Wire Melting Research Articles

Cold Spray Additive Manufacturing: Microstructure Evolution and Bonding Features

ConspectusMost metal additive manufacturing (AM) methods involve the melting or sintering of feedstock powder or wire using an energy source (laser, electron beam, or electric arc). Solid-state AM, sometimes also known as non-beam-based AM, is a process in which the deposited material does not melt and is built up layer-by-layer, typically through severe plastic deformation. Initially considered to be a coating technique, by virtue of its high deposition rate, cold spray additive manufacturing (CSAM) is attractive as a solid-state AM technology. In the CSAM process, metal powder particles are impacted onto a substrate at a supersonic velocity and relatively low temperature. The CSAM process reduces or eliminates many problems associated with melting or beam-based AM methods, making the CSAM technically attractive for a wide range of applications, such as in the aerospace, automobile, marine, biomedical, machinery, and energy sectors.In this Account, the author briefly reviews the setup of a cold spray system and discusses the strengths and drawbacks of CSAM compared with thermal sprays and beam-based AM processes, as well as applications in relevant industries. The author summarizes the bonding mechanisms proposed for the cold spray process. The focus of this Account is to review the microstructure evolution of several typical model metals (copper, nickel, aluminum, and titanium) during the cold spray process. The author shows a large variety of microstructure characteristics (recrystallized grains, annealing twins, shear bands, submicron-sized grains, deformation twins, and nanometer-sized grains) in cold-sprayed copper, dynamic recrystallization of cold-sprayed nickel, and the formation of refined grains even below 10 nm in size in cold-sprayed aluminum. The magnitude of the stacking fault energy of as-sprayed materials considerably influences the microstructure after cold spray and postprocessing. Due to the relatively low thermal conductivity and ductility of titanium, cold spraying of titanium shows both a high deposition efficiency and relatively high porosity of as-sprayed parts, as well as a heterogeneous microstructure. Moreover, the Account introduces the postprocessing heat treatment and nanoindentation characterization of cold-sprayed materials. A fundamental understanding of microstructure evolution during and after the cold spray process is essential for optimal mechanical properties. Lastly, the author provides a perspective on applying old lessons and taking advantage of new techniques and materials, including powder characterization, hybrid additive manufacturing, machine learning for searching process windows, nanomechanical testing, and emerging alloys (high-entropy alloys, nanocrystalline alloys, and quasicrystals), to advance the research and applications of CSAM.

Optimization of Process Conditions for Additive Manufacturing Technology Combining High-Power Diode Laser and Hot Wire

A high-efficiency additive manufacturing technology that combines a high-power diode laser with a large-rectangle spot (beam width of 11 mm) and a hot-wire system was developed. The hot-wire system can generate Joule heat by wire current and heat a filler to its melting point independently from the main heat source of a high-power diode laser. A simple calculation method to derive the appropriate hot-wire current of Z3321-YS308L was proposed with verification by hot-wire feeding experiments without laser irradiation at various wire currents. The effect of process parameters, such as laser power, process speed, and the wire feeding rate (wire feeding speed/process speed) on bead characteristics was investigated by cross-sectional evaluations on three-layer depositions. High-speed imaging observations of wire melting and molten pool formation showed that the energy density input and the wire feeding rate were dominant parameters in terms of bead formation and hot-wire feeding stability. A 50-mm-high, 8-mm-wide, and 250-mm-long sample was fabricated by using appropriate process conditions, and tensile tests were performed by using a sub-sample from the large sample.

Review of Aluminum Alloy Development for Wire Arc Additive Manufacturing.

Processing of aluminum alloys by wire arc additive manufacturing (WAAM) gained significant attention from industry and academia in the last decade. With the possibility to create large and relatively complex parts at low investment and operational expenses, WAAM is well-suited for implementation in a range of industries. The process nature involves fusion melting of a feedstock wire by an electric arc where metal droplets are strategically deposited in a layer-by-layer fashion to create the final shape. The inherent fusion and solidification characteristics in WAAM are governing several aspects of the final material, herein process-related defects such as porosity and cracking, microstructure, properties, and performance. Coupled to all mentioned aspects is the alloy composition, which at present is highly restricted for WAAM of aluminum but received considerable attention in later years. This review article describes common quality issues related to WAAM of aluminum, i.e., porosity, residual stresses, and cracking. Measures to combat these challenges are further outlined, with special attention to the alloy composition. The state-of-the-art of aluminum alloy selection and measures to further enhance the performance of aluminum WAAM materials are presented. Strategies for further development of new alloys are discussed, with attention on the importance of reducing crack susceptibility and grain refinement.

