Wire-arc additive manufacturing of structures with overhang: Experimental results depositing material onto fixed substrate

As additive manufacturing (AM) technology grows both more advanced and more available, the challenges and limitations are also made more evident. Most existing solutions for AM build structures layer by layer using strictly vertical material deposition. As each layer must vertically adhere to the previous layer, support structures must be added if there are to be any kinds of overhangs. For methods requiring the build to be performed within a chamber, the size of the structure is also very limited. The research presented in this paper explores possible solutions to these challenges, focusing on wire-arc additive manufacturing in order to effectively build structures that can not easily be constructed using in-box, layer-based methods for AM. By non-vertical material deposition using an industrial robot manipulator, metal structures with overhangs are built onto a fixed, horizontal surface without any support structures. Cross sections of two different structures are examined by optical microscopy and hardness measurements to reveal potential differences between the areas with and without intersections or overhang.publishedVersio

Depth profiling at a steel-aluminum interface using slow-flow direct current glow discharge mass spectrometry

Direct current glow discharge mass spectrometry (dc-GDMS), which relies on sector field mass analyzers, is not commonly used for depth profiling applications because of its slow data acquisition. Nevertheless, dc-GDMS has good reproducibility and low limits of detection, which are analytical features that are encouraging for investigating the potential of dc-GDMS for depth profiling applications. In this work, the diffusion of traces of chromium and nickel was profiled at the interface of a steel-aluminum bilayer using a new sensitive dc-GDMS instrument. The depth profile of the non-treated sample was compared with that of a heat-treated specimen at 400°C for 30 min. Scanning electron micrographs, energy dispersive X-ray spectroscopy (EDS), and electron probe microanalysis (EPMA) were used to study the diffusion process. The results of the study show that both chromium and nickel are enriched at the steel-aluminum interface, with higher concentrations of both elements for the heat-treated specimen. Two peaks for both chromium and nickel were clearly present at the interface, with a high concentration of chromium in the aluminum layer. This observation is likely a consequence of elemental diffusion from the interface towards the aluminum layer. The presence of the third layer, steel beneath the aluminum layer, might also have contributed to this observation.acceptedVersio

Effect of preheating and preplaced filler wire on microstructure and toughness in laser-arc hybrid welding of thick steel

Acicular ferrite (AF) is the most important microstructural constituent to achieve high toughness at low temperatures in weld metal of steels. This is due to the relatively small grain size and large misorientation angles. AF is known to form at non-metallic inclusions (NMIs), but under high cooling rates, as in deep and narrow laser-arc hybrid welding (LAHW), this phenomenon is scarcely studied. In deep and narrow LAHW, insufficient transportation of filler wire to the root results in low amount of NMIs, thus bainite-martensite mainly forms due to fast cooling. In this work, a 45 mm thick high strength low alloy steel was welded by double-sided LAHW using different groove preparations. The effect of different cooling times on the microstructure in the weld metal and the heat-affected zone was studied. A low fraction of AF and high hardness were achieved in the root of weld metal when using standard LAHW. This was related to a rapid cooling time (Δt8/5 35 J) was achieved at −50 °C by combining preheating and preplaced filler wire, and up to 45 % fraction content of AF was reached. However, many NMIs were still inactive due to a small diameter (< 200 nm) and unfavorable chemical composition related to the high cooling rate. The external methods had no influence on the occurrence of weld centerline cracks in the root, which will require further attention to secure mechanical properties and integrity.publishedVersio

