Evaluation of interfacial bonding in dissimilar materials of YSZ-alumina composites to 6061 aluminium alloy using friction welding

The interfacial microstructures characteristics of alumina ceramic body reinforced with yttria stabilized zirconia (YSZ) was evaluated after friction welding to 6061 aluminum alloy using optical and electron microscopy. Alumina rods containing 25 and 50 wt% yttria stabilized zirconia were fabricated by slip casting in plaster of Paris (POP) molds and subsequently sintered at 1600 °C. On the other hand, aluminum rods were machine down to the required dimension using a lathe machine. The diameter of the ceramic and the metal rods was 16 mm. Rotational speeds for the friction welding were varied between 900 and 1800 rpm. The friction pressure was maintained at 7 MPa for a friction time of 30 s. Optical and scanning electron microscopy was used to analyze the microstructure of the resultant joints, particularly at the interface. The joints were also examined with EDX line (energy dispersive X-ray) in order to determine the phases formed during the welding. The mechanical properties of the friction welded YSZ-Al₂O₃ composite to 6061 alloy were determined with a four-point bend test and Vickers microhardness. The experimental results showed the degree of deformation varied significantly for the 6061 Al alloy than the ceramic composite part. The mechanical strength of friction-welded ceramic composite/6061 Al alloy components were obviously affected by joining rotational speed selected which decreases in strength with increasing rotational speed.     

Investigation of fracture for friction welded joint between pure nickel and pure aluminium with post-weld heat treatment

The present paper described the investigation of the fracture of friction welded joint between pure nickel (Ni) and pure aluminium (Al) with post-weld heat treatment (PWHT). Most of joints autogenously fractured from the adjacent portion of the intermediate layer (interlayer) consisting of intermetallic compound (IMC) on the weld interface due to growing of that after long heating time during the cooling process after PWHT. The IMC interlayer was composed with mainly NiAl, and that grew at the weld interface with PWHT. The joint fracture temperature increased with increasing width of the IMC interlayer in the axial direction of the joint. That is, the fracture of the joint occurred at the interface between NiAl layer and Al base metal. The fractured surface was covered with a little Ni₂Al₃ and/or NiAl₃, and that was like as disbonding. Furthermore, when the width of the IMC interlayer was smaller than approximately 40 μm, the joint fracture temperature of the joint was under about 470 K. However, when the width of the IMC interlayer exceeded 50 μm, the joint fracture temperature drastically increased up to about 800 K. Hence, it was able to be estimate that the joint fracture temperature increased with increasing width of the IMC interlayer. Therefore, one of the main reasons for the fracture of the joint could be concluded as remarkable decreasing of the bonding strength between NiAl layer and Al base metal, which was produced with PWHT.     

Evolution of grain structure across joints in friction stir welded EN AW 5083 H111 plates during thermal exposure

Abstract

The mechanisms involved in the coarsening of the grain structure across FSW joints produced in EN AW 5083 H111 plates with different pin profiles were investigated. The grain coarsening across the FSW joints invariably starts inside the shoulder flow zones and involves both abnormal growth of the grains inside the dynamically recrystallised regions as well as recrystallisation of the deformed grains outside these regions. The elliptical nugget zones produced with the threaded circular pins appear to be more stable with respect to the basin shaped counterparts obtained with the triangular pin. It takes an annealing temperature of 550°C for the former to become fully coarse grained across the joint while the joint produced with the triangular pin is largely covered with coarse grains starting at 500°C. The higher resistance of the nugget zone obtained with a threaded circular pin to grain coarsening appears to be critical in the selection of the pin profiles when the manufacturing cycle of the welded EN AW 5083 plates involves post-welding heat treatments.     

