Microstructure and mechanical properties of porous Si3N4/Invar joints brazed with Ag-Cu-Ti+Mo/Cu/Ag-Cu multi-layered composite filler

Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler was successfully designed to braze porous Si₃N₄ and Invar alloy. To further reduce the CTE mismatch between the porous Si₃N₄ and brazing filler, Mo particles were introduced into Ag-Cu-Ti. The effects of the Mo addition on the microstructure and mechanical properties of the brazed joints were studied. The results showed that, the addition of Mo particles into Ag-Cu-Ti lowered the CTE mismatch and improved the joint strength to a certain degree. However, an excessive content was harmful. The Mo particles could absorb Ti at high temperature, causing Ti shortage in the reaction with the ceramic. When cooling down, the absorbed Ti was released. The released Ti could react with Cu to generate Cu-Ti phase. So, additional Ti was adopted in the brazing filler as a supplement. When the Ti content was 5 wt%, the reaction layer on the ceramic interface was too thin to transfer enough load. However, when it reached 15 wt%, the Cu interlayer dissolved completely and Fe-Ti and Ni-Ti phases appeared. The maximum joint shear strength (83 MPa) was obtained with 10 wt% Ti and 5 vol% Mo, which had exceeded 90% of the porous Si₃N₄ and was 56% higher than the joint brazed without Mo particles.     

Microstructural evolution and mechanical properties of dissimilar Al–Cu joints produced by friction stir welding

5A02 aluminum alloy and pure copper were joined by friction stir welding (FSW). A defect-free joint was obtained when one of process parameters, i.e. the traverse speed was lowered from 40 mm/min to 20 mm/min. A good mixing of Al and Cu was observed in the weld nugget zone (WNZ). A large amount of fine Cu particles were dispersed in the upper part of the WNZ producing a composite-like structure. In the lower part, nano-scaled intercalations were observed and identified by transmission electron microscopy (TEM). These layered structures were subsequently confirmed as Al₄Cu₉ (γ), Al₂Cu₃ (ε), Al₂Cu (θ), respectively. Formation of these microstructures caused an inhomogeneous hardness profile. Particularly, a distinct rise in hardness was noticed at the Al/Cu interface. Excellent metallurgical bonding between Al and Cu gave rise to good behaviors in the tensile and bending strength.     

ZrOCl2·8H2O as an environmentally friendly and recyclable catalyst for the chemoselective synthesis of 2-aryloxazolines and bis-oxazolines under thermal conditions and microwave irradiation

The reactions of arylnitriles with β-aminoalcohols to prepare 2-aryloxazolines has been efficiently carried out in the presence of catalytic amounts of ZrOCl₂·8H₂O using heat or microwave irradiation. Synthesis of mono- and bis-oxazolines from dicyanobenzenes was accomplished with high selectivity using this catalyst. Chemoselective conversion of arylnitriles to their corresponding 2-oxazolines in the presence of alkylnitriles was also achieved by this method. The use of ZrOCl₂·8H₂O catalyst is feasible because of its easy availability, easy handling, stability, easy recovery, reusability, good activity and eco-friendly.     

Numerical simulation to study the effect of tack welds and root gap on welding deformations and residual stresses of a pipe-flange joint

This paper presents a three dimensional sequentially coupled non-linear transient thermo-mechanical analysis to investigate the effect of tack weld positions and root gap on welding distortions and residual stresses in a pipe-flange joint. Single-pass MIG welding for a single ‘V’ butt-weld joint geometry of a 100 mm diameter pipe with compatible weld-neck ANSI flange class # 300 of low carbon steel is simulated. Two tack welds at circumferentially opposite locations, with the crucial effect of the tack weld's orientation from the weld start position is the focus in this study. Four different angular positions of tack welds (0 and 180°, 45 and 225°, 90 and 270°, 135 and 315°) are analyzed. In addition, four cases for root gaps (0.8, 1.2, 1.6, 2.0 mm) are considered and computational results are compared. A basic FE model is also validated with experimental data for temperature distribution and deformations. From the results, the axial displacement and tilt of the flange face are found to be strongly dependent on the tack weld orientation and weakly dependent on the root gap.     

Evaluation of microstructures and mechanical properties of friction stir welded lap joints of Inconel 600/SS 400

The microstructures and mechanical properties of friction stir welded Inconel 600 and SS 400 lap joints were evaluated in this study. Friction stir welding was carried out at a tool rotation speed of 200 rpm and a welding speed of 100 mm/min. Application of friction stir welding was notably effective in reducing the grain size of the stir zone, as a result, the average grain size of Inconel 600 was reduced from 20 μm in the base material to 8.5 μm in the stir zone. The joint interface between Inconel 600 and SS 400 was soundly welded without voids and cracks, and MC carbides with a size of 50 nm were partially formed in the region of the lap joint interface in Inconel 600. In addition, a hook from SS 400 was formed on the advancing side of the Inconel 600 alloy, which directly affected an increase in the peel strength of the weld. In this study, we systematically discussed the effect of friction stir welding on the evolution of the microstructures and mechanical properties of friction stir lap jointed Inconel 600 and SS 400.     

Conditions for spontaneous cracking of a brittle matrix due to the presence of thermoelastic stresses

The stress intensity factor is estimated for a circular crack associated with a spherical particle subjected to thermoelastic stress field. It is shown that the stress intensity factor is a function of particle size, elastic and fracture properties of the two phases and preexisting crack length. Spontaneous matrix cracking will occur when the particle size exceeds critical value D_c. Close agreement between calculated and measured value for D_c is obtained. The model is applicable to all particulate composites where there is volume increase of a particle induced either by phase transformation or thermal expansion mismatch (α_m >α_p). The results of the present analysis fit very well the observations made on spontaneous cracking of a single phase materials possessing thermal expansion anisotropy or two phase materials possessing thermal expansion mismatch.     

Carbothermal reduction reaction enhanced wettability and brazing strength of AgCuTi-SiO2f/SiO2 system

A carbothermal reduction reaction (CRR) approach was developed in this research to tailor the surface phase structure of the SiO_2f/SiO₂ composites with high chemical reactivity to replace the original inert surface. Results show that SiC can form after CRR treatments. For AgCuTi-SiO_2f/SiO₂ wetting interfaces, TiC and residual pyrolytic carbon layer can be found inside the reaction layer, which was the key, promoting the wettability of the AgCuTi-SiO_2f/SiO₂ system. The contact angle of the AgCuTi-SiO_2f/SiO₂ system dropped from 127° to 43° after the CRR treatments. The reliability of the bonded AgCuTi-SiO_2f/SiO₂ interface was also characterized by putting 3 different systems into comparison, i.e., the original AgCuTi-SiO_2f/SiO₂ system, the AgCuTi-SiO_2f/SiO₂ system with CRR treatments (SiC formation) and the AgCuTi-SiO_2f/SiO₂ system coated with powdered carbon (no SiC formation). The shear strength of the SiO_2f/SiO₂-AgCuTi-SiO_2f/SiO₂ system with CRR treatments was the highest, which was 3 times that of the other 2 brazing systems.