Features of Filler Wire Melting and Transferring in Wire-Arc Additive Manufacturing of Metal Workpieces.

In this paper, we present the results of a study on droplet transferring with arc space short circuits during wire-arc additive manufacturing (WAAM GMAW). Experiments were conducted on cladding of single beads with variable welding current and voltage parameters. The obtained oscillograms and video recordings were analyzed in order to compare the time parameters of short circuit and arc burning, the average process peak current, as well as the droplets size. Following the experiments conducted, 2.5D objects were built-up to determine the influence of electrode stickout and welding torch travel speed to identify the droplet transferring and formation features. Moreover, the current–voltage characteristics of the arc were investigated with varying WAAM parameters. Process parameters have been determined that make it possible to increase the stability of the formation of the built-up walls, without the use of specialized equipment for forced droplet transfer. In the course of the research, the following conclusions were established: the most stable drop transfer occurs at an arc length of 1.1–1.2 mm, reverse polarity provides the best drop formation result, the stickout of the electrode wire affects the drop transfer process and the quality of the deposited layers. The dependence of the formation of beads on the number of short circuits per unit length is noted.

Source term analysis and sorting scheme in post-irradiation spherical pyrometric elements

The 10 MW high-temperature gas-cooled reactor (HTR-10) is the most advanced of the fourth generation reactors, and has become a good test platform for very high-temperature operations. Spherical fuel elements are utilized in HTR-10, and the maximum temperature of the fuel element is one of the primary technical factors restricting the outlet temperature of the core. In our experiment, a pyrometric element with different wire melting points, is added to the HTR-10 reactor core to measure the maximum temperature distribution. How to sort out the pyrometric element, and analyze the source terms in post-irradiation spherical pyrometric elements when cycled out of the core, are the key issues of this study. This study can provide technical support for the internal temperature measurement, and analysis of the core, as well as for very high-temperature operation of HTR-10.

Formation technology and structure of titanium nickelide-based alloys in double-arc melting

Alloys based on titanium nickelides have high mechanical properties, as well as a set of special properties: shape memory effect, superelasticity, high level of damping and corrosion resistance, biocompatibility. To obtain titanium nickelides, various technologies are used: smelting in arc and induction vacuum furnaces, electrolysis of molten media, sintering, selfpropagating high-temperature synthesis, which are distinguished by high energy consumption of processes and significant cost. To reduce the cost of obtaining alloys based on titanium nickelide, a technology of double-arc melting in argon using electrode wires made of titanium and nickel was proposed, which makes it possible to obtain not only blanks and parts of the required shape from these alloys, but also deposited layers on the surface of finished products. The conducted studies of processes of titanium nickelides formation by double-arc melting showed that the chemical composition of samples is determined by modes of electric arc melting of electrode wires and, first of all, by the ratio of the feed rates of nickel and titanium wires. The nickel content in obtained samples varied within 34.1–60.1%. The structure of the samples was determined by the ratio of feed rates of electrode wires and was represented by phases: α(Ti); NiTi2; NiTi. The structure based on NiTi2 and Ti is formed when the ratio of the nickel wire feed rate to thetitanium wire feed rate is equal to 0.38. With the feed rate ratio in the range of 0.44–0.86, the structure of samples is represented by phases NiTi2 + NiTi. A single-phase structure based on titanium nickelide (NiTi) during double-arc melting in an argon atmosphere is formed at the feed rate ratio of the order of 1–1.14 of nickel and titanium wires with the same diameter.

Simulation and control of metal droplet transfer in bypass coupling wire arc additive manufacturing

The influences of bypass current and wire feed speed are utilized to change the metal droplet transfer mode for bypass coupling wire-arcing additive manufacturing (BC-WAAW). To fully understand the influence of experimental factors on the wire melting process, this study establishes a mathematical model for the wire melting process, and several simulation experiments are carried out to prove the effectiveness of this mathematical model. The influence of changing the filler wire process on the stability of the wire melting process is simulated. A negative feedback control method with a single input and single output is proposed, which can control the stability of the wire melting process. The results show that the established mathematical model can well reflect the wire melting process in bypass coupling arcing additive manufacturing. The wire feed speed has a strong influence on the wire extension. The negative feedback control method can realize real-time control of filler wire process stability. The deposition process is more stable in the arcing additive manufacturing process, and the forming accuracy of a single multilayer deposition wall is further improved.

