Showing papers on "Filler metal published in 2008"

Microstructure of Al–Mg dissimilar weld made by cold metal transfer MIG welding

Abstract: A modified metal inert gas welding process called cold metal transfer was successfully applied to dissimilar Mg and Al welding with AlSi5 filler metal. Optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction were used to characterise the microstructure, alloy element distribution and phase constituents of the welded joint. The microhardness distribution was determined by microhardness tester on the fusion zone. The super low heat input and the addition of Si inhibited the creation and growth of brittle intermetallic compound in weld metal, which improved the strength of the joint. An obvious multilayer microstructure, which comprised of solid solution, eutectic structure, Mg17Al12, and Mg2Al3 layer, was observed in the fusion zone adjacent to the Mg substrate. It degraded the strength, leading to the fracture of the joint in tensile test. The fracture morphology presented the typical brittleness fracture mode.

Dissimilar Welding of Al and Mg Alloys by FSW

Abstract: Microstructures and mechanical properties of dissimilar welding joint between Al alloy and Mg alloy by Friction Stir Welding (FSW) were investigated in comparison with laser welding of the same combination. Dissimilar joint of Al and Mg alloy by laser welding was very brittle because of building up Mg 17 Al 12 inter metallic compounds in fusion zone. On the other hand, FSW is anticipated to welding dissimilar alloys with enough joint strength because it is a solid-state process without melting. In this paper, FSW was carried out to make dissimilar butt joints of Al alloy and AZ31 magnesium alloy with various tool rotational speed and welding speed. These joints showed higher hardness in their stir zones than that of parent AZ31 alloy because of Mg-Al inter metallic compound formation. However, the hardness of stir zone was lower than that of fusion zone of laser welding, and was changed with the welding parameters of tool rotational speed and welding speed (i.e. heat input ratio of FSW). The optimum welding conditions of Mg and Al dissimilar FSW joint and the influence of inter metallic compound distribution with mixing of materials in stir zone were discussed.

Influence of brazing parameters and alloy composition on interface morphology of brazed diamond

Abstract: Active brazing is an effective technique for joining diamond or cBN grit to metallic substrates. This technique is currently used to manufacture superabrasive, high-performance tools. The investigation of interface reactions between diamond and active brazing alloys plays an important role in understanding and improving the brazing process and the resultant tool performance. Focused ion beam (FIB) milling enabled the high resolution investigation of these extremely difficult to prepare metal–diamond joints. The interfacial nanostructure is characterized by the formation of two layers of TiC with different morphologies. First a cuboidal layer forms directly on the diamond and reaches a thickness of approximately 70 nm. Then a second layer with columnar TiC crystals grows on the first layer into the brazing filler metal by a diffusion-controlled process. The combined thickness of both TiC layers varies between 50 nm and 600 nm depending on the brazing temperature and holding time.

Underwater Welding - A Review

Abstract: Underwater Welding - A Review The paper describes principles of underwater welding and recent trends in research works undertaken for enhance welding technology and properties of underwater welds. Department of Materials Technology and Welding at Gdansk University of Technology (GUT) has been involved in underwater welding research for over 25 years. Investigations include technology of underwater welding, and weld properties examinations. All tests have been performed with the use of self designed stands allow to perform welds in shallow depths as well as the depths up to 1000 m. The main investigation directions performed at the Department of Materials Technology and Welding are presented: Weldability of HSLA steel and factors influencing susceptibility to cold cracking of welded joints. The effects of wet welding conditions on diffusible hydrogen amount in the welds. The effects of heat input, underwater welding depths and composition of shielded gases on welds toughness.

Analysis of circumferentially arc welded thin-walled cylinders to investigate the residual stress fields

Abstract: The control of weld-induced imperfections like welding deformations and residual stresses is of critical importance in circumferentially welded thin-walled cylinders due to their wide utilization in high tech engineering applications in aerospace and aeronautical structures, pressure vessels and nuclear engineering fields. The paper presents a computational procedure for the analysis of temperature distributions and the subsequent residual stress fields during the course of arc welding of thin-walled cylinders of low carbon steel. Parametric studies based on numerical simulations are conducted to investigate the effects of critical welding process parameter on weld-induced residual stresses. Temperature-dependent thermo-mechanical behavior for low carbon steel, filler metal deposition along with double ellipsoidal heat source model is incorporated. The accuracy of the developed finite element simulation strategy is validated for transient temperature distributions and residual stress fields through full-scale shop floor welding experiments with proper instrumentation for data measurement. The aim is to present data to confirm the validity of in-process circumferential welding technology for thin-walled cylinders so that the in service failures of these structures due to process specific inherent stresses may be minimized.

