Masters Theses

Kevin T. Zysk

Date of Award

5-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

T. Dwayne McCay

Committee Members

Mary Helen McCay, Jim-Mou Jong

Abstract

Welding of Inconel 718 by conventional methods produces microfissures within the heat affected zone (HAZ) which weakens the weld. This study investigated the effects of pulsed CO2 laser welding upon the HAZ to determine the suitability of this process for commercialization.

Samples of Inconel 718 in the as-received (AR) and solution-treated (ST) condition were pulse CO2 laser welded (bead-on-plate) with a fixed pulse on-time and a varying off-time. Laser cavity peak power, shield gas flow rate, beam focus, and welding speed were held constant. Welded ST samples were subjected to furnace aging to simulate conditions used on most of the Space Shuttle Main Engine welds. A limited number of continuous wave (CW) welds were done on AR material for comparison.

Microhardness in the weld pool and base metal, top weld bead width, bottom weld bead width, cross-sectional area, percentage pore area, pore diameter, and pore spacing were measured. The number of microfissures per weld (cross-section) was quantified and the total microfissure length per weld (cross-section) was measured.

The HAZ was found not to extend beyond 0.2 mm from the weld pool. Porosity did not exist in the CW and short pulse off-time samples. Microfissures were not found in the pulse welded ST samples. Post-weld heat treatment induced microfissures in 1/3 of the pulse welded samples. All CW welded samples had microfissures.

The effect of subsequent pulses during welding shifts the tensile stress characteristics of the solidifying weld pool such that any induced residual stress is below the yield stress of the Inconel 718, thus eliminating microfissuring. Pulsed CO2 laser welding has been shown to be an effective method of joining Inconel 718.

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