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Additively manufactured TiAl alloys present alternating dendritic grain (DG) and fully lamellar colony (FLC) microstructure, which is detrimental to mechanical properties. This work proposes a novel strategy for eliminating alternating DG and FLC and in-situ obtaining fine FLC by performing arc rescanning (AR) on each deposition layer to increase its thermal cycling number during additive manufacturing. The AR can not only eliminate its mechanical anisotropy, but enhance its mechanical properties by tailoring fine FLC microstructure. [ABSTRACT FROM AUTHOR]

The directed energy deposition-arc (DED-arc) process is gaining popularity for cost-effective production of large parts, especially in the marine industry. However, precise shaping control, especially for parts with sharp corners, has been challenging. Nickel aluminum bronze (NAB) alloy, widely used in the marine field, is an expensive material, making it crucial to improve its forming accuracy. To tackle this challenge, the Angle-Driven Parameter Control Model (APCM) has been introduced. This model is designed to enhance corner paths in the DED-arc process for NAB alloy. By adjusting the travel speed, the APCM aims to reduce height errors at corners. To validate the effectiveness of the APCM, single bead depositions and a multi-layer thin-wall component with seven different angles (10°, 20°, 30°, 45°, 60°, 75°, and 90°) were tested. Results indicate that the APCM has a more significant optimization effect at smaller angles. For angles above 60°, relatively small height deviations can be achieved without parameter control. For the 15-layer thin-wall component, the APCM control reduced the height deviation at the 10° angle from 7.72 to 2.09 mm. Therefore, the proposed APCM is ideal for corner paths in the DED-arc process of NAB alloy. [ABSTRACT FROM AUTHOR]

Ti-Al(48at.%) alloy was deposited by twin-wire plasma arc welding additive. The preheating temperature was 560, 620 and 680 °C, respectively. The results show that when the preheating temperature was 560 and 620 °C respectively, cracks appeared at the arc-starting part of the lower of the sample. The preheating temperature was 680 °C, and the sample had no cracks. The numerical simulation results show that the transient stress decreased with the increased of the preheating temperature. The transient stress at the lower of the specimen was greater than that at the middle and top, and the YZ shear stress and Z-direction stress at the front end were greater than those at the middle and end, resulted in cracks at the arcing starting part of the lower. [ABSTRACT FROM AUTHOR]

TiAl alloy becomes a promising high-temperature structural material due to excellent mechanical properties at elevated temperature. However, the inherent brittleness makes it difficult to be processed by traditional technologies. Therefore, an innovative twin-wire plasma arc additive manufacturing (TW-PAAM) process is developed to fabricate TiAl alloy. Substrate heating is indispensable during additively manufactured TiAl alloy, which can alleviate its crack tendency. In this paper, crack-free TiAl alloy samples are fabricated using TW-PAAM, and the effect of substrate temperature (560, 620 and 680 °C) on as-deposited TiAl alloy is investigated in detail. With the increase in substrate temperature, microstructural lamellar spacing and colony size exhibit the tendency of increase. Also, α2 phase content, recrystallization degree and high Schmid factor frequency present the decrease tendency. These variations of microstructure characteristics further lead to the decrease in both microhardness and tensile properties of the deposit. These findings provide a valuable reference for optimizing microstructures and mechanical properties of additively manufactured TiAl alloys. [ABSTRACT FROM AUTHOR]

Herein, the deposition of a Ti–Al(48 at%) alloy via twin‐wire arc additive manufacturing (WAAM) using plasma arc welding (PAW) and tungsten inert gas (TIG) welding arc sources is presented. The microstructure and the phase composition of different regions of the alloy are analyzed using metalloscopy and X‐ray diffraction. The transient temperature field and residual stress distribution are measured before and after the process, respectively. A transient thermostress model is established using the finite‐element method. Results show that the alloy is composed primarily of α2‐Ti3Al and γ‐TiAl phases, while the microstructure evolution during the Ti–Al(48 at%) alloy deposition process is described. The thermal conductivity in the lower region of the alloy far exceeds that in the middle and upper regions. The thermal conductivity is smaller in the upper region and the midregion, resulting in the increase in heat accumulation. Due to arc shrinkage and reduced heat input, the PAW process reduces the heat accumulation and stress distribution differences more effectively than the TIG process. [ABSTRACT FROM AUTHOR]