Wire-feed assisted A-TIG welding of dissimilar steels

This study investigates the activated flux-tungsten inert gas welding (A-TIG) welding of dissimilar P92 steel-304H ASS using ‘wire feed’ (patent pending) in terms of weld pool mixing behavior, microstructure and mechanical properties of the weld joint. ErNiCrMo-3 wire was fed during welding and three-wire feeding configurations were analyzed in view of filler wire melting and weld pool mixing behavior. The metal from filler wire is transferred into the weld pool in the form of ‘interrupted liquid bridge’ and ‘uninterrupted liquid bridge.’ The ‘uninterrupted liquid bridge’ melting resulted in homogeneous mixing of filler wire into the weld pool. The weld joint produced using the best wire feeding configuration was characterized and compared with the weld joint developed without wire feed. Microstructure alterations were realized with the use of wire feed. The weld zone with wire feed exhibited a fully austenitic structure, whereas; the completely martensitic structure was obtained using A-TIG welding without wire feed. The microstructural transformation led to the improvement of ductility and impact toughness of weld joint without substantial loss of tensile strength. The total elongation and impact toughness of the A-TIG weld joint with wire feed was 45.9% and (89 ± 2) J, respectively, which were significantly higher in contrast to the A-TIG weld joint without wire feed [total elongation: 37.4% and impact toughness: (30 ± 2) J].

Electron beam wire cladding of nickel alloys and stainless steel on a reactor pressure vessel steel

Claddings made of nickel-base alloys or stainless steel are frequently used in the oil, chemical, and power generation industries to protect low-alloy steels from corrosion. Arc weld overlays, which are commonly deposited, often exhibit microstructures near the fusion line that are susceptible to cracking. Due to its very localized and controlled heat input, electron beam welding has the potential to produce claddings with a more resistant microstructure. In this study, multi-layer nickel-base alloy and stainless steel claddings were fabricated by wire-feed electron beam additive manufacturing. Dilution was high because of the specific wire melting technique. It was found that the degree of dilution had a considerable influence on the precipitation of secondary phases, which, however, did not impair the integrity of the claddings. The influence of dilution on the formation of precipitates could be rationalized with thermodynamic calculations. In the nickel-base alloy claddings, the fusion line was sharp without any interfacial martensite. Only a few potentially detrimental type-II grain boundaries were found. Instead, epitaxial solidification was prevalent. This was attributed to metastable melting that was caused by high heating rates. Thus, electron beam-based manufacturing could produce claddings that are more resistant to disbonding than standard arc weld overlays.

Numerical simulation of the spheroidization process with cored wire injection in liquid iron using finite volume method

We develop a finite volume method for the simulation of the liquid iron cored wire feeding spheroidization technology, which can realize the visualization of the spheroidization process and provide guidance for parameter optimization. In this work, we first define a two-dimensional unsteady heat-transfer mathematical model for single-core structure cored wire melting in liquid iron and then discretize the governing equation numerically. The present solver is validated by comparing it with the data obtained from experimental studies. Interestingly, in the numerical results we found that the absorption velocity of the magnesium element can be improved to the greatest extent to get the most ideal spheroidizing effect. To further show the melting and boiling characteristics of spheroidization, we also explore the relationship between melting and boiling depth, wire feeding speed, and temperature of liquid iron in the feeding process, and the wire feeding speed–depth function was also obtained.

Microstructure and Properties of Al-6.0Mg-0.3Sc Alloy Deposited by Double-Wire Arc Additive Manufacturing.

Al-6.0Mg-0.3Sc alloy deposits are prepared by means of a double-wire arc additive manufacturing process. The formation, porosity, metallographic structure, type of precipitated phase, and mechanical properties of the deposit are studied. Double-wire arc forming affords precision advantages over single-wire-arc forming, which is evidenced by the increased surface uniformity of the deposit. Compared with the deposit of single-wire-arc formed, the deposit of double-wire arc formed exhibits only fewer and smaller pores, and the lower process heat yields rapid solidification and tiny precipitate sizes. A larger amount of Mg and Mn is observed to be dissolved in the Al matrix of double-wire arc-formed deposit, which increases the alloy strength, and smaller primary Al3Sc phase, which exhibits excellent grain refinement. Furthermore, the presence of a high amount of Sc solid solution in the matrix of double-wire arc-formed deposit strengthens the alloy, and the melting of the rear wire "heat-treats" the substrate formed by the front wire, promotes secondary Al3Sc phase precipitation, and further strengthens the alloy. Compared with the deposit of single-wire-arc formed, the mechanical properties of double-wire arc-formed deposit show an improvement: the tensile strength, yield strength, and elongation of the horizontally oriented specimens are estimated as 363 MPa, 258 MPa, and 26%, respectively. This successful implementation of the cold metal transfer + pulse process to prepare Al-Mg alloy parts can pave the way to their industrial production. The proposed method can find wide utility in the fields of aviation, aerospace, military, and shipbuilding.

