (welding speed, rotational speed etc.), which results in good quality weld depends on thermal boundary conditions at the bottom of the work piece. These thermal boundary conditions are governed by the back plate material used. This paper critically reviews several effects of back plate material on microstructure and mechanical properties of friction stir welded aluminium joints.
Liquation
Fusion welding
Friction Stir Welding
Friction Welding
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The conventional manual enclosed arc welding process, which has been. employed as a field welding process for rails, has shortcomings that the liquation cracks are likely to occur at the boundaries of coarsened austenite grains in the heat-affected zone.This paper describes the susceptibility of HAZ liquation cracks for rails and the causes of crack incidence. Experiments have been done to define the critical carbon content of weld metal and critical welding condition necessary for avoiding the cracks by employing some electrodes with various carbon content. And also, in order to discuss the crack susceptibility and the crack causes of rail steels comparing to those of general structural steels, the synthetic test has been done to simulate the HAZ liquation cracks.The main experimental results are as follows :(1) The crack incidence increases with decreasing weld metal carbon content when the difference in the carbon content between the weld metal and rail steel is 0.3% or more.The cracks are eliminated when the former difference is less than 0.2%.(2) The cracking tendency increases with increasing electrode diameter and weld current or weld heat input.(3) The liquation crack susceptibility of rail steel is nearly equal to that of plain carbon steel, for which the crack dose not become a subject of discussion in actual welded joint.(4) The main cause of HAZ liquation crack for rail enclosed-arc welding is the considerable difference of liquidous & solidous temperature depended on the difference of carbon content between the weld metal and base metal.
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Flash welding
Carbon steel
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In fusion welding, cast magnesium alloys typically present higher sensitivity to liquation cracking than wrought alloys. In this study, a fluid-mechanics-based analytical model was established to investigate an unusual phenomenon: a lack of cracking in cast magnesium alloy while cracking took place in wrought magnesium alloy. This was observed in resistance spot welding, in which the pressure within the molten nugget allows the liquid to feed through the interface between the fusion zone and the partial melting zone, an aspect which is not generally considered in the typical criteria for liquation cracking. The analytical model shows that the liquid is easy to feed into the inter-grain zone of a cast coarse-grained metal but difficult to feed into wrought metal. The liquid feeding effect changes the composition of the inter-grain region, causing the semi-solid grains to bond to each other tightly in the cast magnesium alloy. Electro-probe microanalysis illustrates the mass transfer pattern of liquid feeding at the interface between the fusion zone and the partial melting zone. Magnesium alloys AZ31, AZ91, and ZK61 were selected to carry out resistance spot welding tests of which results match the theory raised in this work well.
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Spot welding
Fusion welding
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Liquation
Microprobe
Void (composites)
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The main obstacle for the application of high strength 7××× series aluminum alloys is that these alloys are susceptible to hot cracking during fusion welding. This study presents the liquation cracking susceptibility of the novel T-Mg32(AlZn)49 phase strengthened Al-Mg-Zn alloy with a Zn/Mg ratio below 1.0 by a circular-patch welding test, and compared the liquation cracking tendency with η-MgZn2 phase strengthened 7××× series alloys whose Zn/Mg ratios are above 1.0. It was found that all these novel Al-Mg-Zn alloys still have as low a liquation cracking susceptibility as traditional 5××× series alloys, surpassing that of traditional 7××× series alloys substantially. It was noticed that the increase of the Zn/Mg ratio will result in a larger difference between the fraction solids of the fusion zone and the partially melted zone during the terminal solidification stage, which can lead to a wider crack healing disparity between these two areas and thus result in different liquation cracking susceptibilities in different alloys.
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Fusion welding
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Liquation
Weldability
Fusion welding
Friction Stir Welding
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In this paper effect of electron beam welding heat input on the microstructure and mechanical properties of Inconel 718 was studied. Sheet of 2 mm thickness was used with an average grain size of 30 μm. Five different heat inputs ranging from 36 J/mm to 180 J/mm were used to make full penetration welds. Welding current was varied from 18 mA to 90 mA and other welding parameters were kept constant. Change in heat input altered the geometry of the weld (width to depth ratio) and the microstructure of fusion zone (FZ) and heat affected zone (HAZ) also changed. As the heat input increased the interdentric spacing in FZ was increased from 3.5 μm to 6 μm. Grain boundary liquation was found in all the welds. Intensity of liquation increased with decrease in heat input and the microfissuring was observed only in the lowest heat input weld. Change in heat input did not significantly affect the tensile properties of the weld.
Liquation
Inconel
Electron beam welding
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