Date

September 1986

Document Type

Thesis

Degree Name

M.S.

Department

Dept. of Materials Science and Engineering

Institution

Oregon Graduate Center

Abstract

Capacitor discharge welding (CDW) is unique among joining processes as it is the only one capable of producing joints up to 12 mm diameter at cooling rates exceeding 10[superscript 6] K/s under a unidirectional temperature gradient. The potential for microstructural modification by CDW was therefore applied to joining of stainless steels and FeAl ordered alloy to study solidification phenomena and to optimize CDW in producing high cooling rates. The materials used in this investigation included AISI grade 316, 304 and 308 having Cr Eq./Ni Eq. ratios of 1.49, 1.59, and 1.76 respectively, and the high temperature ordered alloy Fe- 40at% Al. The alloys were welded at different solidification rates by CDW and by conventional gas tungsten arc welding for reference. Crystal and dendrite growth morphologies were studied by optical and transmission electron microscopy. Under rapid cooling conditions, 316 and 304 alloys solidified in a cellular/cellular dendritic mode as fcc austenite from the melt, while 308 solidified in a similar mode but as partly austenite and bcc ferrite. Subsequent transformation of the 308 alloy took place completely to austenite by a massive transformation. The bcc alloy FeAl also solidified in a single phase cellular structure. Cooling rates during CDW varied between 10[superscript 6] and 5 X 10[superscript 7] K/s in stainless steel. Estimated growth rates were between 9.3 and 33.4 cm/s in stainless steel and between 4.8 and 11 cm/s in FeA1. Solute Peclet numbers were between 16 and 46 for stainless steels and between 1 and 2 for FeAl. Primary and secondary dendritic spacings (λ1 and λ2 respectively) showed a power relation in stainless steels : λ2 propto λ1[superscript 0.39] λ1 and λ2 converged upon extrapolation at a dendrite arm spacing of 0.27 µm. The variation of solute Peclet number with growth rate also indicated solidification close to absolute stability and convergence of primary and secondary dendritic spacings was attributed to proximity of the dendritic to cellular transition in stainless steels. Dendrite morphology depended upon crystallographic orientation relative to the unidirectional gradient vector. Dendrite arm spacing and tendency for side arm growth increased with angle between primary dendrite/cell orientation and gradient vector. Equiaxed dendrites with an unusual morphology observed in FeAl welds, were nucleated by oxide inclusions present from prior powder consolidation processing. Key variables in producing rapid cooling rates were identified as capacitance, voltage, stud diameter and ignition tip length. As an arc process, CDW was found to be a unique way to study rapid solidification phenomena.

Identifier

doi:10.6083/M4X63JV7

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