Peigang He

Interfacial phenomena of cold metal transfer (CMT) welding of zinc coated steel and wrought aluminium

Abstract The interfacial microstructures and intermetallic compounds produced by cold metal transfer (CMT) welding of wrought aluminium to zinc coated steel have been investigated. The results showed that the reaction layer mainly consists of FeAl3 phase and the thickness of the layer was decreased to 7–8 μm owing to the low heat input of the CMT method. And the thin reaction layer guarantees the tensile property of the joint. At last, according to the interface characteristic of the dissimilar metal joint, the interfacial reaction process was described in detail.

Microstructure, thermodynamics and compressive properties of AlCrCuNixTi (x = 0, 1) high entropy alloys

Two equiatomic high entropy alloys, AlCrCuNixTi (x = 0, 1), were prepared by an arc furnace. Their microstructure, thermodynamics and compressive properties were investigated in as-cast state. The AlCrCuTi alloy consists of a face centred cubic (fcc) phase, two body centred cubic (bcc) phases and an Al4Cu9-like phase, while the AlCrCuNiTi alloy contains an fcc phase and two bcc phases. Thermodynamic expressions based on mixing enthalpy matrix facilitate the thermodynamic calculation. The element Cr takes severe segregation during solidification and forms Cr rich phases in both alloys. The addition of Ni to the AlCrCuNiTi alloy inhibits the formation of intermetallic compounds and enhances the yield strength, compressive strength and ultimate strain, but degrades Vickers hardness.

Microstructure of alumina ceramic/Ag–Cu–Ti brazing alloy/Kovar alloy joint

Abstract The microstructure of the alumina ceramic/Kovar alloy joint brazed with Ag–35·2Cu–1·8Ti (wt-%) was studied. The effects of brazing temperature on the microstructure were also discussed. It was found that the microstructure of the joint brazed at 1173 K for 5 min was TiO + TiNi3 + TiFe/eutectic Ag–Cu/TiFe2 + TiNi3/TiFe2 + Cu (s.s) +Ag (s.s). When the brazing temperature was >1193 K, there was no TiO formed on the alumina ceramic/brazing alloy interface.

Energy absorption mechanisms of modified double-aluminum layers under low-velocity impact

Laminated multiple metal or alloy sheets have been widely used in protective structures. However, energy absorption mechanism remains unclear for those laminates with different interface and surface conditions under low-velocity impact. This study investigates the effect of interface and surface modified double aluminum sheets under drop-weight loading. The experimental results showed that epoxy-bonded double sheets did not dissipate more energy than free-contact sample. The simulation results agree well with the experimental data at low cohesive stress of the epoxy adhesive, and friction plays an important role in absorbing impact energy for free-contact sample. However, at high interface cohesive stress as simulated, epoxy-bonded samples absorb more energy than free-contact ones. Further experiments indicated that sample with grease spread front surface is more sensitive in improving energy absorption than sample with grease applied in between two layers. These results are important reference for designing laminated composites to improve impact resistance.

Microstructures, mechanical properties and oxidation resistance of SiBCN ceramics with the addition of MgO, ZrO2 and SiO2 (MZS) as sintering additives

Nano-crystalline SiBCN ceramics were prepared by mechanical alloying (MA) plus hot pressing (HP) with the addition of MZS1 (MgO, ZrO2 and SiO2) and MZS2 (ZrSiO4 and SiO2) as the sintering additives. The effects of the two additives on the microstructure, mechanical properties and oxidation resistance of SiBCN ceramics were carefully evaluated. The addition of MZS1 and MZS2 additives in the SiBCN matrix can boost elemental diffusion and matrix densification. Owing to the effective densification caused by the sintering additives, the mechanical properties of SM1 and SM2 samples are far superior to monolithic SiBCN. The oxide layers of SM1 and SM2 samples remain relatively dense and continuous, and have a strong binding capacity with the matrix after oxidation testing at 1500 °C for 20 h. The residual excess carbon in the SiBCN matrix should be responsible for the formation of ZrC and the outermost oxide layer structure is comprised of SiO2 and ZrC after the oxidation test. The flexural strength, Young’s modulus, fracture toughness and Vicker’s hardness of the SM2 sample are much higher than monolithic SiBCN ceramics, reaching to 394.2 ± 41.7 MPa, 152.9 ± 16.0 GPa, 5.86 ± 0.86 MPa m1/2 and 8.3 ± 0.6 GPa, respectively.