Interface and Bond Strength of Brazing Cemented Carbide K20 to Alloy Steel AISI 4140 by High-Frequency Induction

High-frequency induction brazing of cemented carbide (WC–Co, K20) and alloy steel (AISI 4140) using Cu–Zn base filler metal was carried out. The relationship between microstructure and performance of the welding joint was investigated. It was found that the filler metal exhibited excellent wettability and metallurgical bond in the welding surface. As the heating rate reduced, welding joint appeared smooth without any visible crack. In the diffusion layer, some intermetallic compounds were observed, which were produced by the reaction of diffusion atoms. The microhardness in the middle of the welding seam was 168 Hv and it increased gradually when approaching to the edge of welding seam. With brazing temperature increased or heating rate decreased, the shear strength of welding joint increased first and then decreased. The machining test clearly revealed that the cutting temperature and the flank wear increased with the cutting speed rose. The welding joint had good shear strength when the temperature was below 500°C and the shear strength decreased seriously when the temperature exceeded 500°C.     

两台同组板翅式换热器的偏流研究

简介空分设备中两台并联板翅式换热器的偏流量的计算,比较不同偏流量所带来的换热效果差别,得出同组两台换热器的偏流量与流体出口温度的关系曲线,最后提出三种解决偏流的方法及其优缺点。     

Effect of heat treatment on small scale fragmentation of aluminium alloy

Abstract

Small scale explosions, using a detonator, of 7075 aluminium alloy cylinders, 15–100 mm outside diameter, were carried out to investigate the effects of heat treatment on fragmentation. This was the finest for the strongest as received alloy and coarsest for the softest overaged alloy. This effect was similar to that seen in investigations of the fragmentation of steel. Cylinders of 50 and 100 mm in diameter did not fragment but plastically deformed with maximum deformation at the cylinder bottom. Fragmentation of 33 and 42 mm diameter cylinders produced long fragments typical of the break-up of thick walled cylinders. At smaller diameters, break-up gave fragments of several shapes, finer fragments being largely associated with the smallest diameter cylinders and the highest strength alloys. Results followed those seen in large scale studies of cylinder break-up and suggest the possibility of using small scale fragmentation experiments in the investigation of the effects of composition, heat treatment and processing on natural fragmentation.     

Effects of surface roughness on the diffusion bonding of Al alloy 6061 in air

The effect of surface roughness on the properties of Al6061 joints fabricated by diffusion bonding in air at 450°C was studied. It was found that rougher surfaces yield superior ultimate tensile strength and linearized bonded ratio. Joints with ultimate tensile strength comparable to that of bulk metal were obtained for holding times of 75 min using rougher surfaces; however, the maximum linearized bonded ratio obtained was only about 75%. Incomplete bonding was attributed to air entrapment along the bond interface. This was due to good bonding occurring at the periphery of the bonded specimens during the early part of the bonding process.     

铝合金管与碳钢管板连接工艺

列管式换热器的换热管束与管板连接一般均采用焊接加胀接的密封固定工艺。本文介绍了铝合金换热管与碳钢管板的连接固定密封工艺。     

Heat transfer characteristics of accumulator heat exchangers under various geometric and operating conditions

To minimize the possibility of flash gas generation at the inlet of the expansion device in multi air-conditioners, an accumulator heat exchanger is usually adopted. In this study, the heat transfer rate and subcooling difference in an accumulator heat exchanger were measured under various operating and geometric conditions. The effect of the operating conditions on the performance of the accumulator heat exchanger was analyzed for R-22, R-134a, and R-410A. The performance of the accumulator heat exchanger was also analyzed by varying diverse geometric parameters: the inner volume and the ratio of the diameter to height of the accumulator, and the diameter, length, and bending diameter of the inner heat exchanger. In addition, the flow of the refrigerant-oil mixture inside the accumulator was visualized. Based on the measured data, a dimensionless correlation for predicting the subcooling difference across the accumulator heat exchanger was developed.     

