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All sources NASA NTRS DOE OSTI

NASA NTRS2000

Fatigue Crack Growth Rate Test Results for Al-Li 2195 Parent Metal, Variable Polarity Plasma Arc Welds and Friction Stir Welds

The fatigue crack growth rate of aluminum-lithium (Al-Li) alloy 2195 plate and weldments was determined at 200-F, ambient temperature and -320-F. The effects of stress ratio (R), welding process, orientation and thickness were studied. Results are compared with plate data from the Space Shuttle Super Lightweight Tank (SLWT) allowables program. Data from the current series of tests, both plate and weldment, falls within the range of data generated during the SLWT allowables program.

Hafley, Robert A.↗

DOE OSTI2016

Hydrogen accelerated fatigue crack growth of friction stir welded X52 steel pipe

Friction stir welded steel pipelines were tested in high pressure hydrogen gas to examine the effects of hydrogen accelerated fatigue crack growth. Fatigue crack growth rate (da/dN) vs. stress-intensity factor range (ΔK) relationships were measured for an X52 friction stir welded pipe tested in 21 MPa hydrogen gas at a frequency of 1 Hz and R = 0.5. Tests were performed on three regions: base metal (BM), center of friction stir weld (FSW), and 15 mm off-center of the weld. For all three material regions, tests in hydrogen exhibited accelerated fatigue crack growth rates that exceeded an order of magnitude compared to companion tests in air. Among tests in hydrogen, fatigue crack growth rates were modestly higher in the FSW than the BM and 15 mm off-center tests. Select regions of the fracture surfaces associated with specified ΔK levels were examined which revealed intergranular fracture in the BM and 15 mm off-center specimens but an absence of intergranular features in the FSW specimens. In conclusion, the X52 friction stir weld and base metal tested in hydrogen exhibited fatigue crack growth rate relationships that are comparable to those for conventional arc welded steel pipeline of similar strength found in the literature.

08 HYDROGEN↗

DOE OSTI2014

Comparing Laser Welding Technologies with Friction Stir Welding for Production of Aluminum Tailor-Welded Blanks

A comparison of welding techniques was performed to determine the most effective method for producing aluminum tailor-welded blanks for high volume automotive applications. Aluminum sheet was joined with an emphasis on post weld formability, surface quality and weld speed. Comparative results from several laser based welding techniques along with friction stir welding are presented. The results of this study demonstrate a quantitative comparison of weld methodologies in preparing tailor-welded aluminum stampings for high volume production in the automotive industry. Evaluation of nearly a dozen welding variations ultimately led to down selecting a single process based on post-weld quality and performance.

aluminum↗

NASA NTRS1999

Flow Trajectories in Friction Stir Welding

In the Friction Stir Welding (FSW) process, a rotating, shouldered tool with a threaded pin is inserted under pressure along the seam of two pieces of metal which are tightly clamped together, and secured against a rigid anvil underneath. The rotating pin travels along the seam and through a combination of pressure and friction heating produces a zone of plastic deformation around the pin within the workpiece on either side of the seam. As the pin is moved in the direction of welding, the plasticised material moves around the tool and bonds together behind it. The elements of the material flow behavior are a combination of three elements. There is a rotational transport of material being carried around the tool, extrusion of material being forced around the pin on both sides into the cavity created behind it, and a lifting and dropping of material as it is stirred and mixed by the rotating action of the pin. It was assumed that rotational motion of the plastic zone is the primary mechanism for transport of material around the welding tool. A kinematic mathematical model was used to compute trajectories of material movement for various distributions of rotational slip within the plastic zone. These trajectories were then compared with the results of an experiment that produced radiographs of markers embedded in a workpiece that was welded with the FSW process. It was assumed that the copper wire markers retained their original length as the aluminum material flowed around them. The kinematic model included a constraint so that the displacements were such that the total length of the wire markers did not increase after deformation. There was good agreement between the calculated trajectories for the case of localized slip at the outer surface of the plastic deformation zone and the radiographs of the copper wire markers. The trajectories differed markedly from the radiograph traces when a distributed slip zone was assumed. It was concluded that the flow field could be described generally by a rotating plug model with slip occurring on the surface at the outer boundary of the plastic zone.