The Mechanism of Ductility Dip Cracking in Nickel-Chromium Alloys : Subsolidus cracking results from global stresses produced during fusion welding and local stresses generated when coherent or partially coherent second phases form

Abstract: High-chromium (-30 wt-%) nickel-alloy filler metals are desirable for use in nuclear power systems due to their outstanding resistance to corrosion and stress corrosion cracking. However, these alloys are susceptible to welding defects, especially to subsolidus intergranular cracking commonly known as ductility dip cracking (DDC). In order to develop a high-chromium filler metal that is resistant to as-welded defects, a series of Ni-Cr alloys between 16 wt-% and 34 wt-% chromium were assessed for their susceptibility to cracking. Each alloy was evaluated by fabricating a restrained, multipass, automatic gas tungsten arc, V-groove weld, and counting the number of cracks per unit area observable at 50x. The type of cracking (subsolidus DDC or solidification cracking) was further differentiated via scanning electron microscopy. The results from these welds, coupled with microstructural characterization, chemical analyses, mechanical testing, microstructural modeling, and finite element modeling indicate that DDC in Ni-Cr alloys is caused by the combination of macroscopic thermal and solidification stresses induced during welding and local grain boundary stresses generated during precipitation of partially coherent (Cr,Fe) 23 C 6 carbides. Cracking can be mitigated by alloying to minimize (Cr,Fe) 23 C 6 precipitation (e.g., by Nb and Ti additions), lessening the misfit between the matrix and these precipitates (lowering the Cr and Fe concentration), and by minimizing welding-induced stresses. This mechanism of precipitation-induced cracking (PIC) via misfit stresses is consistent with subsolidus cracking in other alloy systems including superalloys, nickel-copper alloys, titanium alloys, and ferritic steels where ductility loss corresponds to the time/temperature regime where partially coherent or fully coherent second phases form.

Pressureless brazing of zirconia to stainless steel with Ag–Cu filler metal and TiH2 powder

Abstract: Partially stabilized zirconia was joined to stainless steel by pressureless active brazing with Ag–Cu filler metal and TiH2 powder. Microstructures, microchemistry and reaction products of the brazing seam were analyzed. The effects of brazing temperature and holding time on the joint shear strength were also investigated. The results showed that there existed three zones in the brazing seam and a double-layer structure at the ZrO2/filler interface. Due to the difference in brazing condition, the microstructures including the thicknesses and compositions of the three zones and two layers were different. It is further found that Ti originated from TiH2 coating diffused into the whole interlayer, resulting in the reaction products such as CuTi3, Ti3Cu3O, Cu4Ti3, NiTi2, Ni3Ti and Ti. The maximum joint shear strength of over 90 MPa was obtained due to the improved interface bonding.

Effect of Martensite Start and Finish Temperature on Residual Stress Development in Structural Steel Welds

Abstract: Martensite start and finish temperatures are very important in structural steel welding because they control the residual stresses in a weld. Tensile residual stresses amplify the effect of applied tensile stress. On the other hand, compressive residual stresses are algebraically added to the applied tensile stresses to result in a lower net stress level experienced by a weld, thus inhibiting crack initiation and increasing the fatigue life of the welded component. The residual stress state, i.e., whether compressive or tensile, and its magnitude will depend on the expansion that accompanies the austenite-to-martensite transformation and the thermal shrinkage due to cooling. High martensite start temperature and low martensite finish temperature will both minimize the effect of transformation-induced compressive stress generation. To obtain a full martensitic structure in a weld metal within an optimal range of temperatures will depend mainly on the filler metal composition. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied. In this study, several low-transformation-temperature welding (LTTW) wires have been developed and investigated to determine the martensite start and finish temperatures of the welds. Also studied was the effect of the martensite start and finish temperatures on microstructural development and hardness in single- and multi-pass weldments.

Role of electroplated interlayer in continuous drive friction welding of AA6061 to AISI 304 dissimilar metals

Abstract: Aluminium austenitic stainless steel joints find application in cryogenic engines, spacecrafts and automobiles. This dissimilar material combination is unweldable by conventional fusion welding due to a tendency for brittle intermetallic formation. Solid state welding processes such as friction welding are reported to be employed in such situations. This paper deals with continuous drive friction welding of AA6061 Al alloy to AISI 304 austenitic stainless steel. Direct welding of this combination resulted in brittle joints with 0° bend angle due to the formation of Fe2Al5. To alleviate this problem welding was carried out by incorporating Cu, Ni and Ag as diffusion barrier interlayers. The interlayer was incorporated by electroplating. Welds with Cu and Ni interlayer were also brittle due to the presence of CuAl2 and NiAl3. Ag acted as an effective diffusion barrier for Fe avoiding the formation of Fe2Al5. Therefore welds with Ag interlayer were stronger and ductile and could be bent to an angle o...

