Ni3Al-based superalloys have excellent mechanical properties which have been widely used in civilian and military fields. In this study, the mechanical properties of the face-centred cubic structure ...Ni3Al were investigated by a first principles study based on density functional theory (DFT), and the generalized gradient approximation (GGA) was used as the exchange-correlation function. The bulk modulus, Young’s modulus, shear modulus and Poisson’s ratio of Ni3Al polycrystal were calculated by Voigt-Reuss approximation method, which are in good agreement with the existing experimental values. Moreover, directional dependences of bulk modulus, Young’s modulus, shear modulus and Poisson’s ratio of Ni3Al single crystal were explored. In addition, the thermodynamic properties (e.g., Debye temperature) of Ni3Al were investigated based on the calculated elastic constants, indicating an improved accuracy in this study, verified with a small deviation from the previous experimental value.
The undercut is an undesired defect in high-speed hybrid laser-arc welding, which will deteriorate the mechanical properties of the weld. Laser power has a significant impact on undercut formation, ...while research on the corresponding mechanism is insufficient. Exploring the mechanism of laser power influence on undercut has an important theoretical guiding significance for industrial production. In this study, the variation law of undercut is investigated by analyzing the arc characteristic, droplet transfer, and the flow state of the molten pool surface assisted by a visual sensing system. We analyze the change of arc pressure and droplet impact in detail momentum to explain the behavior of molten metal in the arc gouge region. Furthermore, combined with the arc drag force, fluctuating state, and the solidification direction of the weld pool, we discuss the flow state of liquid metal at the weld toe. The results show that the increase of laser power keeps more molten metal in the arc gouge region and slows down the backward flow of liquid metal. High laser power is beneficial for molten metal to fill the weld toe and suppressed undercut effectively. By increasing the laser power to 10 kW, the forming quality of the upper and lower surfaces of welds can reach the B-level in welding standard: ISO13919-2:2021.
Stationary shoulder friction stir welding has been used to weld 4-mm-thick 2219-T6 aluminium alloy at high rotation speeds. Strain plastic damage was applied to demonstrate the formation mechanism of ...welding defects at high rotation speeds above 2000 rpm. A three-way converging zone in the joint, in which materials of different microstructure characteristics converged from three directions during high tool rotation speed welding, was found. At the relatively high tool rotation speed, the significant differences in the microstructures would result in weld defects in this zone. It could be attributed to material toughness damage at high strain rate. With increasing tool rotation speed, the tensile strength of the joint constantly decreased. When the tool rotation speed varied from 2000 to 2600 rpm, the tensile strength decreased from 305 MPa (68.2% of the BM) to 238 MPa (53.2% of the BM).
The twin-wire CMT with Al-Si filler was used as weld/brazing process for dissimilar joining of stainless steel and aluminum. It is shown that this approach can ensure a satisfactory performance of ...the joint and the dissimilar joint exhibits typical characteristics of common known steel/aluminum weld brazing joints. Intermetallic compounds (IMCs) were detected next to the steel as a layer of Fe
2
Al
5
, as a layer of Fe
3
Al
2
Si
4
adjacent to the aluminum side and as a scattered phase of Fe
4
Al
13
in between. The total thickness of the IMCs was determined with 2–4 μm. The maximum tensile strength of the weld joints without reinforcement was up to 96 MPa and cross tensile test specimens fractured at the brazing interface at about 35% of the strength of the aluminum. An increase of the wire feeding rate and decrease of the welding speed thereby outperform beneficial in respect to wetting characteristics. However, excessive heat input (e.g., wire feeding rate larger than 7.5 m/min) causes a reduction of the mechanical properties. A groove gap about 1 mm ensures furthermore a good back appearance.
Q690E high strength low alloy (HSLA) steel plays an important role in offshore structures. In addition, underwater local cavity welding (ULCW) technique was widely used to repair important offshore ...constructions. However, the high cooling rate of ULCW joints results in bad welding quality compared with underwater dry welding (UDW) joints. Q690E high strength low alloy steels were welded by multi-pass UDW and ULCW techniques, to study the microstructural evolution and mechanical properties of underwater welded joints. The microstructure and fracture morphology of welded joints were observed by scanning electron microscope and optical microscope. The elemental distribution in the microstructure was determined with an Electron Probe Microanalyzer. The results indicated that the microstructure of both two welded joints was similar. However, martensite and martensite-austenite components were significantly different with different underwater welding methods such that the micro-hardness of the HAZ and FZ in the ULCW specimen was higher than that of the corresponding regions in UDW joint. The yield strength and ultimate tensile strength of the ULCW specimen are 109 MPa lower and 77 MPa lower, respectively, than those of the UDW joint. The impact toughness of the UDW joint was superior to those of the ULCW joint.
