C. M. Cheng

Preparation and characterization of carbon nanofluid by a plasma arc nanoparticles synthesis system

Heat dissipation from electrical appliances is a significant issue with contemporary electrical devices. One factor in the improvement of heat dissipation is the heat transfer performance of the working fluid. In this study, we used plasma arc technology to produce a nanofluid of carbon nanoparticles dispersed in distilled water. In a one-step synthesis, carbon was simultaneously heated and vaporized in the chamber, the carbon vapor and particles were then carried to a collector, where cooling furnished the desired carbon/water nanofluid. The particle size and shape were determined using the light-scattering size analyzer, SEM, and TEM. Crystal morphology was examined by XRD. Finally, the characterization include thermal conductivity, viscosity, density and electric conductivity were evaluated by suitable instruments under different temperatures. The thermal conductivity of carbon/water nanofluid increased by about 25% at 50°C compared to distilled water. The experimental results demonstrated excellent thermal conductivity and feasibility for manufacturing of carbon/water nanofluids.

Hot cracking of welds on heat treatable aluminium alloys

Abstract The Spot Varestraint test was used to evaluate the hot cracking susceptibility of several aluminium alloys namely 6061-T6, 6061-T6 (H), 7075-T6, 7075-T6 (H). The effects of augment strain, the number of thermal cycles and cold working (rolling) on the cracking susceptibility were investigated, and the total crack length was used to evaluate the hot cracking susceptibility. The results indicate that the number of thermal cycles is irrelevant to the hot cracking susceptibility in the weld fusion zone, but does affect this susceptibility in the heat affected zone (HAZ). More thermal cycles correspond to larger hot cracks in the HAZ, especially in the weld metal HAZ. The hot cracking susceptibility of materials increased with augment strain in both the fusion zone and the HAZ. Cold working of the materials can reduce their hot cracking susceptibility. The hot cracking susceptibility of 7075-T6 aluminium alloys is higher than that of 6061-T6. There was significant Cu segregation in the HAZ of 7075-T6 aluminium alloy, resulting in a higher susceptibility to hot cracking in this zone.

Effect of aging treatment on the mechanical properties of C-250 maraging steel by flow forming

The technique of forward flow forming has been used to produce a long, thin walled tube made of C-250 maraging steel. The forward flow forming can save raw material, increase strength, and reduce the production process time. Because the work hardening effect on solution-treated C-250 using flow forming is minimal, the flow-formed tube requires an additional heat treatment to obtain higher hardness and strength. With the direct aging treatment, low elongation values are obtained, making this treatment unsuitable for the engineering design. It was found that the 540 °C/6 h/AC over-aging treatment results in better strength and elongation values. The strengthening phase of the flow-formed C-250 maraging steel was found to be the intermetallic compound of Fe3Mo.

Evolution of microstructures and mechanical properties of AZ31B magnesium alloy weldment with active oxide fluxes and GTAW process

In this study, the effect of active oxide fluxes with gas tungsten arc welding on the microstructure and mechanical properties of AZ31B magnesium alloy weldment was investigated. The gas tungsten arc welding process through a flux spray layer was applied to an AZ31B magnesium alloy sheet to produce a bead-on-plate specimen. Oxide (TiO2, SiO2, Fe2O3, Al2O3, and ZrO2) powders were used as the activating fluxes. The macrographs and micrographs of the weld beads were examined using an optical microscope and a scanning electron microscope. The specimens with SiO2 and Fe2O3 fluxes had high depth-to-width ratio welds, followed by those with TiO2 and ZrO2 fluxes and while that with Al2O3 flux had the low ratio weld. The use of 70 A welding current for the specimens with different fluxes produced complete penetration, whereas the specimen without any flux required a 90 A welding current to produce complete penetration. The weld bead microstructure was affected by the activating fluxes, which created different thermal effects that changed the convection direction and promoted the formation of various precipitates in the fusion zone during solidification. Three types of precipitates were found in the fusion zones, that is, a long layer-shaped TiAlMg precipitate with TiO2 flux, a spherical AlMgZn precipitate with Al2O3 flux, and an oval-shaped MgAlMn precipitate with all types of fluxes. The mechanical properties of AZ31B magnesium alloy were measured by tensile testing in the rolling direction. Fractures occurred in the fusion zone near the heat-affected zone interface of specimens welded with TiO2 flux, revealing a brittle fracture with trans-granular cleavage facets and a large number of small, bright dimples at the center. Such brittle fractures also occurred in the fusion zone of specimens welded with Al2O3, ZrO2, SiO2, and Fe2O3 fluxes. Similarly, the specimens welded with Al2O3 exhibited a brittle fracture with trans-granular facets, whereas the other specimens revealed a brittle fracture with inter-granular cleavage facets.