Predicting the dilution of plasma transferred arc hardfacing of stellite on carbon steel using response surface methodology

Abstract: Control of dilution is important in hardfacing, where low dilution is typically desirable. At present, most fabrication industries use shielded metal are welding, gas metal arc welding, gas tungsten arc welding and submerged are welding processes for hardfacing purposes. In these processes, the percentage of the dilution level is higher, ranging between 10% and 30%. In Plasma Transferred Arc (PTA) hardfacing, a solidified metallurgical bond between the deposit and the substrate is obtained with minimum dilution (less than 10%). This paper highlights the application of response surface methodology to predict and optimize the percentage of the dilution of a cobalt-based hardfaced surface produced by the PTA process. Experiments were conducted based on a fully replicable five-factor, five-level central composite rotatable design and a mathematical model was developed using response surface methodology. Furthermore, the response surface methodology was used to optimize the process parameters that yield the lowest percentage of dilution.

Keyhole gas tungsten arc welding of commercially pure zirconium

Abstract: Keyhole gas tungsten arc welding (K-GTAW), a novel variant of GTAW, has been used to join commercially pure zirconium. The process enables single pass welding of 6˙35 mm thick zirconium using conventional GTAW equipment and a high current torch, without expensive filler metal addition or joint preparation. The mechanical properties and the microstructure of the resulting joints were characterised. It is concluded that the K-GTAW process, with its high productivity combined with low capital investment requirements, can be successfully used for welding relatively heavy section zirconium.

Infrared brazing of Ti-6Al-4V and 17-4 PH stainless steel with (Ni)/Cr barrier layer(s)

Abstract: Infrared brazing of Ti-6Al-4V and 17-4 PH stainless steel using the BAg-8 filler metal was performed in this study. A nickel barrier layer 10 µm thick was introduced on the 17-4 PH stainless steel before infrared brazing. For the specimen that was infrared brazed at 800 °C and 850 °C for less than 300 seconds, the Ni layer served as an effective barrier layer to prevent the formation of Ti-Fe intermetallics. Experimental results show that the average shear strength of the joint can be greatly improved for the specimen by Ni plating. Comparing the specimens with and without electroless-plated Ni film, the former has no Ti-Fe intermetallic compound, but interfacial CuNiTi and NiPTi phases are observed in the latter. The fractured location of the joint after the shear test is changed from the interfacial TiFe (without Ni plating) into the TiCu reaction layer (with Ni plating). The plated Ni layer is consumed for the specimen that was infrared brazed at 880 °C for 300 seconds, and its bonding strength is impaired. Consequently, a lower brazing temperature and/or time are still preferred even though a plated Ni barrier layer is applied.

Effect of GMAW Process and Material Conditions on DP 780 and TRIP 780 Welds

Abstract: The drive to reduce vehicle weight and improve crash performance has led automotive manufacturers to introduce higher-strength grades of advanced high-strength steels (AHSS). For these materials to be used effectively, the influence of material and process conditions on gas metal arc (GMA) weld properties must be understood. The objective of this work was to characterize the effects of material prestrain, cooling rate conditions (welding heat input and fixture heat sink), filler metal selection, dilution, and postbaking on the microstructure and mechanical properties of GMA welds on coated dual-phase (DP) and transformation-induced plasticity (TRIP) steels. The primary materials studied were DP 780 and TRIP 780; for comparison purposes a limited amount of work was conducted with DP 980. The DP steels showed varying degrees of heat-affected zone (HAZ) hardening and softening depending on the material grade, prestrain, and cooling rate condition. The relatively high aluminum content of the TRIP 780 allowed retained ferrite to be present in all regions of the HAZ, along with a continuous region of coarse ferrite along the weld interface. This resulted in the TRIP 780 having lower peak HAZ hardness than the DP 780. Fusion zone microstructure and hardness were found to be affected by the base metal chemistry, the cooling rate condition, and the filler metal composition. Filler metal strength did not affect the static or dynamic tensile properties of either the TRIP 780 lap or butt joint welds, or the DP 780 butt joint welds. All of the TRIP 780 and DP 780 butt joints failed in the soft HAZ. The results of the lap joint tests showed a greater variation in strength that is attributed to porosity at the root of the weld.