Wire Arc Additive Manufacturing of Aluminium Alloys

Summary Additive manufacturing of metals has gained considerable attention from the industry and academia in the last decade. The attributes to create stand-alone components with novel designs and enhanced properties are main drivers to implement additive manufacturing into the industrial value chain. Wire arc additive manufacturing, hereafter referred to as WAAM, utilise the inherent properties of arc welding to manufacture components. WAAM is considered a high efficiency, low-cost method with large build envelopes. Big components can therefore be made in a reasonable amount of time. The method is highly suited for a range of industrial sectors for manufacturing of all kinds of metals, including aluminium. The WAAM process is still maturing for industrial exploitation. One objective for further development of WAAM is to benchmark the process against established forming processes. One of these processes is casting. Thus, two different casting methods, i.e., steel mould casting and sand mould casting, were compared against WAAM. Al-Si alloys with near-eutectic composition were used as reference. The materials were assessed in terms of porosity content, secondary dendrite arm spacing, grain size, tensile strength and hardness. The Al-Si alloy processed by WAAM showed superior properties compared to both casting methods. The enhanced properties were accounted for by a higher cooling rate during solidification and hence a finer microstructure. The comparative study demonstrated that a fine microstructure is key for enhanced properties. A finer microstructure implies better mechanical integrity in terms of strength, ductility, anisotropy, among others. Extensive grain refinement can also depress the cracking susceptibility that pester several aluminium alloys during solidification. Thus, measures to refine the microstructure of WAAM aluminium alloys were investigated. In particular, addition of ceramic nanoparticles to refine the aluminium microstructure and to provide particle strengthening was identified as a promising solution. In an attempt to pursue this solution, a new production route for aluminium alloys reinforced with ceramic nanoparticles was developed. The novel solid-state process metal screw extrusion was examined as an all-in-one solution to mix, disperse and extrude wires for WAAM. Produced materials through metal screw extrusion before and after WAAM deposition were examined by a range of techniques. This included light optical microscopy, scanning electron microscopy, hardness testing, computed tomography, tensile testing and hydrogen measurements. Alloys of AA4043 Al-Si, AA5183 Al-Mg, and AA6082 Al-Mg-Si were mixed with nanoparticle additions (TiC, TiN, TiO$_2$) through metal screw extrusion. The ceramic compounds did not survive the melting cycle in WAAM, and various morphologies of the Al$_3$Ti intermetallic were observed. In high-silicon alloys like AA4043, TiC decomposed to Ti$_7$Al$_5$Si$_{12}$. AA5183 mixed with TiC exhibited significant grain refinement after WAAM, due to heterogeneous nucleation on Al$_3$Ti. Remaining TiC provided particle strengthening. All investigated materials with ceramic additions exhibited a highly porous structure after WAAM. Thermogravimetric measurements of nanoparticle powders showed significant mass loss at elevated temperatures, indicating presence of volatile compounds such as moisture. Metal screw extruded materials were therefore produced with an elevated content of hydrogen, which is the primary cause for porosity in aluminium alloys. Vacuum heat treatment of nanoparticles prior to metal screw extrusion was investigated but failed to lower the pore density after WAAM. Measures to protect the input materials and improve the material quality are briefly discussed in this work. In summary, WAAM of aluminium alloys shows great potential for future industrial implementation. WAAM have attractive attributes which include reasonably high deposition rate, large design freedom, and sound mechanical properties. However, the properties can be further enhanced, and the aluminium alloy selection must be expanded. Addition of ceramic nanoparticles to enhance strength and refine the microstructure is a promising, yet challenging, solution. Metal screw extrusion show promising results to produce wires with low carbon footprint and with novel compositions and mixtures. As WAAM requires high-quality materials for successful deposition, the metal screw extrusion setup needs further development related to protection of the input materials

Optimization of Electrical Conductivity in Screw Extruded Wires

The novel production method of screw extrusion was used to produce an electrical conducting wire of commercial pure aluminium. To optimize electrical properties, effects of impurity atoms in solid solution were examined by resistance measurements and microstructural analysis in LOM, SEM and TEM. Performed actions revealed precipitation of iron to be a major contributor to increasing electrical conductivity in aluminium. Precipitation kinetics, summarized in a newly assessed TTT-diagram, showed the most rapid precipitation around 450 degree Celcius. The produced screw extruded wire had a fine sub-grain structure arising from repeated deformation, recrystallization and concurrent precipitation during extrusion. The final product performed very well in terms of ultimate tensile strength and electrical conductivity; 61 MPa and 64.30 %IACS, respectively. Tempering after screw extrusion resulted in a decreased conductivity, but a slight enhancement of the tensile strength

Comparative study of eutectic Al-Si alloys manufactured by WAAM and casting

Wire and arc additive manufacturing (WAAM) of metallic materials is expected to become part of the new industrial revolution. The possibilities for complex designs and superior mechanical properties can in many cases replace traditional manufacturing processes such as casting. In order to benchmark the properties of aluminium WAAM components, a comparative study was performed with two different casting techniques: permanent casting with steel mould and sand mould casting. Aluminium-silicon alloys with near eutectic composition were used for the comparison. Porosity levels, secondary dendrite arm spacing, grain size distribution, tensile strength and microhardness were considered for the comparison. The WAAM material exhibited superior mechanical properties originating from a finer dendritic and eutectic microstructure compared with the castings. A slight anisotropy in tensile ductility was observed in the WAAM material, probably due to a coarse microstructural zone between individual beads. All investigated materials had low levels of porosity, < 1% by area fraction. The comparative study has shown that WAAM of aluminium-silicon alloys is well suited for high-integrity applications.publishedVersio

