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Friction Stir Welding (FSW) is being investigated as a method to fabricate a partial penetration closure weld of the steel vessel of a copper-coated used fuel container. The hemi-head is made of A516 Grade 70 steel and the cylinder body is made of A106 Grade C steel. In this initial feasibility study, the objective is to use FSW to demonstrate the merits of FSW using flat steel plate in concert with a closure weld joint designed specifically for a cylindrical container. To complete this objective, there are two initial feasibility demonstrations. First, demonstrate the capability of FSW to create defect free welds in a corner joint design with specific dimensions (10 mm deep weld nugget). Subsequently, verify the weld quality by ultrasonic inspection and metallography. Further, characterize weld zone properties by establishing mechanical properties and hardness at room temperature, and impact toughness at-5°C. Second, demonstrate the ability to use FSW to repair defects that might occur in the initial friction stir weld. Weld repair was accomplished by intentionally creating tunnel defects by FSW, performing metallography and ultrasonic inspection to characterize the morphology of the defects, performing a second friction stir weld repair over the defects, and verifying the subsequent weld integrity by repeating the metallography and ultrasonic inspections. Results on these initial two phases of this program are presented herein.

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.

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.

This study evaluates the degree of sensitization (DOS) of 304 stainless steel joined by friction stir welding (FSW). Single-loop electrochemical potentiokinetic reactivation tests were performed using a 0.5 mol/L H2SO4 + 0.01 mol/L KSCN solution. Sensitization was promoted by exposition of the stainless steel at temperatures between 400°C and 850°C. The microstructure was characterized using optical microscopy to identify the weld zone and the base metal. The samples treated at 550°C showed the most severe intergranular corrosion. The DOS was lower in the weld zone than in the base metal after heat treatments. This reduction in the DOS for the weld zone indicates that FSW is a beneficial process in joining stainless steel.

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.

The authors study a new solid-state spot joining process, friction bit joining (FBJ), which relies on the use of a consumable joining bit. It has been reported that FBJ is feasible for the joining of steel/steel and aluminum/steel, but the metallurgical characteristics of the joint for enhancement of the properties and reliability remain unclear. Therefore, this study produced friction bit joints in DP980 steel and then examined the microstructures in the joint precisely. In this article, the microstructure distribution associated with hardness in the friction-bit-joined DP980 steel and the microstructural evolution during FBJ are reported.

The use of high strength steels (HSS) and ultra high strength steels (UHSS) is increasing in automotive applications to reduce vehicle weight and to improve structural performance. This study focused on the properties and microstructures of friction stir welded steel sheets, as a function of welding conditions. Tooling feeds and speeds had a significant impact on mechanical properties and microstructures of the welds. For example, at constant tool speeds higher feedrates produced softer welds with large amounts of primary ferrite, compared with lower feedrates which produced harder welds and a greater fraction of martensite. These results illustrate the potential of adjusting weld properties and microstructures by using different friction stir welding process conditions for HSS and UHSS alloys.

A new spot joining technology relying on a consumable joining bit has been developed and evaluated on dual phase (DP) 980 steel and a dissimilar combination of aluminium alloy 5754-O and DP 980. Th...

Friction stir welding (FSW) has been widely used to metals with moderate melting temperatures, primarily Al alloys. Recently, tool materials that withstand high stresses and temperatures necessary for FSW of materials with high melting temperatures have been developed. In the present study, polycrystalline cubic boron nitride (PCBN) tool was used for partially penetrated FSW of Inconel Alloy 600, and a defect-free weld was successfully produced. Microstructural characteristics, mechanical and corrosion properties in the weld were examined. The weld had better mechanical properties than the base material due to formation of fine grain structure in the stir zone, but exhibited slightly the lower corrosion resistance in a part of the stir zone and heat-affected zone (HAZ).