Friction Stir Knead Welding of steel aluminium butt joints

Abstract: To develop steel aluminium-tailored hybrids in a butt joint for sheets in a thickness of about 1 mm conventional Friction Stir Welding is not feasible due to a high distortion of the welded specimen. Contrary to Friction Stir Welding the tool used for Friction Stir Knead Welding has no pin wherefore higher welding speeds can be realised. Due to the fact that this is a newer process, applied for patent in 2005, the cut contours of the edges and their variations have to be optimised by numerical analysis to transfer a maximum of load in order to improve the formability. The examined materials in this paper are steel DC04, as well as the aluminium alloys AA5182 and AA6016 in sheet thicknesses of 1 mm. Accompanying experimental investigations, as tensile tests, will evaluate the quality of the welding mechanism. As the mechanics of the new welding technology is not fundamentally investigated until now, metallographic investigations are performed, and additionally micro hardness measurements are carried out to verify the changes in the hardness distribution in the welding zone after stirring and welding.

Dual-beam laser welding of ultra-thin AA 5052-H19 aluminum

Abstract: Welding of 0.05 mm (0.002 inch) thin AA 5052-H19 aluminum samples in lap-joint configuration was conducted autogenously (no filler metal) using dual lasers that included Nd:YAG and diode with a zero inter-beam spacing. The 70-ns pulsed Nd:YAG (1064 nm) laser acted as the welding tool while the continuous wave diode (810 nm) laser with interaction times of 40–120 ms served to improve the light absorption characteristics of aluminum through preheating and oxidation effects. The microstructure, composition, flaws, and hardness of the joint were evaluated by scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and micro-indentation hardness test. The dual-beam welding technique was also compared with single-beam (Nd:YAG) welding. Results of parametric effects are displayed in the form of processing maps. Deeper penetration, better weld quality (less humping and cutting), and increased hardness were observed in dual-beam welds when compared with single-beam welds. The most astounding result was a nearly 200% increase in hardness over the base metal in dual-beam welding. This can be explained by the oxygen pickup in a dual-beam weld due to longer heating and amorphous microstructure of aluminum oxide as revealed by energy dispersive X-ray spectrum and X-ray diffraction respectively.

Ductility-Dip Cracking in High Chromium, Ni-Base Filler Metals

Abstract: Ductility-dip cracking (DDC) is an elevated temperature, solid-state cracking phenomenon that is observed in austenitic weld metals. In this study, the DDC susceptibility of several high-chromium, nickel-base filler metals was evaluated using the strain-to-fracture (STF) test technique. These filler metals were of the Ni-30Cr type and included INCONEL® Filler Metals 52 and 52M supplied by Special Metals Welding Products Company, and Sanicro 68HP® and Sanicro 69HP® supplied by Sandvik AB. In addition, two experimental Ni-30Cr filler metals were evaluated which contained variations in other alloy additions, including niobium additions up to 2.5 wt% and molybdenum additions of 4 wt%. A wide range in DDC susceptibility was observed with these filler metals, including large heat-to-heat variations in filler metals with similar compositions. The experimental filler metals containing Nb and Mo were found to be remarkably resistant to DDC. Cracking susceptibility is primarily associated with the type and form of precipitate that forms along the weld metal migrated grain boundaries. The formation of Nb-rich, M(C,N) at the end of solidification has the most profound effect on DDC, since these precipitates are most effective in pinning the boundaries. The formation of M23C6 carbides during weld cooling or subsequent reheating can also affect DDC susceptibility in these filler metals.

Brazing sheet of aluminum alloy

Abstract: A brazing sheet of aluminum alloy composed of a core material and a first brazing filler metal covering one surface of the core material. The core material contains as an essential component 0.2-1.0 mass % of Cu and as optional components at least one species of no more than 1.5 mass % of Si, no more than 1.8 mass % of Mn, no more than 0.35 mass % of Ti, and no more than 0.5 mass % of Mg, with the remainder being Al and inevitable impurities. The first brazing filler metal has a liquid phase ratio (X %) at 600° C. and a thickness (Y μm) such that X and Y satisfy the following relationship: (1) 30≦X≦80, (2) Y≧25, and (3) 1000≦X×Y≦24000. The brazing sheet provides good brazeability and maintains high corrosion resistance after brazing on the surface cladded with the brazing filler metal.