Bead-on-plate laser welds were made on an industrially produced DP780 steel to determine the effect of normalized welding heat input on the microstructure and mechanical properties within the weld fusion zone (FZ) and heat affected zone (HAZ) with reference to the base material (BM) mechanical properties. Normalized welding heat input was calculated using an established model from the literature utilizing measurements from the weld cross-section microstructures along with known materials properties. Microhardness profiles and optical microscopy were employed to evaluate materials properties and microstructural changes across the various microstructural zones of each weld. The mechanical properties of the welds were evaluated globally through standard ASTM tensile specimens as well as through a series of specialized mechanical testing sample geometries which examined the properties of individual microstructural zones. These specialized sample geometries included non-standard uniaxial and plain strain tension where effective stress and effective strains were used to compare the mechanical properties across samples.
It was determined that there was a good correlation between ASTM standard samples and the specialized sample geometries employed in this study and that the UTS and YS values obtained in both cases were comparable. Sigmoidal decay behaviour was observed in the UTS and YS with increasing heat input for both the FZ and HAZ of all welds. It was found that welds with heat inputs greater than 60 J/mm2 had both a UTS and YS which were significantly depressed in the FZ and HAZ when compared to the base material values. Conversely, welds with heat inputs below 36.3 J/mm2 were found to have a UTS and YS in both the FZ and HAZ microstructural zones which were above the values determined for the BM. When global weld properties were tested, it was found that welds with a heat input greater than 60.0 J/mm2 failed within the HAZ while welds with heat inputs below 36.3 J/mm2 failed within the BM. It has been shown that there is a significant correlation between the heat inputs of laser welded DP steels and both the mechanical properties and microstructural features of the various microstructural zones as well as the location of failure during weld tensile testing. It has also been demonstrated that the mechanical properties of weld microstructural zones can be qualitatively evaluated using specialized tensile testing geometries. / Thesis / Master of Applied Science (MASc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/16761 |
| Date | January 2015 |
| Creators | Smith, Heather |
| Contributors | McDermid, Joesph R., Materials Science and Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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