Effects of scanning strategy during powder bed fusion electron beam additive manufacturing (PBF-EB AM) on microstructure, nano-mechanical properties, and creep behavior of Ti6Al4V alloys were ...compared. Results show that PBF-EB AM Ti6Al4V alloy with linear scanning without rotation strategy was composed of 96.9% α-Ti and 2.7% β-Ti, and has a nanoindentation range of 4.11–6.31 GPa with the strain rate ranging from 0.001 to 1 s−1, and possesses a strain-rate sensitivity exponent of 0.053 ± 0.014. While PBF-EB AM Ti6Al4V alloy with linear and 90° rotate scanning strategy was composed of 98.1% α-Ti and 1.9% β-Ti and has a nanoindentation range of 3.98–5.52 GPa with the strain rate ranging from 0.001 to 1 s−1, and possesses a strain-rate sensitivity exponent of 0.047 ± 0.009. The nanohardness increased with increasing strain rate, and creep displacement increased with the increasing maximum holding loads. The creep behavior was mainly dominated by dislocation motion during deformation induced by the indenter. The PBF-EB AM Ti6Al4V alloy with only the linear scanning strategy has a higher nanohardness and better creep resistance properties than the alloy with linear scanning and 90° rotation strategy. These results could contribute to understanding the creep behavior of Ti6Al4V alloy and are significant for PBF-EB AM of Ti6Al4V and other alloys.
The temperature evolution during the joining process of high thermally conductive copper-diamond joints was characterized. Experiments and numerical simulations were carried out on the copper-diamond ...joints, which were joined by using self-propagating reactive multilayers sandwiched by solders. Two different kinds of solders were used, namely Sn-3at%Ag-0.5at%Cu and Sn-9at%Zn. Experimental results showed that the obtained joints had a high shear strength of 32.1 MPa and high thermal conductivity of 38.15 W/(<inline-formula> <tex-math notation="LaTeX">\text{m}\cdot \text{K} </tex-math></inline-formula>) based upon averages from statistical computations. Numerical simulation results showed that the heat released by the self-propagating reaction of multilayers was quite concentrated. When the reaction finished, the instantaneous temperature of the multilayers had reached nearly 1300 °C. However, the temperature of the copper and diamond never exceeded 400 °C. The heat-affected region of the copper-diamond joint was very localized, and the high-temperature area was mainly distributed within 0.25 mm. There was no significant thermal impact on other nearby components.
Single-crystalline Ni3Al-based superalloys have been widely used in aviation, aerospace, and military fields because of their excellent mechanical properties, especially at extremely high ...temperatures. Usually, single-crystalline Ni3Al-based superalloys are welded together by a Ni3Al-based polycrystalline alloy via transient liquid phase (TLP) bonding. In this study, the elastic constants of single-crystalline Ni3Al were calculated via density functional theory (DFT) and the elastic modulus, shear modulus, and Poisson’s ratio of the polycrystalline Ni3Al were evaluated by the Voigt–Reuss approximation method. The results are in good agreement with previously reported experimental values. Based on the calculated mechanical properties of single-crystalline and polycrystalline Ni3Al, three-dimensional finite element analysis (FEA) was used to characterize the mechanical behavior of the TLP bonded joint of single-crystalline Ni3Al. The simulation results reveal obvious stress concentration in the joint because of the different states of crystal orientation between single crystals and polycrystals, which may induce failure in the polycrystalline Ni3Al and weaken the mechanical strength of the TLP bonded joint. Furthermore, results also show that the decrease in the elastic modulus of the intermediate layer (i.e., polycrystalline Ni3Al) can relieve the stress concentration and improve the mechanical strength in the TLP bonded joint.
Flux-cored arc welding (FCAW) of duplex stainless steels has been widely used in offshore engineering, nuclear power engineering, and petroleum pipe lines. But underwater wet welding repair ...technology is still impossible for avoiding pore defects in welds. To study how pores perform in underwater wet welds, underwater wet flux-cored arc welding of S32101 duplex stainless steel was carried out in a hyperbaric chamber. The evolution of porosity and microstructure with welding parameters in 20 and 60 m water depth was analyzed for comparison. The relationship between porosity and austenite morphology in microstructures was discussed. Decreased welding parameters can increase the cooling rate. As a result, the ferrite to austenite transformation can be promoted and the resultant microstructures can be refined. Lower welding speed prolongs the high-temperature retention time, which is helpful to gas escaping from the molten pool. As a result, the porosity of the welds decreases. Reduced welding voltage with finer microstructure prevents the porosity in both water depths. It was found that the coarser the austenite microstructure was, the higher the porosity was. Additionally, the welds conducted with bigger welding current in 20 m water depth have lower porosity, the same as the welding speed, while the situation is reversed in 60 m water depth for a coarser austenite microstructure.
Abstract
The influences of Er content on the interfacial microstructure shear properties and creep properties of Sn58Bi joints were investigated in this study. The intermetallic compound composition ...of Sn58Bi-xEr/Cu was Cu6Sn5 compound. The addition of Er suppressed the activity of Sn element, decreased the driving force for the growth of Cu6Sn5 intermetallic compound and decreased the thickness of Cu6Sn5 intermetallic compound layer. The shear properties and creep durability of Sn58Bi-xEr/Cu welded joints were improved to a certain extent. At the Er content of 0.1%, the shear strength and creep durability properties of the solder alloy are relatively optimal. When wt%Er was more than 0.1%, with the increasing Er content of rare earth elements, the internal organization of the joint interface is coarsened, and the flatness of the IMC layer at the interface is reduced, which leads to the decrease of the creep performance of the final joint.
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