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Saunders N.,Brigham Young University | Miles M.,Brigham Young University | Hartman T.,Brigham Young University | Hovanski Y.,Pacific Northwest National Laboratory | And 2 more authors.
International Journal of Precision Engineering and Manufacturing | Year: 2014

High speed friction stir spot welding was applied to 1.2 mm thick DP 980 steel sheets under different welding conditions, using PCBN tools. The range of vertical feed rates used during welding was 2.5∼102 mm per minute, while the range of spindle speeds was 2500∼6000 rpm. Extended testing was carried out for five different sets of welding conditions, until tool failure. These welding conditions resulted in vertical welding loads of 3.6∼8.2 kN and lap shear tension failure loads of 8.9∼11.1 kN. PCBN tools were shown, in the best case, to provide lap shear tension failure loads at or above 9 kN for 900 spot welds, after which tool failure caused a rapid drop in joint strength. Joint strength was shown to be strongly correlated to bond area, which was measured from weld cross sections. Failure modes of the tested joints were a function of bond area and softening that occurred in the heat-affected zone. © 2014 Korean Society for Precision Engineering and Springer-Verlag Berlin Heidelberg. Source


Hartman T.,Brigham Young University | Miles M.P.,Brigham Young University | Hong S.-T.,University of Ulsan | Steel R.,Megastir Technologies | Kelly S.,Element Six
Wear | Year: 2015

Friction stir spot welding of advanced high strength steel, like dual phase (DP) 980, is a very demanding application in terms of tool wear. The best candidate material currently being tested is polycrystalline cubic boron nitride (PCBN). The purpose of the current work is to study the effect of PCBN material grade, as specified primarily by CBN grain size, on weld quality and tool life, for the spot welding of DP 980 steel. With a nominal composition of 90% CBN, three tool grades were tested. Grade A had CBN grain sizes of 3-6. μm, grade B had grain sizes of 12-15. μm, and grade C had a multimodal distribution, with grain sizes ranging from 4 to 40. μm. The best performance in terms of joint strength and tool life was achieved with grade A. The effect of fine CBN grain size was less adhesion of DP 980 on the tool surface over time, less abrasive wear, and higher lap shear failure loads of the welds that were produced, compared to the other grades. A-type tools were the most consistent in both the number of welds per tool, and the number of welds that reached acceptable lap shear failure loads. B-type tools, performed slightly better than C-type tools in terms of wear, but neither of them was able to achieve consistent, acceptable lap shear failure load values after the first 200 welds. In fact only one out of five C-type tools was able to produce acceptable lap shear failure loads after the first 100 welds. Based on the experimental evidence, tool wear was likely dominated by grain pullout. This is consistent with the correlation showing that larger CBN grain size was related to shorter tool life. © 2015 Elsevier B.V. Source


Miles M.,Brigham Young University | Hartman T.,Brigham Young University | Steel R.,Megastir Technologies | Kelly S.,Element Six | Hong S.-T.,University of Ulsan
CAMX 2014 - Composites and Advanced Materials Expo: Combined Strength. Unsurpassed Innovation. | Year: 2014

Polycrystalline cubic boron nitride (PCBN) tool grade had a significant impact on weld quality, and tool failure in friction stir spot welded dual phase (DP) 980 steel sheets. With a nominal composition of 90% CBN, three tool grades were tested. Grade A had CBN grain sizes of 3-6 microns, grade B had grain sizes of 12-15 microns, and grade C had a multimodal distribution, with grain sizes ranging from 4-40 microns. The best performance in terms of joint strength and tool life was achieved with grade A. The effect of fine CBN grain size was less adhesion of DP 980 on the tool surface over time, less abrasive wear, and higher lap shear failure loads of the welds that were produced, compared to the other grades. Source


Steel R.J.,Megastir Technologies | Peterson J.,Megastir Technologies | Packer S.M.,Megastir Technologies
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2011

Friction stir welding (FSW) is a solid state joining process providing many advantages over typical arc welding processes. Although initially developed for low melting point materials such as aluminum alloys, recent developments in tool materials have enabled FSW to be applied to higher melting temperature materials such as alloys of steel and stainless steel. Though there are similarities between FSW in both aluminum and steel, there are significant differences necessitated by the higher temperature required for steel and stainless steel. This paper presents tool and processing differences required for full penetration welding in steel and stainless steel friction stir welds. Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE). Source


Young G.W.,NAVFAC ESC | Stewart W.C.,U.S. Navy | Steel R.,Megastir Technologies | Babb J.,Megastir Technologies | And 2 more authors.
TMS Annual Meeting | Year: 2013

Microstructures and mechanical property changes associated with Friction Stir Processing (FSP) of HY-80 steel both dry and under seawater were examined. FSP on HY-80 plates employed a PCBN / tungsten rhenium tool operating at 400 rpm and 2 ipm. Microstructural characterization of the as-received HY-80 plate emphasized the differences in the distribution of constituent phases and inclusions in the rolling, transverse and normal planes using optical and scanning electron microscopy. Microstructural details of transverse sections as well as the plan section at the tool extraction sites of processed plates were also evaluated. Mechanical properties of these samples were evaluated by tensile tests, microhardness tests and Charpy V-notch impact tests. Residual hydrogen content was also evaluated. Stir zones exhibited untempered bainitic / martensitic microstructures with minimal hydrogen pick up but distinct property gradients from stir zone to base metal. The influence of a post-FSP tempering treatment also is summarized. Source

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