Wire + arc additive manufacture of new and multiple materials

Wire + Arc Additive Manufacture (WAAM) features high deposition rates, short lead times and low equipment investment. WAAM is especially suitable for producing large-scale parts made from highly demanding and expensive materials and provides design freedom for multi-material structures. Although WAAM has been successfully applied to titanium alloys, its feasibility to deposit various other single material and multiple materials are yet to be studied, and the resulting WAAM material properties are largely unknown. Maraging steel and Inconel 718 are highly advanced alloys widely used for critical components in aerospace, and Inconel 625 cladded steel pipes are extensively applied in Oil & Gas industry, all of which require an alternative manufacturing process to forging and casting. The present work aims at utilizing WAAM to deposit maraging steel, Inconel 718, and Inconel 625/low alloy steel multimaterial structures complying with the industrial standard. The objective for the single material is to develop the strength comparable to wrought alloy, while dilution control is the concern for the multi-material deposition. Material characterization was carried out to understand the WAAM microstructure resulting from the inherent thermal history and explain the strength inferiority to the wrought material from a crystallographic scale. Industry standard heat treatment and interpass cold rolling were applied to improve the strength of WAAM material to the wrought level based on the understanding of the strengthening mechanisms of a particular material. Besides, environmental factors affecting the deposition feasibility was studied, and strategies controlling the dilution level for multi-material deposition was developed. The research contributes to the WAAM material database of maraging steel and Inconel 718 and proves the capability of building multi-material structures using WAAM. A research pattern for applying WAAM to various new materials and applications is established, and a scientific understanding of the relationship between the WAAM process and material properties was gained

Enhancing mechanical properties of wire + arc additively manufactured INCONEL 718 superalloy through in-process thermomechanical processing

Wire + arc additive manufacture (WAAM) was applied to produce INCONEL 718 superalloy (IN718) components in a layer by layer manner; further, interpass cold rolling was introduced to generate in-process thermomechanical processing effect during the deposition process. Mechanical testing showed that with rolling applied, the strength of the solution plus aging treated WAAM IN718 was improved from 1056 MPa (unrolled) to 1351 MPa (rolled) which met the wrought standard (1276 MPa), and the material anisotropy was eliminated. The unrolled IN718 featured large columnar grains developing along the building direction, with the length and width as large as 11 mm and 0.8 mm respectively; rolling induced plastic deformation triggered a non-uniform recrystallization upon successive depositions, which produced a recrystallized core with small columnar grains and numerous finely equiaxed grains with the grain size of 12.7 μm. The overall strengthening produced by interpass rolling was attributed mostly (76%) to the rolling induced recrystallization which produced grain size reduction strengthening and created larger grain boundary area to allow more precipitation at the grain boundaries, and partially (24%) due to the improved aging response of the recrystallized grain structur

Improving mechanical properties of wire + arc additively manufactured maraging steel through plastic deformation enhanced aging response

Maraging steel gains ultrahigh strength through aging; however, wire + arc additively manufactured maraging steel features a columnar-dendritic structure with associated segregation and shows a much less pronounced aging response. In this paper, plastic deformation was introduced through interpass cold rolling during the layer-by-layer deposition process. After aging, mechanical testing showed a substantial strength improvement from 1410MPa (unrolled) to 1750MPa (50kN rolled). Rolling induced partial recrystallisation to break the dendritic structure and form high-angle grain boundaries, which promoted the atoms diffusion to enable a more uniform solutionizing process and improved the subsequent aging response by 105-110%. The main contribution of overall strengthening of the rolled alloy was attributed to the effective aging process, accounting for more than 95% of the entire strength increase

Investigation of process factors affecting mechanical properties of INCONEL 718 superalloy in wire plus arc additive manufacture process

This paper systematically evaluated the effect of oxide, wire source and heat treatment on the mechanical properties of wire + arc additively manufactured (WAAM) INCONEL 718. Comparison of the as deposited grain structure was made with laser-powder based AM and wrought INCONEL 718. Results showed that oxides formed during deposition had no effect on the mechanical properties since a 0.5μm thick passivation layer consisting of Cr2O3 and Al2O3 formed upon deposition and prevented further oxides from forming inside the bulk. Wires from different suppliers resulted in around 50 MPa difference in UTS possibly due to the slight compositional variation and uncertainties in TiN inclusion. Standard heat treatment improved the strength from 824 MPa to 1110 MPa in the horizontal direction, but the average strength was 105 MPa lower than the wrought alloy. The as deposited WAAM INCONEL718 featured large columnar grains and numerous Laves phase, as compared to the fine grains of laser powder bed fusion and wrought INCONEL 718. This starting microstructure led to less favourable and less numerous precipitates forming during heat treatment, which is the main reason for the strength mismatch. A different heat treatment would not help due to the starting microstructure

