Heat-affected zone

About: Heat-affected zone is a research topic. Over the lifetime, 18787 publications have been published within this topic receiving 231744 citations.

Mechanism of ripple formation during weld solidification

Abstract: The formation of ripples on the surfaces of GTA spot and seam weld surfaces in thin metal sheet was investigated by high speed motion pictures. The ripples are observed to form solely due to oscillation of the weld pool during solidification; no other mechanism for ripple formation was found. Weld pools were melted through thin metal sheets while a high speed camera observed the melting and solidification events on the bottom surface of the pool opposite the arc. When welding power is supplied from storage batteries, the weld pool surface shows virtually no oscillation while the arc is on. Shutting off of this arc suddenly releases the plasma pressure which was stretching the pool surfaces, setting the pool into oscillation like a struck drumskin. Solidification during this oscillation results in rippled surfaces. When a single phase full-wave rectified conventional dc welding power supply is used, the pool surface is observed to oscillate at 120 hertz. When this arc is shut off, the pool changes oscillation frequency in a fraction of a second from the frequency imposed by the pulsating plasma pressure to its own natural frequency. Pool oscillation periods after the arc is shut off are measured on the film strips. If the pool is considered to be a stretched membrane with surface tension providing the stretching force, a theoretical surface tension can be calculated from the oscillation period, pool mass and pool geometry. Agreement of the calculated surface tension values with published surface tension values for several metals demonstrates that the stretched membrane model adequately describes the pool oscillations. By counting the number of pool oscillations in the films and counting the number of ripples on solidified spot welds afterwards, a one-to-one correlation is established between ripples and pool oscillations. When a seam weld is made using storage batteries as the welding random disturbances of the pool surface are observed. Each disturbance leaves a few ripples before it damps out.

Spatially resolved x-ray diffraction mapping of phase transformations in the heat-affected zone of carbon-manganese steel arc welds

Abstract: Phase transformations that occur in the heat-affected zone (HAZ) of gas tungsten arc welds in AISI 1005 carbon-manganese steel were investigated using spatially resolved X-ray diffraction (SRXRD) at the Stanford Synchrotron Radiation Laboratory. In situ SRXRD experiments were performed to probe the phases present in the HAZ during welding of cylindrical steel bars. These real-time observations of the phases present in the HAZ were used to construct a phase transformation map that identifies five principal phase regions between the liquid weld pool and the unaffected base metal: (1) α-ferrite that is undergoing annealing, recrystallization, and/or grain growth at subcritical temperatures, (2) partially transformed α-ferrite co-existing with γ-austenite at intercritical temperatures, (3) single-phase γ-austenite at austenitizing temperatures, (4) δ-ferrite at temperatures near the liquidus temperature, and (5) back transformed α-ferrite co-existing with residual austenite at subcritical temperatures behind the weld. The SRXRD experimental results were combined with a heat flow model of the weld to investigate transformation kinetics under both positive and negative temperature gradients in the HAZ. Results show that the transformation from ferrite to austenite on heating requires 3 seconds and 158°C of superheat to attain completion under a heating rate of 102°C/s. The reverse transformation from austenite to ferrite on cooling was shown to require 3.3 seconds at a cooling rate of 45 °C/s to transform the majority of the austenite back to ferrite; however, some residual austenite was observed in the microstructure as far as 17 mm behind the weld.

Welding metallurgy of martensitic advanced high strength steels during resistance spot welding

Abstract: The paper addresses the process–microstructure–performance relationships in resistance spot welding of a martensitic advanced high strength steel. Significant softening was observed in the heat affected zone (HAZ) due to allotriomorphic ferrite formation in the inter-critical HAZ and tempering of martensite in sub-critical HAZ (SCHAZ), with the latter plays more important role in mechanical properties of the spot welds. The strain concentration associated with the HAZ softening promotes initiation of pullout failure from the soft SCHAZ. While, the peak load in the interfacial failure mode is governed by the fusion zone size, that of the pullout mode is significantly affected by the HAZ softening. To improve weldability of martensitic steels, the HAZ softening should be minimised via modifications in welding process or steel chemistry.