Commercial Waspaloy plates with two different initial microstructures (annealed and aged) were joined by Friction Stir Welding (FSW). This study presents the evolution of the grain structure, and precipitation, dissolution and reprecipitation of the γ' phase during FSW using Optical Microscopy, Scanning Electron Microscopy and Transmission Electron Microscopy. The relationship between microstructure and microhardness is also discussed.

The microstructure and mechanical properties of friction stir (FS) welded SAF 2507 super duplex stainless steel were examined. High-quality, full-penetration welds were successfully produced in the super duplex stainless steel by friction stir welding (FSW) using polycrystalline cubic boron nitride (PCBN) tool. The base material had a microstructure consisting of the ferrite matrix with austenite islands, but FSW refined grains of the ferrite and austenite phases in the stir zone through dynamic recrystallisation. Ferrite content was held between 50 and 60% throughout the weld. The smaller grain sizes of the ferrite and austenite phases caused increase in hardness and strength within the stir zone. Welded transverse tensile specimen failed near the border between the stir zone and TMAZ at the retreating side as the weld had roughly the same strengths as the base material.

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. Micro structural 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. Micro structural 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 micro structure s 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.

In prior FSW studies, consistent full penetration in pipeline steel has proved to be a difficult goal when using a portable FSW system capable of operation in the field [1]. In a previous study, metallography, mechanical testing (tensile and Charpy impact), and workmanship testing (root bend) demonstrated that full penetration can be achieved through much of the weld length, and when full penetration is achieved, mechanical properties are excellent. However, at times there remained sections of the weld that did not exhibit full penetration. As an example, metallography and root bend tests of the weld root have shown full penetration at many locations around the pipe circumference while from the same weld, locations are identified where remnant faying surfaces remain and full penetration was not achieved. Further, some welds exhibit full penetration accompanied by a continuous oxide path that remains at the weld root. Conversely, if the FSW tool penetrates into the support anvil, anvil material is drawn into the weld nugget. Thus, different approaches need to be developed to assure consistent full penetration without anvil contact.

Friction stir spot welding has been shown to be a viable method of joining ultra high strength steel (UHSS), both in terms of joint strength and process cycle time. However, the cost of tooling must be reasonable in order for this method to be adopted as an industrial process. Several tooling materials have been evaluated in prior studies, including silicon nitride and polycrystalline cubic boron nitride (PCBN). Recently a new tool alloy has been developed, where a blend of PCBN and tungsten rhenium (W-Re) was used in order to improve the toughness of the tool. Wear testing results are presented for two of these alloys: one with a composition of 60% PCBN and 40% W-Re (designated as Q60), and one with 70% PCBN and 30% W-Re (designated at Q70). The sheet material used for all wear testing was DP 980. Tool profiles were measured periodically during the testing process in order to show the progression of wear as a function of the number of spots produced. Lap shear testing was done each time a tool profile was taken in order to show the relationship between tool wear and joint strength. For the welding parameters chosen for this study the Q70more » tool provided the best combination of wear resistance and joint strength.« less

Friction stir welding (FSW) is a relatively new joining technology invented by The Welding Institute in 1991. The general use has been for welding soft, low melting materials such as aluminum. Advances in tool technology has allowed for the implementation of FSW into steel, stainless steel and nickel base alloys. The benefits of FSW include low heat input, elimination of solidification defects, the ability to join difficult and historically unweldable materials all while improving the mechanical properties over conventional welding processes. The oil and gas industry utilizes many specialized materials for performance and corrosion resistance. This paper will present and report progress on the different material properties and applications of FSW within the oil and gas industry.

An exploratory study was conducted to investigate the feasibility of friction stir spot welding advanced high-strength steel sheet metals. The fixed pin approach was used to weld 600MPa dual phase steel and 1310MPa martensitic steel. A single tool, made of polycrystalline cubic boron nitride, survived over one hundred welding trials without noticeable degradation and wear. Solid-state metallurgical bonding was produced with welding time in the range of 2 to 3 seconds, although the bonding ligament width was relatively small. The microstructures and hardness variations in the weld regions are discussed. The results from tensile-shear and cross-tensile tests are also presented.

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