Wire based plasma arc and laser hybrid additive manufacture of Ti-6Al-4V

In this study, a novel wire based plasma transferred arc (PTA)-laser hybrid additive manufacture process was proposed for deposition of large-scale titanium parts with high deposition rate and near-net shape. The optimum processing conditions, including the heat source configuration, wire feeding position, and arc-to-laser separation distance, were investigated. The benefits of using the hybrid process over the single PTA and laser deposition processes on their own were studied. The results show that compared to the single PTA process, the hybrid process has extended energy distribution and melt pool size, giving more interaction time of the wire with the heat sources and therefore a higher deposition rate. Compared to the laser process, the hybrid process has a much higher wire melting efficiency and tolerance to wire positioning accuracy. Owing to more distributed energy across the two heat sources, the likelihood of keyhole formation in the hybrid process is lower than that in the single PTA process. The best configuration for the hybrid process is the PTA leading, combined with front feeding of the wire. In this configuration, the PTA is used to melt the feedstock and the laser is used to control the melt pool size, which allows independent control of deposition rate and bead shape. A set of multi-layer walls was built to demonstrate the feasibility of this process for the manufacture of engineering parts. The results show that the achieved flat beads are very desirable for low surface waviness and lead to near-net-shape deposition. The main limitation of the hybrid process is remelting into the underlying layer. To overcome this, a multi-energy source process with more evenly distributed energy has been proposed.

Microstructure and Properties of 304 Stainless Steel Specimen Manufactured by Cold Metal Transfer + Pulse Composite Arc Additive

304 stainless steel test block was fabricated by continuous melting wire with CMT and pulse mixed mode, and the path of additive manufacturing is layered slice S-shaped. The relationship between microstructure and properties of the specimen was investigated by microscope, SEM, EBSD, XRD, tensile, impact and electrochemical experiments. The results show that molding between weld and weld is very good, and the microstructure is mainly Austenite, Ferrite and a little of σ, and there are three kinds of Ferrite morphology: cellular, wormlike and lath. σ phase precipitates easily in regions with high ferrite content and is distributed at the boundary between austenite and ferrite. The specimen has good low temperature toughness. The microscopic fracture surface is mainly dimple, and the precipitates in the fracture surface are mainly fine carbide particles. The tensile strength of the additive manufacturing 304 specimen is higher than the forged specimen, and the type of fracture is ductile fracture. The electrochemical analysis of 304 stainless steel specimens and forgings shows that CMT and pulse arc additive manufacturing specimen has excellent corrosion resistance and its corrosion current is slightly lower than the forging.

Observation of Arc Behaviour in TIG/MIG Hybrid Welding Process

In this project, the influence of TIG currents’s variation on arc stability of TIG/MIG hybrid welding was studied by comparing with MIG welding process. The welding current-voltage waveform was analyzed to characterize the arc stability of the MIG arc. From the observations, the introduction of TIG arc as low as 60 A of current significantly change the MIG arc stability in TIG/MIG hybrid welding. The length of MIG arc in TIG/MIG welding increased with the introduction of the TIG arc as compared with MIG welding. The increase of arc length was due to the arc interaction between the TIG arc and MIG arc, which is affecting the wire melting rate. At the maximum TIG current, the diameter of the molten droplet decreased with the increment of droplet transfer frequency.

Effects of Filler Wire Intervention on Gas Tungsten Arc: Part II - Dynamic Behaviors of Liquid Droplets

In gas tungsten arc welding (GTAW), the filler wire in-creases the deposition efficiency and influences the welding stability. Its interactions with the gas tungsten arc (GTA) are significant to better understand the welding process and to monitor and control weld quality. In view of this, the first part of the work, Effects of Filler Wire Intervention on Gas Tungsten Arc: Part I — Mechanism, explained the inter-action mechanisms between the filler wire and the gas tungsten arc based on the proposed arc-sensing method of detecting probe voltage (i.e., the voltage signal between the filler wire and the tungsten electrode/workpiece). In this second part of the work, experiments were designed to make the filler wire melt in different areas of the arc to study the dynamic behaviors of the droplet and its effect on the arc. Typical metal transfer modes are discussed, and droplet oscillation is geometrically characterized through image processing and then analyzed in the time domain and time-frequency domain. The results show that the liquid droplet affects the arc through its transfer to the weld pool, its oscillation, and occupying the arc space. Information about these dynamic behaviors can be easily reflected in the probe voltage, which would be a valuable signal to monitor the process stability in GTAW with filler wire. This work shows the potential of the proposed sensing method for monitoring and controlling weld quality in all welding positions, GTA-based additive manufacturing, etc.