• Metal evaporation flux across Knudsen layer is discussed in laser keyhole welding; • A high-gas pressure state is indicated at the keyhole bottom; • Evaporation flux decreases with the keyhole depth due to the high-gas pressure; • The decrease of evaporation flux suppresses the element loss in the fusion zone. This study aims to calculate the metal evaporation flux across the Knudsen layer inside the keyhole in laser welding of Al–Mg alloys. The pressure balance at the keyhole wall was analysed and used as the boundary condition in the calculation. Based on the extracted keyhole shape parameters from high-speed images, the surface temperature and gas pressure inside the keyhole were calculated, and the evaporation fluxes for both Al and Mg were acquired. The results show that the front wall of the keyhole absorbs more laser energy than the rear wall. The gas pressure inside the keyhole increases and the evaporation flux decreases with increasing keyhole depth. In addition, the evaporation flux of Mg is much higher than that of the base element Al, which can result in the loss of Mg inside the melt pool. The distribution of the Mg evaporation flux inside the keyhole agrees with the measured Mg loss in the welded section of the Al–Mg alloy. Thus, the calculation method of the metal evaporation flux inside the keyhole can be applied in future simulations of element loss in laser keyhole welding. [ABSTRACT FROM AUTHOR]

Due to low density and good high temperature performance, titanium aluminide has been continuously attractive for aerospace and automotive industries. In recent years, additive manufacturing of titanium aluminide has become a popular research area. The present work has first time fabricated commercial graded TiAl-4822 alloy using an innovative twin-wire directed energy deposition-arc (TW-DED-arc) process, which uses a unique twin-wire feeding method to produce the target alloy in-situ. According to experimental results, TW-DED-arc fabricated TiAl-4822 alloy possesses direct full density, homogeneous chemical composition, uniformed microstructure, and stable mechanical property. Also, the formation mechanism of layered microstructure consisting of alternative columnar and equiaxed grain regions has been investigated in detail. Even under as-fabricated condition, TW-DED-arc fabricated TiAl-4822 performs competitive tensile properties compared to the same alloy produced by casting and other additive manufacturing methods. Generally, successful TW-DED-arc buildup of TiAl-4822 provides a high-efficiency and low-cost route for the further development of titanium aluminide production. [ABSTRACT FROM AUTHOR]

Considering the advantages of low density and excellent high-temperature performance, titanium aluminide is identified as an ideal aerospace structural material. In recent years, twin-wire directed energy deposition-arc (TW-DED-arc), which possesses the characteristics of low cost but high deposition efficiency, has continuously attracted worldwide attention. However, obtaining equiaxed grains with small Al variation for titanium aluminide using TW-DED-arc method is still a key concern. In the present research, Ti–48Al–2Cr–2Nb alloy with equiaxed lamellar colonies and less than 1 at.% of Al content was successfully fabricated by regulating deposition current of TW-DED-arc. Also, the effect of deposition current on microstructure and mechanical properties was systematically investigated. The experimental results that focus on top and middle regions of as-fabricated TiAl-4822 alloys, indicate that increasing current favors in generation of equiaxed grains and γ lamellae, which leads to columnar to equiaxed transition and higher γ phase content, respectively. Also, elemental homogeneity can be significantly homogenized. As deposition current increases, tensile strength increases while degree of tensile anisotropy decreases significantly, benefiting from the equiaxed grains with satisfying size. Importantly, microstructure evolution, tensile anisotropy and fracture characteristics of TW-DED-arc fabricated TiAl-4822 alloys are discussed in detail. Generally, these findings provide an effective process method to optimize microstructure and mechanical properties of TW-DED-arc fabricated TiAl-4822 alloys. [ABSTRACT FROM AUTHOR]