Bernstein, Edward L.↗

DOE OSTI2018

On the Similarity of Deformation Mechanisms During Friction Stir Welding and Sliding Friction of the AA5056 Alloy

A comparative investigation of the structure of an aluminum-manganese alloy is performed after its friction stir welding and sliding friction. Using the methods of optical and electron microscopy, it is shown that during friction identical ultrafine-grained structures are formed in the weld nugget and in the surface layer, in which the grains measure ~ 5 μm irrespective of the initial grain size of the alloy. An assumption is made that the microstructure during both processes under study is formed by the mechanism of rotational plasticity.

36 MATERIALS SCIENCE↗

NASA NTRS2007

Friction stir weld tools

A friction stir weld tool sleeve is supported by an underlying support pin. The pin material is preferably selected for toughness and fracture characteristics. The pin sleeve preferably has a geometry which employs the use of an interrupted thread, a plurality of flutes and/or eccentric path to provide greater flow through. Paddles have been found to assist in imparting friction and directing plastic metal during the welding process.

Carter, Robert W.↗

NASA NTRS2007

Friction Stir Weld Tools

Carter, Robert W.↗

DOE OSTI2017

Analysis on the Fracture of Al-Cu Dissimilar Materials Friction Stir Welding Lap Joint

Friction stir welding (FWS) is regarded as a more plausible alternative to other welding methods for Al-Cu dissimilar joining. However, the structure of an FSW joint is different from others. In this study, lap joints of 6061 aluminum alloy and commercially pure copper were produced by FSW, and the effects of rotation rate on macromorphology, microstructure and mechanical properties were investigated. In addition, a fracture J integral model was used to analyze the effect of microstructure on the mechanical properties. The results revealed that the macrodefect-free joints were obtained at a feed rate of 150 mm/min and 1100 rpm and that the failure load of the joint reached as high as 4.57 kN and only reached 2.91 kN for the 900 rpm, where tunnel defects were identified. Particle-rich zones composed of Cu particles dispersed in an Al matrix, and “Flow tracks” were observed by the EDS. The J integral results showed that the microdefects on the advancing side cause serious stress concentration compared with the microdefects located on the Al-Cu interface, resulting in the fracture of the joints.

36 MATERIALS SCIENCE↗

NASA NTRS1998

A Combined Experimental and Analytical Modeling Approach to Understanding Friction Stir Welding

In the Friction Stir Welding (FSW) process a rotating pin tool joins the sides of a seam by stirring them together. This solid state welding process avoids problems with melting and hot-shortness presented by some difficult-to weld high-performance light alloys. The details of the plastic flow during the process are not well understood and are currently a subject of research. Two candidate models of the FSW process, the Mixed Zone (MZ) and the Single Slip Surface (S3) model are presented and their predictions compared to experimental data.

Nunes, Arthur C., Jr.↗

NASA NTRS2001

Friction Stir Weld System for Welding and Weld Repair

A friction stir weld system for welding and weld repair has a base foundation unit connected to a hydraulically controlled elevation platform and a hydraulically adjustable pin tool. The base foundation unit may be fixably connected to a horizontal surface or may be connected to a mobile support in order to provide mobility to the friction stir welding system. The elevation platform may be utilized to raise and lower the adjustable pin tool about a particular axis. Additional components which may be necessary for the friction stir welding process include back plate tooling, fixturing and/or a roller mechanism.

R. Jeffery Ding↗

NASA NTRS2016

Origins of Line Defects in Self-Reacting Friction Stir Welds and Their Impact on Weld Quality

Friction stir welding (FSWing) is a solid state joining technique which reduces the occurrence of typical defects formed in fusion welds, especially of highly alloyed metals. Although the process is robust for aluminum alloys, occasional reductions in the strength of FSWs have been observed. Shortly after the NASA-MSFC implemented a variation of FSW called self-reacting (SR), low strength properties were observed. At that time, this reduction in strength was attributed to a line defect. At that time, the limited data suggested that the line defect was related to the accumulation of native oxides that form on the weld lands and faying surfaces. Through a series of improved cleaning methods, tool redesign, and process parameter modifications, the reduction in the strength of the SR-FSWs was eliminated. As more data has been collected, the occasional reduction in the strength of SR-FSW still occurs. These occasional reductions indicate a need to reexamine the underlying causes. This study builds off a series of self reacting (SR)-FSWs that were made in 3 different thickness panels of AA2219 (0.95, 1.27 and 1.56 cm) at 2 different weld pitches. A bead on plate SR-FSW was also made in the 1.56 cm thick panel to understand the contribution of the former faying surfaces. Copper tracer studies were used to understand the flow lines associated with the weld tool used. The quality of the SR-FSWs was evaluated from tensile testing at room temperature. Reductions in the tensile strength were observed in some weldments, primarily at higher weld pitch or tool rotations. This study explores possible correlations between line defects and the reduction of strength in SR-FSWs. Results from this study will assist in a better understand of the mechanisms responsible for reduced tensile strength and provide methodology for minimizing their occurrence.