Investigation of filler wire melting and transfer behaviors in laser welding with filler wire

For laser welding with filler wire process, there exist numerous technical parameters about the position between the laser beam and the filler wire, so it is difficult to guarantee the process stability and weld quality. In order to obtain stable welding process and high quality weld, the wire melting and transfer behavior must be controlled as the interaction between the laser beam and filler wire is extremely complicated. In this work, laser welding with filler wire process was studied, with particular attention to the melting dynamics of filler wire, the wire melting and transfer behavior. The high-speed camera system was used to observe the wire melting and transfer behaviors. The direct observation of the welding process by the high-speed camera can help to uncover the mechanism of the process stability of laser welding with filler wire. Systematic experiments were carried out in this study for liquid bridge and droplet transfer mode. The results indicated that the relative distance (Wx) between the filler wire and the laser beam and wire feed rate could influence the wire melting and transfer behaviors even the whole welding stability significantly. Therefore, control the Wx and wire feed rate is the key to obtain stable transfer mode for the liquid bridge. The research findings provided a clearer understanding for improving process stability and weld quality.

분석화학을 통한 전선 용융흔의 분석기법 향상에 관한 연구

With the steady increase in electricity use, electric fires account for more than 20% of the total number of fires each year. Electrical fires are caused by electrical arcs or sparks inside the cover of wires, but they are also caused by external flames melting the cover of wires into contact with each other. In electric fires, melting of wires is usually found, which is commonly referred to as short circuit. The molten marks are divided into the primary one that occurs inside the wire and causes the fire directly, the secondary one caused by external fire, and the heat melt found in the unenergized state. Scientific criteria for the distinction between primary and secondary molten mark is presented by using qualitative and quantitative analysis, through analytical chemistry such as SEM and EDS, XRD and EA.

Effect of bypass coupling current on corrosion resistance of twin-wire indirect arc surfacing layer

Corrosion resistance of stainless steel surfacing layer prepared by twin-wire indirect arc welding (TWIAW) under different bypass coupling current was studied. For bypass coupling TWIAW(BC-TWIAW), the welding arc was a hybrid arc consisting of indirect arc and direct arc burning alternately. Increased cathode wire feeding speed raised current of indirect arc, which was beneficial for promoting wire melting and adjusting solidification rate. With a higher cathode wire feeding speed, weld solidified faster, and the corrosion resistance was significantly improved. Passive film of BC-TWIAW layers owned the characteristics of bipolar semiconductors with less charge carriers.

Effect of welding parameters on weld formation quality and tensile-shear property of laser welded SUS301L stainless steel lap filet weld

Laser welding of lap-filet joints SUS301L austenitic stainless steel plates were carried out with filler wire. Results showed that the weld zone was composed of columnar austenite and a small amount of δ-ferrite. The effect of feed direction, wire distance (H), laser incident position (L) and laser incident angle on the wire melting mechanism, weld pool geometry, metal transfer modes and heat distribution were studied with the assistance of the high speed camera recording the welding process. Better weld quality was obtained under the circumstances of leading direction, wire distance 0.5mm, laser incident position 0.5mm, and laser incident angle 0̊. The fracture was always located in the weld where the thickness of it was the smallest. There was a linear relationship between the weld width and the smallest weld thickness. The maximum tensile-shear force was 24.46kN. The fracture mode was ductile fracture with fracture surface dimples.

Development of a highly productive GMAW hot wire process using a two-dimensional arc deflection

Gas metal arc welding (GMAW) processes are used in a wide range of applications due to their high productivity and flexibility. Nevertheless, the supplied melting wire electrode leads to a coupling of material and heat input. Therefore, an increase of the melting rate correlates with an increase of the heat input by the arc at the same time. A possibility to separate material and heat input is to use an additional wire, which reduces penetration and worsens the wetting behaviour. Consequently, bead irregularities such as bonding defects or insufficient root weldings can occur. In the context of this article, a controlling system for a two-dimensional magnetic arc deflection is presented, which allows to influence arc position as well as material transfer. The analysed GMAW hot wire process is characterised by high melting rates while also realising a sufficient penetration depth and wetting behaviour.