Due to the intrinsic high room temperature brittleness and cold-cracking susceptibility, the fabrication and forming of γ-TiAl intermetallic alloy component is extremely difficult. Therefore, in recent years, a wire-arc additive manufacturing (WAAM) technique has been developed to fabricate the γ-TiAl intermetallic alloy by depositing the Ti and Al wires into a single tungsten arc generated molten pool with specific wire feed ratios. However, the WAAM fabricated γ-TiAl intermetallic alloy has been found having inhomogeneous layer-by-layer microstructure and the excessive heat input of tungsten arc would induce significant residual stress in the bulk sample. In the present paper, the previous WAAM has been further upgraded and an innovative twin-wire plasma arc additive manufacturing (TW-PAAM) process has been developed. Afterwards, a γ-TiAl intermetallic alloy wall component with specific chemical composition of Ti-48Al has been fabricated and the metallography, phase composition and tensile properties are characterized subsequently. It has been found that a significantly more uniform microstructure is obtained in the TW-PAAM fabricated γ-TiAl intermetallic alloy than the previous WAAM technique. The content of α 2 phase, lamellar colony size and lamellar spacing exhibited the tendency of decreasing from the lower to upper part along building direction. And the tensile strength and ductility of the lower section are lower than the middle and top sections. In general, the present TW-PAAM technique has shown promising capability of fabricating γ-TiAl intermetallic alloy with lower cost, and the investigation results would become a valuable reference for understanding the evolution mechanism of microstructure and mechanical properties of the additively manufactured TiAl alloy. [ABSTRACT FROM AUTHOR]

The microstructure of TiAl alloy is extremely sensitive to the thermal history. During additive manufacturing, multiple thermal cycles and partial remelting of the TiAl deposition layers take place. Therefore, the heat input would exert significant influences on microstructural and mechanical characteristics of additively manufactured TiAl alloys. In this study, TiAl alloys were prepared successfully by twin-wire directed energy deposition-arc (TW-DED-arc). Systematic studies were conducted to regulate the microstructure characteristic and the mechanical property of the as-deposited TiAl alloys via the line energy. The results indicate strong effects of the line energy on the microstructure and performance of the TiAl alloys, which show alternative distribution of layer-band like microstructure with the dendritic grains and the fully lamellar colonies along the deposition direction. With the enhancement in the line energy, the layer-band like feature gradually weakens, and the fully lamellar colonies increase. In addition, the discontinuous coarsen phenomenon and the discontinuous dynamic recrystallization occurred, and the anneal twin was also formed. On the other hand, tensile fractures were observed in the dendritic grains region, and the maximum tensile strength was achieved with the medium line energy. However, the elongation and microhardness are proportional to the line energy. With the elevation in the line energy, the elongation and microhardness gradually increases and decreases, respectively. The microstructure formation mechanism of as-fabricated TiAl alloy is analyzed systematically, and the fracture mechanics was also discussed in detail. The findings of this research contribute not only to the methodology for fabrication of TiAl alloys with the novel TW-DED-arc process, but also to the deep understanding on the regulation of the microstructural and mechanical characteristics of the additively manufactured TiAl alloys. • The TiAl alloys are fabricated successfully by twin-wire directed energy deposition-arc. • The microstructure evolution mechanism is analyzed systematically for as-deposited TiAl alloy. • The relationships are established in detail among microstructure, properties and line energy for as-deposited TiAl alloy. • The microstructure and mechanical properties are regulated successfully by the line energy. • The tensile fracture mechanism is also revealed clearly for as-deposited TiAl alloy. [ABSTRACT FROM AUTHOR]