Control of hydrogen cracking in the welded steel using microstructural traps

Hydrogen diffusion into steel can embrittle the material in H2S environments, but this effect can be offset by suitable hydrogen trapping sites in the microstructure. Fine Ti(C,N) inclusions have been studied as the trapping sites in high strength low alloy (API X-70) welds, with Ti additions ranging from 0.004 to 0.16 wt.%. The trapping sites were investigated by electron microscopy and thermal desorption spectroscopy. Manganese sulphide particles were the main initiation sites for hydrogen induced cracking as expected. The optimum Ti addition was around 0.02% with no evidence of cracking in the weld. The estimated values of trapping activation energy for dislocations, microvoids, MnS and Ti(C, N) were approximately 25.9, 34.6, 65.1 and 87.6 kJ mol−1, respectively.

Interface evolution of SiO2 glass ceramic and Ti–6Al–4V alloy joint brazed with Ag–Cu–Ti alloy

Abstract In the present paper, SiO2 glass ceramic and Ti–6Al–4V alloy were successfully brazed with Ag–21Cu–4·5Ti active braze alloy. The interfacial microstructure and evolution course of SiO2 glass ceramic/Ti–6Al–4V joint were studied in detail. According to the experimental results, active element Ti plays a quite important role in the formation of reaction layers on the joint interface. The reaction products of the joint are TiSi2, Ti4O7, TiCu, Cu2Ti4O and Ti2Cu respectively. The interface evolution can be generally described by four stages, which are solution and diffusion of atoms, reaction among atoms, formation of reaction layers and precipitation of solid solution layers respectively.

Interface microstructure analysis of SiO2 glass ceramic and Ti–6Al–4V alloy joint brazed with Ti–Zr–Ni–Cu alloy

Abstract In the present paper, SiO2 glass ceramic was joined to Ti–6Al–4V alloy with 35Ti–35Zr–10Ni–15Cu (wt-%) filler foil. The whole brazing process was performed under vacuum circumstances at different temperatures (850–1000°C) for several holding times (1–30 min). According to results of scanning electron microscopy, energy dispersive spectrometry, electron probe X-ray microanalysis and X-ray diffraction analysis, the reaction products of the interface are Ti2O, Zr3Si2, Ti5Si3, Ti based solid solution and Ti2(Cu,Ni). There is residual TiZrNiCu braze alloy on the SiO2 glass ceramic/Ti–6Al–4V alloy interface after brazing. Besides, the interface evolution model of the joint was described by four stages: diffusion and solution among atoms, formation of reaction products, precipitation and growth of reaction layers respectively.

Effects of graphene oxide on the geopolymerization mechanism determined by quenching the reaction at intermediate states

The effects of graphene oxide on the geopolymerization reaction products at different times were investigated by quenching the reaction. The phase composition and valence bond structure evolution were investigated systematically. The results show that the ethanol/acetone mixture helps isolate reaction products early in the process (0–24 h). RGO bonded well with the geopolymer matrix during the geopolymerization. The degree of densification increased and the amorphous nature of the material decreased with reaction time. The addition of rGO accelerated the conversion of five and six coordinate Al–O sites into four coordinates and Si atoms forming Q4(3Al) network structure.

Experimental investigation on interface phenomena involving spreading of Fe–Cr based filler metal

Fe–Cr based alloy emerges as an economic alternative to Ni based filler metal applied in brazing of steels and high temperature alloys; however, its spreadability and interface microstructure have been very poorly studied. In the present study, wettability and interface microstructure of new type Fe–Cr–Ni–Cu–P–Si filler metal are investigated at different pressures. The molten Fe–Cr–Ni–Cu–P–Si filler metal demonstrated good spreadability both in vacuum and in air. Fingering phenomenon at the spreading edge indicated the dissolution of base metal during spreading and insufficient diffusion. Reactions promoted the fragmentation and removal of the oxide film, and the dissolution of oxide and base metal further improved the spreadability. These mechanisms indicate potential for an excellent brazeability of Fe–Cr–Ni-Cu–P–Si filler metal.