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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.


Cho H.-H.,Seoul National University | Han H.N.,Seoul National University | Hong S.-T.,University of Ulsan | Park J.-H.,University of Ulsan | And 3 more authors.
Materials Science and Engineering A | Year: 2011

High-quality, defect-free welds were successfully produced in 409 ferritic stainless steel by friction stir welding. A remarkably fine-grained microstructure was observed in the stir zone, and the fraction of low angle grain boundary in the stir zone significantly increased as compared to that in the base material. An increase in plunging depth led to an increase of the fraction of low angle grain boundary, a decrease in grain size, and an increase in hardness in the stir zone. © 2010 Elsevier B.V.


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.


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.


Cho H.-H.,Seoul National University | Hong S.-T.,University of Ulsan | Roh J.-H.,University of Ulsan | Choi H.-S.,Seoul National University | And 3 more authors.
Acta Materialia | Year: 2013

A three-dimensional thermomechanical simulation of friction stir welding (FSW) processes is carried out for ferritic stainless steel by utilizing an Eulerian finite volume method under the steady state condition, and the simulation result is compared directly with both the measured temperature histories during FSW and the microstructural changes after FSW. Based on a viscoplastic self-consistent approach for polycrystal, the texture development in the FSWed material is determined from the velocity gradients along the streamlines in the material flow field. The simulation results show that the heat is generated mainly near the interface between the tool and the workpiece, and that the viscosity changes drastically in the vicinity of the boundary between the stir zone and the thermomechanically affected zone. From the predicted streamlines, it can be indicated that the strong material flow mainly develops on the retreating side of the tool. Also, the simulation results show that the shear deformation texture is significantly developed in the FSWed region. The measured temperatures and microstructural characteristics agree fairly well with the predicted data.


Miles M.P.,Brigham Young University | Feng Z.,Oak Ridge National Laboratory | Kohkonen K.,Brigham Young University | Weickum B.,Brigham Young University | And 2 more authors.
Science and Technology of Welding and Joining | Year: 2010

Spot joining of dissimilar metal alloys is difficult because of differences in melting temperature as well as differences in composition. If a fusion welding process like resistance spot welding is employed, then alloys mix in the fusion zone and can create brittle intermetallic compounds during solidification. When self-piercing riveting (SPR) is used, the alloys to be joined must be ductile enough to be formed around the rivet. In this paper, a new approach to joining dissimilar metal alloys has been studied, where a consumable bit has been used to create a spot joint between dissimilar alloys. The resulting joint has both metallurgical and mechanical bonding characteristics and can be made between very soft and very hard alloys, like dual phase (DP) 590 and DP 980 and light metals like AA 5754. Lap shear strengths of joints made by friction bit joining (FBJ) are similar to or better than comparable joints made by self-piercing riveting. © 2010 Institute of Materials, Minerals and Mining.


Steel R.J.,MegaStir Technologies | Peterson J.,MegaStir Technologies | Sanderson S.,MegaStir Technologies | Higgins P.,MegaStir Technologies | Packer S.M.,MegaStir Technologies
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2012

Friction stir welding (FSW) is a solid state joining process providing many advantages over typical arc welding processes. FSW has historically been applied to aluminum and soft low melting temperature materials. However, with the development of tool materials having high temperature hardness and stability, alloys such as steels, stainless steels, and nickel base alloys can now be jointed using FSW. Thicknesses have been limited by tool materials, and the sizes that could be produce. This paper shows the results of tool material development of large PcBN blanks used for FSW of 20mm thickness in steel materials. Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE).


Feng Z.,Oak Ridge National Laboratory | Steel R.,MegaStir Technologies | Packer S.,MegaStir Technologies | David S.A.,Oak Ridge National Laboratory
American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP | Year: 2010

Friction stir welding (FSW), a novel solid-state joining process, was applied to girth weld API 5L Grade 65 steel pipes with an outer diameter of 12.75'' (324 mm) and a wall thickness of 0.25'' (6.35 mm). Fully consolidated single pass butt welds were obtained using a specially designed mechanized portable FSW system suitable for on-site pipe construction welding. The friction stir girth weld shows a slightly overmatched strength and superior impact toughness in comparison with the base metal, a very desirable combination of properties for pipeline weld that can be attributed to the wrought microstructure with refined grains in the stir zone (SZ), the thermal-mechanically affected zone (TMAZ), and the heat-affected zone (HAZ). Copyright © 2009 by ASME.


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).


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.

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