Intrinsic brittleness and cold cracking sensitivity of Fe–Al intermetallic compounds make their fabrication and processing difficult. In recent years, an innovative additive manufacturing (AM) technology, twin-wire directed energy deposition-arc (TW-DED-arc), has been developed to prepare defect-free Fe–Al alloys via in-situ alloying of Fe and Al elements in a gas tungsten arc welding (GTAW) generated molten pool. However, unconstrained plasma of regular tungsten arc leads to excessive heating area and residual stress during AM buildup. And relatively weak arc pressure of GTAW makes composition inhomogeneity a concern in the in-situ alloying molten pool. To solve the two potential drawbacks, plasma arc welding (PAW) possessing higher energy density and arc pressure is used to power the TW-DED-arc process and fabricate Fe–30Al alloy. In the present research, a special wire-feeding strategy for stable twin-wire feeding of Fe and Al wires is successfully developed to overcome the droplet transfer difficulty induced by constrained arc of PAW and large thermophysical properties difference of heterogeneous filler wires. And according to microscopy characterization results, the innovative PAW powered TW-DED-arc process is capable of fabricating defect-free Fe–30Al bulk sample with homogeneous Al content and epitaxial columnar grain structures. The successful application of PAW-powered TW-DED-arc technique in fabrication of iron aluminide provides stabler and more efficient route for iron aluminide component. • Iron aluminide is first time fabricated via PAW powered TW-DED-arc process. • Droplet transfer mode of Fe and Al wires during in-situ alloying is clarified. • Heat accumulation during deposition is found beneficial to Fe–30Al alloy. [ABSTRACT FROM AUTHOR]

Additively manufactured TiAl alloy presents heterogeneous and anisotropic microstructure and mechanical properties, resulting from the characteristic of layer by layer deposition, which is detrimental to its further application. In this work, TiAl alloy is built using an innovative twin-wire directed energy deposition-arc (TW-DED-arc). In order to eliminate the heterogeneity and anisotropy of additively manufactured TiAl alloy, the effect of heat treatment on the TiAl alloy fabricated by TW-DED-arc process is investigated. The experimental results indicate that the suitable heat treatment process (1420 °C, 2 min, furnace cooling) can effectively eliminate the heterogeneity and anisotropy of TiAl alloy fabricated using TW-DED-arc process. After the heat treatment, the layer-band like microstructure transforms into homogeneous fully lamellar colony with relatively smaller size, microstructure homogeneity is significantly improved, and the microstructure is also closer to equilibrium state. Also, the microhardness distribution of sample becomes more homogeneous, whose anisotropy is also effectively weakened. • The TiAl alloy is fabricated successfully by twin-wire directed energy deposition-arc • The additively manufactured TiAl alloy presents heterogeneous and anisotropic microstructure and mechanical properties • The heat treatment method to eliminate the heterogeneity and anisotropy of additively manufactured TiAl alloy is obtained successfully • The layer-band like microstructure transforms into homogeneous fully lamellar colony after the heat treatment • The heterogeneity and anisotropy of mechanical properties are significantly improved after the heat treatment. [ABSTRACT FROM AUTHOR]

Titanium aluminide has continuously attracted the attention of aerospace industry due to its low density and good performance at high temperatures. To overcome the room temperature brittleness induced manufacturing difficulty, additive manufacturing (AM) technology of titanium aluminide has been developing rapidly since last decade. Compared to powder-based AM methods such as electron beam melting (EBM), the twin-wire directed energy deposition-arc (TW-DED-arc) method for titanium aluminide fabrication possesses outstanding material deposition efficiency and much lower cost. However, because of the excessive arc deposition energy, the layer-band like microstructure of TW-DED-arc fabricated titanium aluminide is a great concern affecting the mechanical properties. In the present research, microstructure of as-fabricated TiAl alloy is modified by adjusting the deposition current of TW-DED-arc. Experimental results show that along with the increase of deposition current, the layer-band like microstructure consisting of dendritic grain region and fully lamellar colony region can be mostly homogenized. Although microhardness is decreased because of the inevitable lamella growth, tensile strength of TiAl deposit is on the other hand increased by eliminating the interdendritic γ-phase. In general, the present work has proved that deposition current adjustment is an effective method for homogenizing the microstructure of additively manufactured titanium aluminide. [ABSTRACT FROM AUTHOR]