Study on strengthening mechanism of Ti/Cu electron beam welding

Welding-brazing method is widely used for dissimilar metals welding. However, it is becoming increasingly difficult to further improve the connection strength by controlling the formation of the transition layer. In this study, an innovative welding method referred to as adjacent welding was addressed, which greatly improved the tensile strength of Ti/Cu dissimilar joint. The strength of new joint could reach up to 89% that of copper base metal, compared to the use of a traditional welding-brazing method which strength coefficient is within the limit of 70%. In order to determine the strengthening mechanism of adjacent welding, optical microscopy, SEM, EDS and XRD were applied for the analysis of microstructure and phase structure. Furthermore, tensile strength was also tested. The results show that due to the process of remelting and reverse solidification of intermetallic compounds (IMCs) layer, a less complex and thinner IMCs layer was formed and TiCu (553 HV) with high embrittlement existing in the front of titanium substrate was changed into Ti2Cu (442 HV). Performances of joints were optimized by these changes. An interpretation module was presented for the mechanism

Microstructure and mechanical properties of TOP-TIG-wire and arc additive manufactured super duplex stainless steel (ER2594)

As the excellent combination of mechanical properties and corrosion resistance for super duplex stainless steel, a prospective method – Wire and Arc Additive Manufacturing – for fabricating this material was proposed, and a wall component was deposited in this study. The microstructure of the as-deposited wall was carefully analyzed along with the variation of mechanical properties. The results revealed that, in the wall-body, the austenite/ferrite phase balance was broken by the overgrowing the austenite phase. During this process, the intergranular secondary austenite leading the increase of austenite phase together with some contributions made by the precipitation of intragranular secondary austenite. Propagation of the intermetallic phases, chi and sigma phase, was not the major reason for the low impact toughness in the last layer area and the root region. Instead, the presence of CrN and “inclusions” (Cr2N and impurities) took the main responsibility not only in the impact toughness but also the ductility. The anisotropic analysis revealed that the UTS and elongation appeared distinct difference in vertical and horizontal direction samples. The varieties in YS were eliminated by the nitrogen work hardening effect to a large extent

Oxide accumulation effects on wire + arc layer-by-layer additive manufacture process

A maraging steel wall structure was built layer-by-layer to study oxide accumulation mechanisms and the influence of oxides on the subsequent deposition. An online arc welding camera was also applied to investigate the wetting and spreading behaviour of the deposition on different surface conditions. Two maraging steel walls were deposited under torch shielding only and torch plus tent shielding conditions respectively to study the effect of oxides on the mechanical properties. Upon deposition a mixture of Fe, Al and Ti oxides formed, floated to the weld pool surface and accumulated layer by layer, deteriorating the surface condition such that it was rough and porous, which adversely affected the stability of arc and the wetting and spreading process of the weld pool in subsequent layers. The accumulation of oxides added to the uncertainty of the layer dimension and worsened the surface finish to reduce the structural integrity. Despite that the majority of the oxides floated to the weld pool surface, oxides (up to a few hundred nanometers in diameter) were found to be dispersed in the additively manufactured structure and might be one of the strengthening sources resulting in a 11% increase in UTS and a 19% decrease in elongation compared to the structure built in the torch plus tent shielding condition

Microstructural evolution and mechanical properties of maraging steel produced by wire + arc additive manufacture process

Wire + arc additive manufacture is developed for producing large-scale metallic components. In this paper, maraging steel parts were produced, and the microstructure and mechanical properties were investigated. The microhardness and tensile strength of the as deposited alloy reduced from the bottom to the top due to the transient thermal cycling, which resulted in partial aging and non-uniform formation of intermetallic compounds along the building direction. Solutionizing, followed by 3 h aging, significantly reduced the microstructural heterogeneity and increased the mechanical properties by 24.7% through the formation of large amounts of finely distributed precipitates. The as deposited alloy possessed superior strength to the wrought alloy in solutionized condition but inferior to the later in aged condition, which was attributed to the less pronounced aging response of the low-angle columnar grains characterized microstructure and the presence of retained and reverted austenite