Schneider, Judy↗

DOE OSTI2017

Force measurement-based discontinuity detection during friction stir welding

Here, the objective of this work is to develop a method for detecting the creation of discontinuities ( i.e. , voids, volume defects) during friction stir welding. Friction stir welding is inherently cost effective, however, the need for significant weld inspection can make the process cost prohibitive. A new approach to weld inspection is required in which an in situ characterization of weld quality can be obtained, reducing the need for postprocess inspection. To this end, friction stir welds with subsurface voids and without voids were created. The subsurface voids were generated by reducing the friction stir tool rotation frequency and increasing the tool traverse speed in order to create “colder” welds. Process forces were measured during welding, and the void sizes were measured postprocess by computerized tomography ( i.e. , 3D X-ray imaging). Two parameters, based on frequency domain content and time-domain average of the force signals, were found to be correlated with void size. Criteria for subsurface void detection and size prediction were developed and shown to be in good agreement with experimental observations. Furthermore, with the proper choice of data acquisition system and frequency analyzer the occurrence of subsurface voids can be detected in real time.

47 OTHER INSTRUMENTATION↗

DOE OSTI2014

Modelling of the effect of process parameters on tensile strength of friction stir welded aluminium alloy joints

Friction stir welding (FSW) process is a promising solid-state joining process with the potential to join low-melting point material, particularly, aluminium alloy. The most attractive reason for joining aluminium alloy with this process is the avoidance of the solidification defects formed by conventional fusion welding processes. In this article, an attempt has been made to develop an empirical relationship between FSW variables and tensile strength. Central composite rotatable design was used by considering four factor and five levels, which enables to quantify the direct and interactive effect of four numeric factors, that is, tool rotational speed, welding speed, axial force, and tapered pin diameter on the tensile strength. The developed relationship is useful for prediction of tensile strength in friction stir welded AA6082 aluminium alloy joints at 95% confidence level. It will also be helpful for selection of process variable to obtain desired strength of the joint. Furthermore, the optimized capabilities in design-expert software were used to numerically optimize the input parameters. At the optimal parameters, three experiments were conducted to compare the predicted value of tensile strength with the experimental value.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Impact of initial temper of base metal on microstructure and mechanical properties of friction stir welded AA 7055 alloy

Highlights: • Friction stir welded 7055-T4 alloy exhibits higher tensile properties than T6 alloy. • Friction stir welded 7055-T4 alloy exhibits more uniform hardness distribution. • Some regions in heat affected zone have higher hardness than base metal in T4-joint. • Effect of initial temper on precipitate evolution in different zones is discussed. AA 7055-T6 and AA 7055-T4 aluminium alloy sheets were friction stir welded to investigate the impact of initial base metal temper on microstructure, hardness and tensile properties of the joints. In the joint of alloy in T6 temper (T6-joint), the hardness of the weld zone is lower than that of base metal; in the joint of alloy in T4 temper (T4-joint), however, some regions in the heat affected zone exhibit higher hardness than base metal. The weld nugget zone in T6-joint exhibits slightly lower hardness compared with that in T4-joint. The hardness minima is slightly higher in T4-joint than in T6-joint. T4-joint exhibits higher yield strength, ultimate tensile strength and elongation compared with T6-joint. Based on microstructure examination by transmission electron microscopy, the impact of initial precipitate state on precipitation behavior in the weld nugget zone, thermo-mechanically affected zone and heat affected zone during friction stir welding and mechanical properties of the joints is discussed.