TiAl alloys are considered as promising high temperature structural materials. However, the inherent brittleness makes it difficult to be processed. In the present research, TiAl alloys with different Al content are fabricated successfully using an innovative twin-wire plasma arc additive manufacturing system. The effect of Al level on the phase composition, microstructure characteristics, microhardness and tensile properties of as-deposited TiAl alloy is investigated in detail. The results show that the α 2 phase content exhibits increase tendency with the decrease of Al level, and the α 2 phase content is higher in the top region than that in the middle region. The tetragonal ratio c/a of γ phase tends to reduce with the decrease of Al content. The microstructure characteristics also present obvious difference for as-deposited TiAl alloys. The lamellar spacing gradually decreases with the decrease of Al concentration. With decreasing Al level, the microhardness of alloy tends to increase. In addition, the as-deposited Ti-48 Al (at.%) alloy exhibits much better tensile properties than other TiAl alloys due to optimum microstructure characteristics. The findings provide a valuable reference for additively manufactured TiAl alloy with proper composition ratio. [ABSTRACT FROM AUTHOR]

The formation of intermetallic compounds (IMCs) at the joint interface between dissimilar metals has been the focus of recent research, since joint properties are largely determined by these interfacial IMCs. In this study, FeCoCrNiMn high entropy alloy (HEA) was joined to traditional 1060Al alloy by friction stir lap welding (FSLW) for the purpose of reducing the excessive cost of HEAs. Microstructural characterization shows the interfacial layer is continuous but varies in thickness from 1.3 to 1.7 μm. The interfacial structure can be divided into three types. Close to the Al side are STR I and II, confirmed to be of same monoclinic Al 13 Fe 4 type IMC, but showing sharp difference in composition with simultaneous co-depletion of Cr and Mn and co-enrichment of Co and Ni in STR I. Close to the HEA side is STR III, consisting of parallelly arranged ultra-fine columnar grains, identified to be orthorhombic Al 5 Fe 2 type IMC. [Display omitted] [ABSTRACT FROM AUTHOR]

• Keyhole and plume features were automatically extracted from the image processing method; • Effects of Mg content was helpful to keep the opening of keyhole; • Effects of Mg content leaded to the frequent swelling generation at the rear weldpool. This paper concerns the effects of Mg content on keyhole behaviours during the laser welding process of 5754, 5083 and 5A06 Al-Mg alloys. The features of the vapour plume and keyhole area are extracted from high-speed images with image processing methods. The increase of Mg content leads to the increase of hydrodynamic pressure caused by metal evaporation and decrease of surface tension acting on the keyhole wall. The strong metal evaporation and decrease of surface tension help the formation of keyhole, which both result in a large size of keyhole with deep penetration and low frequency of keyhole collapse. However, the impact of metal vapour could be enhanced by the increase of evaporation and the decrease of both surface tension and viscosity, which leads to the frequent generation of swelling at the rear weldpool and poor formation of the weld bead. [ABSTRACT FROM AUTHOR]

3D printed porous TC4 alloy was joined with ultra-high molecular weight polyethylene (UHMWPE) via friction spot joining (FSpJ) in this study. The Z -axial load (F z) and joining temperature (T J) histories were measured with load cell beneath worktable and K-type thermocouple inserted in joints respectively. Strong joining strength (3000 N, fracture on polymer base material) was achieved after joining. The F z evolution was calculated to be strongly correlated with polymer viscosity (η) during joining (dF z /dt ∝ η). Meanwhile, strong coupling effect between T J and F z (dT J /dt ∝ F z) was found. With optimized joining parameters, good macro−/micro- joining effect were achieved. These findings make it possible to predict joining quality and interface temperature during joining process via collecting F z history. Unlabelled Image • 3D-printed TC4 alloy was joined with UHMWPE via friction spot joining (FSpJ) technique. Strong joint strength (~3000N, 20MPa) was realized. • Sound macroscale filling effect and microscale joining between TC4 and UHMWPE were realized with optimized parameter control. • During FSpJ, the z-axial joining load (F z), viscosity of polymer (η) and joining temperature (T J) were found to have strong relationships. [ABSTRACT FROM AUTHOR]