Fenggui Lu

Investigation on thermal inertia of GMAW-P welding on Al alloy

Abstract Thermal inertia relates to the rate at which the plasma cools and heats, which plays important role in improving the arc stability at base time of pulsed gas metal arc welding (GMAW-P). A three-dimensional (3D) transient model is established to investigate the thermal inertia due to current rapid variation of GMAW-P. The model is validated by comparison of measured and predicted arc profile and voltage. Thermal inertia increases as the current decent rate improves at the same current difference. Two high thermal inertia zones exist in arc region, one locates right under the wire tip, and another is above molten pool surface. The arc region with high temperature is broadened due to thermal inertia compared to constant current, which is more obvious as the current decent rate increases. The results show that thermal inertia remarkably influences the arc temperature and stability while current decent rate is higher than 130 A ms−1.

The Microstructural Evolution of Vacuum Brazed 1Cr18Ni9Ti Using Various Filler Metals

The microstructures and weldability of a brazed joint of 1Cr18Ni9Ti austenitic stainless steel with BNi-2, BNi82CrSiBFe and BMn50NiCuCrCo filler metals in vacuum were investigated. It can be observed that an interdiffusion region existed between the filler metal and the base metal for the brazed joint of Ni-based filler metals. The width of the interdiffusion region was about 10 μm, and the microstructure of the brazed joint of BNi-2 filler metal was dense and free of obvious defects. In the case of the brazed joint of BMn50NiCuCrCo filler metal, there were pits, pores and crack defects in the brazing joint due to insufficient wettability of the filler metal. Crack defects can also be observed in the brazed joint of BNi82CrSiBFe filler metal. Compared with BMn50NiCuCrCo and BNi82CrSiBFe filler metals, BNi-2 filler metal is the best material for 1Cr18Ni9Ti austenitic stainless steel vacuum brazing because of its distinct weldability.

MICROSTRUCTURE AND WEAR RESISTANCE OF CHROMIUM CARBIDE COATING IN SITU SYNTHESIZED BY VEB

In this paper, (Cr, Fe)7C3(M7C3)/γ-Fe composite layer has been in situ fabricated on a low carbon steel surface by vacuum electron beam irradiation (VEB). Three kinds of powder mixtures were placed on a low carbon steel substrate, which was then irradiated with electron beam in vacuum condition. The microstructure and wear resistance of the composite layers has been studied by means of optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM), microhardness tester and tribological tester. The chemical composition of all specimens were carefully analyzed using energy-dispersive X-ray spectroscopy (EDAX) technique. Depending on three different powder mixtures, hypereutectic and hypoeutectic microstructures were obtained on surface composite layers. No pores and cracks were found on the coatings. The amount of carbides formed in the surface composite layer was mainly determined by carbon concentration. The microstructure close to the fusion line was largely primary austenite dendrite. The hardness and wear resistance of the surface composite layer has been greatly improved due to the extensive distribution of carbides.

Study on Laser-TIG Dissimilar Welds of Cemented Carbide and Invar Alloys Welded with CO2 Laser

Invar alloy has lower thermal expansion and is widely used. However, its intensity is too low to meet the requirement of industry. In order to improve its high-temperature strength, WC-Co cemented carbide was welded to invar alloy using high energy Laser-TIG (Tungsten Inert Gas) hybrid welding. WC migration phenomenon was discovered in welding process and its corresponding mechanism was discussed. Microstructure at different interface was discussed and the hardness profile was measured. The results showed: (1) Butt joint of WC-Co/ invar alloy manifested well penetration without any filler materials; (2) WC migration near WC-Co/ weld interface was owed to synthesis action including stirring effects of welds, intrinsic Stress, high pressure of molten materials and ionized shielding gas, interface reaction and surface tension.