Heat transfer characteristics of spirally-coiled circular fin-tube heat exchangers operating under frosting conditions

The objective of this study is to investigate the heat transfer characteristics of spirally-coiled circular fin-tube heat exchangers under frosting conditions. The heat transfer rate, pressure drop, frost thickness, and Nusselt number of the heat exchanger were measured and analyzed by varying the fin pitch and number of tube rows under frosting conditions. In addition, the Nusselt number of the spirally-coiled circular fin-tube exchanger was compared with those of flat plate fin-tube heat exchangers with discrete fins. An empirical correlation of the Nusselt number was developed as a function of the Reynolds number, dimensionless fin pitch normalized by the hydraulic diameter, i.e., D_h/F_p, Fourier number, and number of tube rows. The measured Nusselt number was consistent with the predicted value with mean and average deviations of 3.5% and 0.3%, respectively.     

Influence of heat treatment and cutting speed on chip segmentation of age hardenable aluminium alloy

Abstract

A change in chip shape has been observed as a function of age hardening and cutting speed during high speed milling of the aluminium alloy 7075. In order to study this effect systematically, the aluminium alloy was heat treated to produce different precipitation states and machined under carefully controlled conditions at cutting speeds between 1000 and 7000 m min^-1. The underaged state shows local shearing producing segmented chips. The degree of segmentation increases with cutting speed. In contrast, the overaged state shows continuous chips up to the highest cutting speeds. The chips obtained with the peak aged state show a fluctuation between segmented and continuous parts. These results can be understood in terms of the differing work softening/hardening behaviour of the under- and overaged states owing to the specific interactions between dislocations and precipitates during chip formation.     

Thermal-hydraulic CFD study in louvered fin-and-flat-tube heat exchangers

Heat transfer performance prediction by CFD codes is of major interest. Usually air-side heat transfer characteristics of fin-and-tube heat exchangers are determined from limited experimental data. The ability of CFD code to predict flow patterns and thermal fields allows determining the heat transfer characteristics by performing ‘numerical experiments’. CFD calculations of a 1-row automotive condenser are compared to experimental results and correlations of the literature matching the fin design and the flow conditions. Calculations are performed for different air frontal velocities. 2D models, with uniformly constant fin temperature overestimate significantly the heat transfer coefficient. 3D models, taking into account tube effects, conjugate heat transfer and conduction through the fin are in better agreement with the experimental results. However, even if an offset in noticed between CFD calculations and the experimental results, the trends are comparable and CFD study permits to reach local information, leading to better understanding of the physical phenomena involved in compact heat exchangers. An attempt for 2D unsteady flow has also been performed. Results are discussed in terms of flow pattern and heat transfer coefficient behaviour.     

Properties of sintered-silver heat exchangers

Properties of sintered-silver plugs for use in heat exchangers were measured, including surface areea, electrical resistivity and shrinkage.     

On the performance of copper foaming metal in the heat exchangers of pulse tube refrigerator

As key components in pulse tube refrigerators (PTRs), heat exchangers have great influence on the performance of the PTRs, especially the cold end heat exchangers which dominate the cooling effect between the cold gas and heat load. Filling copper screens are widely used to improve the performance of heat exchange and laminar flow. Whereas, the heat transfer rate of copper screens is still not good enough for the actual requirements of PTRs. Furthermore, the flow resistance of the copper screen is growing up quickly with the increase of screen mesh. In this paper, we propose a new type of copper foaming metal with high heat transfer area and low flow resistance in the heat exchanger instead of the copper screens. The heat transfer performances of the copper screens and the copper foaming metal are firstly compared by theoretical calculation, which shows that the performance of the copper foaming metal with 600 μm pore size is better than that of 20 and 80 mesh copper screens, verified by experimental results. A four valve pulse tube refrigerator (FVPTR) with copper foaming metal of 600 μm pore size as filling material of the heat exchanger achieved 69.5 K, 2.5 K lower than that of using 20 mesh copper screens, 1.7 K lower than that of using 80 mesh copper screens.     

Formation process of the bonding joint in Ti/Al diffusion bonding

The process of the formation of Ti/Al diffusion bonding joints was studied by means of scanning electron microscopy (SEM), X-ray diffractometry (XRD) and shear strength measurement. Pure titanium and pure aluminum were used as bonding couples. The results show that the process of joint formation can be separated into four stages, and the product of the diffusion reaction is only TiAl₃ under a particular range of holding time. There is a delay time t_D before TiAl₃ is generated, which is mainly affected by temperature. The joint strength depends on the metallurgical combination percentage and the interface structure in the diffusion zone, and it can reach or even exceed the strength of pure aluminum after TiAl₃ forms a layer. The position where shear fracture occurs depends on interface structure in the diffusion zone.     