Wire and Arc Additive Manufacturing with TiC-Nanoparticle Reinforced AA5183 Alloy

Wire and arc additive manufacturing of aluminium-ceramic composites shows great potential to produce high strength materials. By incorporation of nanoparticles in the feedstock wire, fine-grained material with low susceptibility for solidification cracking and enhanced strength can be obtained. In fact, this study utilised the novel screw extrusion method to prepare an aluminium alloy containing TiC nanoparticles. The commercial aluminium alloy AA5183 was selected for WAAM to assess and benchmark the effects of screw extrusion and TiC. The materials have been assessed in terms of microstructure, porosity content and mechanical properties. The presence of TiC reduced the average grain diameter by 70%, while Vickers hardness increased with 13%. However, number of pores per unit volume increased by one order of magnitude. The porosity is believed to stem from hydrogen introduced in the AA5183-material through screw extrusion processing, in addition to hydrogen trapping and pore nucleation on TiC nanoparticles.publishedVersio

Wire Arc Additive Manufacturing by Robot Manipulator: Towards Creating Complex Geometries

Additive manufacturing (AM) is the umbrella term that covers a variety of techniques that build up structures layer-by-layer as opposed to machining and other subtracting methods. It keeps evolving as an important technology in prototyping and the development of new devices. However, using AM on a larger scale is still challenging, as traditional methods require the AM machines to be larger than the manufactured structure. The research presented in this paper is a continuation of our work on assessing the possibility of large-scale robotic AM. The focus in this paper is the feasibility of large-scale AM of metallic materials by arc welding. A series of experiments with robotic arc welding using an ABB IRB2400/10 robot are presented and discussed. These experiment will help map some of the challenges that need to be addressed in future work

Wire arc additive manufacturing of AA5183 with TiC nanoparticles

Aluminium alloys processed by wire arc additive manufacturing (WAAM) exhibit a relatively coarse microstructure with a columnar morphology. A powerful measure to refine the microstructure and to enhance mechanical properties is to promote grain refinement during solidification. Addition of ceramic nanoparticles has shown great potential as grain refiner and strengthening phase in aluminium alloys. Thus, an Al-Mg alloy mixed with TiC nanoparticles was manufactured by the novel metal screw extrusion method to a wire and subsequently deposited by WAAM. Measures to restrict oxidation of magnesium during metal screw extrusion were examined. Purging of CO2 gas into the extrusion chamber resulted in a remarkable reduction in formation of MgO and Mg(OH)2. TiC decomposed to Al3Ti during WAAM deposition, leading to a significant grain refinement of 93% compared to a commercial benchmark. The presence of remaining TiC nanoparticles accounted for an increased hardness of the WAAM material through thermal expansion mismatch strengthening and Orowan strengthening. Exposure of TiC to moisture in air during metal screw extrusion increased the internal hydrogen content significantly, and a highly porous structure was seen after WAAM deposition

Effects of iron precipitation and novel metal screw extrusion on electrical conductivity and properties of AA1370 aluminium

In order to develop well-performing aluminium electrical conductors, the role of iron and processing method on electrical and mechanical properties was studied for an AA1370 alloy. Firstly, Ø3 mm cold drawn wires were subjected to a solid solution heat treatment (640 °C/1 h) followed by artificial aging at various temperatures in order to reveal the representative time-temperature-transformation (TTT)-diagram for Fe-rich precipitates. The highest precipitation rate occurred at 450 °C. Secondly, an identical AA1370 alloy was produced by the novel metal continuous screw extruder (MCSE) process into a new Ø3 mm wire. The as produced wire had a recrystallised outer layer and an elongated fibrous structure having a typical 〈0 0 1〉 texture in the center region. TEM investigations revealed Fe-rich precipitates at grain boundaries thus impeding grain growth to some extent. The screw extruded wire processed at 450 °C had a high conductivity (64.2%IACS) while being softer (UTS 65 MPa) than the cold drawn wire (UTS 164 MPa, 61.9%IACS). The correspondence between strength and electrical conductivity for cold drawn and screw extruded wires was compared to literature data for pure and dilute alloys. The screw extruded wire outperformed other alloys as to electrical conductivity, while being among the materials having lowest strength