36 MATERIALS SCIENCE↗

NASA NTRS2001

Friction Stir Welding of Steel Alloys

The friction stir welding process has been developed primarily for the welding of aluminum alloys. Other higher melting allows such, as steels are much more difficult to join. Special attention must be given to pin tool material selection and welding techniques. This paper addresses the joining of steels and other high melting point materials using the friction stir welding process. Pin tool material and welding parameters will be presented. Mechanical properties of weldments will also be presented. Significance: There are many applications for the friction stir welding process other than low melting aluminum alloys. The FSW process can be expanded for use with high melting alloys in the pressure vessel, railroad and ship building industries.

Ding, R. Jeffrey↗

NASA NTRS2004

Heat Control via Torque Control in Friction Stir Welding

In a proposed advance in friction stir welding, the torque exerted on the workpiece by the friction stir pin would be measured and controlled in an effort to measure and control the total heat input to the workpiece. The total heat input to the workpiece is an important parameter of any welding process (fusion or friction stir welding). In fusion welding, measurement and control of heat input is a difficult problem. However, in friction stir welding, the basic principle of operation affords the potential of a straightforward solution: Neglecting thermal losses through the pin and the spindle that supports it, the rate of heat input to the workpiece is the product of the torque and the speed of rotation of the friction stir weld pin and, hence, of the spindle. Therefore, if one acquires and suitably processes data on torque and rotation and controls the torque, the rotation, or both, one should be able to control the heat input into the workpiece. In conventional practice in friction stir welding, one uses feedback control of the spindle motor to maintain a constant speed of rotation. According to the proposal, one would not maintain a constant speed of rotation: Instead, one would use feedback control to maintain a constant torque and would measure the speed of rotation while allowing it to vary. The torque exerted on the workpiece would be estimated as the product of (1) the torque-multiplication ratio of the spindle belt and/or gear drive, (2) the force measured by a load cell mechanically coupled to the spindle motor, and (3) the moment arm of the load cell. Hence, the output of the load cell would be used as a feedback signal for controlling the torque (see figure).

Venable, Richard↗

DOE OSTI2018

Friction stir welding of multi-walled carbon nanotubes reinforced Al matrix composites

Highlights: • Al/MWCNT composites were successfully welded using friction stir welding. • After FSW, the composites possessed an excellent balance of mechanical properties. • High tensile strength and elongation over BM owing to MWCNTs after FSW. • Microstructures were investigated according to MWCNT contents and FSW conditions. - Abstract: In this study, we prepared composites between Al and multi-walled carbon nanotubes (MWCNTs) using the friction stir welding (FSW) method and investigated their microstructural evolution and mechanical properties by controlling the amount of MWCNT (1 and 3 vol%) and the plunging load (400 and 600 kg) used. After the FSW process, MWCNT showed slight coarsening of grain with random shear textures and accumulated dislocations; the differences obtained by varying the plunging load and MWCNT amount were insignificant. The friction-stir welded composite obtained using 3 vol% MWCNT and a plunging load of 600 kg possessed two-times better balance between strength and ductility than the base metal, due to the presence of both MWCNTs and the partially decomposed Al{sub 4}C{sub 3} in the composite.

36 MATERIALS SCIENCE↗

NASA NTRS2002

NDE of Friction Stir Welds in Aerospace Applications

Friction Stir Welding (FSW) is a solid state joining process, which utilizes a cylindrical, shouldered pin tool with a radiused tip that is rotated and plunged into the weld joint. Frictional heating beneath the shoulder, and surrounding the pin tip causes the material to plasticize, intermix and consolidate into a weldment without melting the parent material. FSW in aluminum alloys has many advantages such as low distortion and shrinkage, excellent mechanical properties, and no porosity. However, the propensity of the FSW process to create detrimental defects does exist, and is dependent on FSW parameter limits and controls. Inspection processes for FSW must also be selected and implemented concurrent with the new weld process. This paper describes the efforts by Lockheed Martin and NASA to find proper NDE techniques for detecting and characterizing the anomalies that may be caused by operating outside the envelope of optimized FSW parameters. Potential defects are identified and the results of the exploration of numerous NDE techniques including visual, liquid penetrant, multiple ultrasonic methods, eddy current and conductivity are discussed.

Kinchen, David G.↗