A review on application of carbonaceous materials and carbon matrix composites for heat exchangers and heat sinks

Carbonaceous materials and carbon matrix composites (CAMCs) have potential to be used in heat exchangers and heat sinks for a number of thermal management applications related to HVAC&R systems, especially in high-temperature and corrosive environments. Recent developments in carbonaceous materials, such as new, natural graphite, carbon foam, carbon nanotubes, and CAMCs, open opportunities for new heat exchanger designs for compact and lightweight applications. The property data of various monolithic carbonaceous materials and CAMCs and their applications in liquid-to-liquid heat exchangers, liquid-to-gas heat exchangers, gas-to-gas heat exchangers and heat sinks are reviewed in this paper. While it is clear that these materials do hold promise for use in the construction of heat exchangers in different applications, additional research is still required in material properties, life-time behavior, structural design and manufacturing cost reduction.     

An approximate method for transient behavior of finned-tube cross-flow heat exchangers

A closed, approximate, analytical approach is obtained for transient behavior of finned-tube, liquid/gas cross flow heat exchangers for the step change in the inlet temperature of the hot fluid. In this two-solid heat capacity approach, the heat capacities of the hot and cold fluids and the heat capacity of the wall and the fins are lumped separately. The temperature variation of both fluids between inlet and outlet is assumed to be linear. It is also assumed that flow rates and inlet conditions remain fixed for both fluids, except for the step change imposed on the inlet temperature of the hot fluid. Energy equations for the hot and cold fluids, fins and walls for both approaches are derived and solved analytically. The variation of the exit temperatures of both fluids with time are obtained for a step change in the inlet temperature of the hot fluid. The dynamic behavior of the heat exchanger is characterized by time constant, delay time and gain. This approach is easier to apply and can easily be modified for other heat exchanger types. Results are compared with the experimental and numerical results given in the literature.     

Microstructure and mechanical properties of aluminium/steel dissimilar weldments: effect of heat input

Lap joints of aluminium alloy A6061-T6 and galvanised steel DP800 were produced by an advanced gas metal arc welding process with AlSi3Mn filler. Four heat inputs designated as A (110?J?mm^?1), B (120?J?mm^?1), C (130?J?mm^?1) and D (140?J?mm^?1) were used during the process and the joints made were subjected to analysis of the microstructures, mechanical properties, residual stresses and corrosion behaviour. Microstructural analysis shows the formation and variation of Fe–Al intermetallic (IM) layer. It is also found that the joints made using the heat input of 120?J?mm^?1 exhibited highest tensile strength and better corrosion resistance. Electron microscopy and X-ray diffraction analysis revealed the presence of different IM phases with other strengthening precipitates.     

Kinetics of transient liquid phase diffusion bonding process for joining aluminium metal matrix composite

Abstract

The mechanism and kinetics of the transient liquid phase diffusion bonding process in a 6061–15 wt-%SiCp composite at 570°C, 0·2 MPa, with 200 μm thick copper foil interlayer, has been investigated by microstructural characterisation of the bond region using optical microscopy, scanning electron microscopy and electron probe microanalysis. The kinetics of isothermal solidification, representing the displacement of the solid/liquid interface y (in micrometres) as a function of time t (in seconds), followed a power law relationship y?=?157t^0·07. According to this kinetic equation, the effective diffusivity of copper in the composite system was found to be ～10⁶ times higher than the lattice diffusivity, indicating the dominance of short circuit diffusion through the defect rich particle/matrix interface.     

铝制板翅式换热器事故的分析与防范

主要分析铝制板翅式换热器在空分设备中应用时被损坏的原因,介绍换热器在制造、维修和使用过程中应采取的必要技术措施。