Partial Penetration Fiber Laser Welding on Austenitic Stainless Steel

Author: Reiter, Matt J.
Degree: Master of Science, Ohio State University, Welding Engineering, 2009.
Committee / Advisors: Dave Farson PhD (Advisor) Sudarsanam Suresh Babu PhD (Committee Member)

Abstract

Penetration depth fluctuations of single mode fiber laser partial penetration welds with long focal length optics of 150mm were studied. It was found that welds having high aspect ratios and penetrations greater than 0.381mm were subject to the spiking discontinuity. Deeper welds contained spikes of greater magnitude. It was found that several methods could improve partial penetration welds containing severe spiking. Sinusoidal laser power modulations of 10% and 25% reduced the rate of occurrence and the magnitude of the spiking at frequencies between 900Hz to 3kHz; however, it was found that modulation was not robust and lacked repeatability. High travel speed welding at 400-450mm s-1 significantly reduced the magnitude of spiking compared to similar penetrations made at slower travel speeds. Spiking was not completely suppressible only increasing travel speeds but was found to be much more predictable and repeatable than modulation techniques. Laser beam circular oscillation techniques repeatably and robustly eliminated spiking from the penetration profile of single mode fiber laser welds at oscillation diameters of 0.1mm, 0.2mm and 0.35mm with corresponding frequencies of 2kHz, 1.5kHz, and 1kHz respectively. Decreasing the f# of the lens by reducing the focal length to 80mm also was found to decrease the magnitude of spiking and increased the threshold penetration depth for the formation of spiking as compared to 150mm focal length optics. A series of experiments was conducted to compare the fit up tolerability to gap, mismatch, and edge break between a traditional long focal length optic setup and a shorter f-θ galvanometer setup. It was found that even with the added bead width provided by the galvanometer, gap still remains a very difficult fit up variable to overcome. Edge Break fit up condition was improved from 0.076mm to 0.254mm using oscillation diameters of 0.2mm and 0.35mm. Mismatch fit up condition was improved from 0.076mm to 0.635mm using oscillation diameters of 0.1mm, 0.2mm, and 0.35mm.

Finite element analysis was performed on a pin/plate joint to evaluate the displacement caused by thermal distortions only during welding. It was found that a welding schedule of 10ms weld time creating a 0.46mm spot weld was not capable of creating permanent plastic deformation of the pin relative to the plate. Fit up tolerance between the pin and plate is the important variable controlling displacement of the pin at the end of welding.

Long term power stability was studied on a multimode fiber laser power supply. It was found that over an 11 week period the output power of the laser, measured at the work, varied 2.0%, leading to a 5% variation in penetration relative to the average penetration of the sample set. Here's the link to the .pdf:

http://search.ohiolink.edu/etd/send-pdf.cgi/Reiter%20Matt%20J.pdf?osu1243339754

Enjoy the reading... I know I will!!! :) :) ;)

Respectfully,
Henry