Hot Cracking in AZ31 and AZ61 Magnesium Alloy

This paper examined the impact of the number of thermal cycles and augmented strain on hot cracking in AZ31 and AZ61 magnesium alloy. Statistical analyses were performed. Following observation using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS) was used for component analysis. Results showed that AI content in magnesium alloy has an effect on hot cracking susceptibility. In addition, the nonequilibrium solidification process produced segregation in AI content, causing higher liquid Mg-alloy rich AI content at grain boundaries, and resulting into liquefied grain boundaries of partially melted zone (PMZ). In summary, under multiple thermal cycles AZ61 produced serious liquation cracking. AZ61 has higher (6 wt%) AI content and produced much liquefied Mg17Al12 at grain boundaries under multiple thermal cycles. The liquefied Mg17Al12 were pulled apart and hot cracks formed at weld metal HAZ due to the augmented strain. Since AZ31 had half the AI content of AZ61, its hot-cracking susceptibility was lower than AZ61. In addition, AZ61 showed longer total crack length (TCL) in one thermal cycle compared to that in three thermal cycles. This phenomenon was possibly due to high-temperature gasification of AI during the welding process, which resulted in lower overall AI content. Consequently, shorter hot cracks exhibited in three thermal cycles. It was found the AI content of AZ31 and AZ61 can be used to assess the hot-cracking susceptibility.

Effects of Flux Precoating and Process Parameter on Welding Performance of Inconel 718 Alloy TIG Welds

The purpose of this study is to investigate the effect of activating flux on the depth-to-width ratio (DWR) and hot cracking susceptibility of Inconel 718 alloy tungsten inert gas (TIG) welds. The Taguchi method is employed to investigate the welding parameters that affect the DWR of weld bead and to achieve optimal conditions in the TIG welds that are coated with activating flux in TIG (A-TIG) process. There are eight single-component fluxes used in the initial experiment to evaluate the penetration capability of A-TIG welds. The experimental results show that the Inconel 718 alloy welds precoated with 50% SiO2 and 50% MoO3 flux were provided with better welding performance such as DWR and hot cracking susceptibility. The experimental procedure of TIG welding process using mixed-component flux and optimal conditions not only produces a significant increase in DWR of weld bead, but also decreases the hot cracking susceptibility of Inconel 718 alloy welds.

The Influence of Aluminum Content of AZ61 and AZ80 Magnesium Alloys on Hot Cracking

This study aims to investigate how aluminum content in magnesium alloys AZ61 and AZ80 impacts the hot cracking susceptibility of magnesium alloys. Differences in aluminum content are known to influence the total crack length of hot cracking. Magnesium alloy AZ61s total crack length was the longest in one thermal cycle, while AZ80s total crack length increased as the number of thermal cycles increased. The most significant difference between AZ61 and AZ80 was the hot crack at the heat-affected zone (HAZ). As the number of heat inputs increased, the grain would coarsen in the HAZ and precipitation started, which resulted in the accumulation of hot cracks at weld metal HAZ (W. M. HAZ). During the solidification of AZ80, which has higher aluminum content, the segregation of aluminum at the grain boundary caused Mg17Al12 to liquefy, increasing the length of hot cracks. Augmented strain caused miniature cracks between Mg17Al12 and grains. Therefore, aluminum content and augmented strain were found causes of hot cracking susceptibility in magnesium alloys.

Effect of stress relief on microstructure and mechanical properties of flow formed maraging steel weldment by electron beam welding

AbstractThe technique of forward flow forming has been used to produce a long and thin wall tube of C-250 maraging steel. The work hardening effect of solution treated C-250 by flow forming is small. Therefore aging treatment after flow forming is required to increase hardness and strength. Final formed C-250 tubes are usually manufactured by combining aging treatment and electron beam welding (EBW). The hardness and tensile strength of aged C-250 maraging steel weldment decreased to the corresponding values obtained in the solution annealed state. Thus, the formed tube by EBW requires stress relieving treatment (and similarly for theaged condition) to increase hardness and strength and thus meet the required specification. The strength requirement was successfully achieved but the elongation of the aged, EBW, and stress relieved specimen was 88.5% lower than that of the unwelded specimen.

Characterization of Hot Cracking Due to Welding of High-Strength Aluminum Alloys

The “Spot-Varestraint Test” was applied to assess the sensitivity of three aluminum alloys–A2024-T351, A2219-T87, and A7050-T6–to hot cracking from welding. The results indicate that the number of cracks increases with increasing augmented strain. This phenomenon occurs in both the fusion and the heat-affected zones. The number of thermal cycles also has a significant influence on the heat-affected zone; the number of hot cracks increases, especially in the heat-affected zone of the metal weld, with increasing number of thermal cycles. The compositions of these three alloys show that A2024 and A7050 have similar tendencies to be subject to hot cracking, greater than A2219. With increasing number of thermal cycles, the hot cracks show the same tendency, A2024 > A7050 > 2219.

Relaxing welding residual stresses by diamond like carbon coating on duplex titanium alloys

Abstract The distribution of welding residual stresses in titanium alloy plates composed of Ti–6Al–4V (Ti-64) and Ti–4˙5Al–3V–2Mo–2Fe (SP700) alloys was measured and calculated under the same heat input. Ti-64 alloy exhibited a higher residual tensile stress than the SP700 alloy due to the low thermal conductivity and thermal diffusivity of the former alloy, in addition to its high thermal expansion coefficient. Diamond like carbon (DLC) thin film deposited on the as welded titanium alloy plates reduced the magnitude of the welding residual stress caused by the cathodic arc physical vapour deposition (PVD) process. The distribution of the measured welding residual stresses agreed with the finite element method (FEM) simulated results very well.