Brian T. Gibson

Effect of Pin Length and Rotation Rate on the Tensile Strength of a Friction Stir Spot-Welded Al Alloy: A Contribution to Automated Production

Friction stir spot welding (FSSW) is performed on thin plates of an aluminum alloy in a lap joint configuration with tools of different pin lengths and various rotation rates. The effects these process parameters have on the joint properties of the welds are investigated. The tensile strength of the welds decreased when the rotation rate was increased. The tensile strength of welds made with a pinless tool is on average 90% the strength of the full penetration spot welds. Intermediate pin lengths were tested between these two extremes. It was found that the tensile strength decreases as the pin length increases from pinless to 10% bottom plate penetration. Three distinct failure modes were identified when the welds were placed under tensile loading: shear mode, mixed mode, and nugget-pullout mode. The dependence of static joint strength on these process parameters is discussed.

Investigation of force-controlled friction stir welding for manufacturing and automation

Abstract Friction stir welding (FSW) is a solid-state joining process for materials with low melting points. The process uses a rotating tool that consists of a shoulder and a pin. The tool plastically deforms the material with its pin and then forges together the parent materials underneath the shoulder. Past research has established that the axial force on the tool that creates the forging pressure is a function of plunge depth, traverse speed, and rotation speed. Historically, force control of FSW has been accomplished by varying the plunge depth of the tool. The research presented in this paper examines the force control of FSW by varying each of the process parameters separately. A force controller was implemented on a retrofitted milling machine. The closed-loop proportional—integral—derivative (PID) control architecture was tuned using the Ziegler—Nichols method. Welding experiments were conducted by butt welding ¼ inch (6.35 mm) × 1½ inch (38.1 mm) × 8 inch (203.2 mm) samples of aluminium 6061-T6511 with a ¼ inch (6.35 mm) FSW tool. The results indicate that force control via traverse speed is the most accurate and, as a by-product, heat distribution control along the weld seam occurs. Force control via plunge depth is the least accurate but it compensates for machine and robot deflection. Tensile test data show that greater strength can be obtained through force control via rotation speed. It is concluded that force is maintained by keeping the amount of tool surface area in contract with the workpiece constant throughout the welding process when plunge depth is used as the controlling variable. Force is maintained by varying the rate of heat generation when rotation speed is used as the controlling variable. Lastly, force is maintained by changing the amount of heat deposited per unit length along the weld seam when traverse speed is used as the controlling variable. Successful robotic FSW requires to be selected the appropriate controlling variable and the sensitivity of the interaction between the tool and the workpiece to be reduced.

A Comparative Evaluation of the Wear Resistance of Various Tool Materials in Friction Stir Welding of Metal Matrix Composites

Friction stir welding (FSW) is the preferred joining method for metal-matrix composites (MMCs). As a solid-state process, it precludes formation of the intermetallic precipitates responsible for degradation of mechanical properties in fusion welds of MMCs. The major barrier to FSW of MMCs is the rapid and severe wear of the welding pin tool, a consequence of prolonged contact between the tool and the harder reinforcements which give the material its enhanced strength. This study evaluates the effectiveness of harder tool materials to combat wear in the FSW of MMCs. The tool materials considered are O1 steel, cemented carbide (WC-Co) of the micrograin and submicrograin varieties, and WC-Co coated with diamond. The challenges which accompany the application of harder tool materials and diamond coatings in FSW are also discussed. This study represents the first use of diamond-coated tools in FSW and the first comparative evaluation of tool materials for this application.

Adaptive torque control of friction stir welding for the purpose of estimating tool wear

In this paper, an adaptive torque controller for friction stir welding (FSW) that can estimate parameters such as probe radius which may be changing throughout the welding process is presented. Implementing an adaptive controller with this capability would be of interest to industry sectors in which FSW is performed on high melting point alloys or metal matrix composites (MMC). Welding these materials has shown a greatly accelerated rate of tool wear. Simulations were conducted to examine how extreme tool wear would affect controller performance and how accurately the controller could estimate the probe radius. A simplified wear model consisting of a linear decrease in probe radius was used to verify controller performance. Next, a wear model consistent with wear patterns seen in the welding of highly abrasive materials was developed. Results indicate that torque is controlled effectively while a change in system dynamics is experienced, as would be expected with adaptive control, but also that the tool profile is accurately estimated after an initial identification period.

A Phenomenological Model for Tool Wear in Friction Stir Welding of Metal Matrix Composites

Friction stir welding (FSW) of metal matrix composites (MMCs) is advantageous because the solid-state nature of the process precludes formation of deleterious intermetallic phases which accompany melting. FSW of MMCs is complicated by rapid and severe wear of the welding tool, a consequence of contact between the tool and the much harder abrasive reinforcement which gives the workpiece material its enhanced strength. The current article demonstrates that Nunes’s rotating plug model of material flow in FSW, which has been successfully applied in many other contexts, can also help us understand wear in FSW of MMCs. An equation for predicting the amount of wear in this application is developed and compared with experimental data. This phenomenological model explains the relationship between wear and FSW process parameters documented in previous studies.

Dimensional analysis and a potential classification algorithm for prediction of wear in friction stir welding of metal matrix composites

The objective of this study is to develop a dimensionless parameter which can be used to estimate the amount of volumetric wear, a friction stir welding tool will experience in joining a metal matrix composite. Metal matrix composites are strong, lightweight materials consisting of a metal matrix (often an aluminum alloy) reinforced with ceramic particles or fibers. This study derives a dimensionless number based on three major process variables in friction stir welding: rotation speed, traverse speed, and length of weld. This number is correlated with wear data collected from experiments in which a steel friction stir welding tool was used to join Al 359/SiC/20p. The use of the dimensionless number as a classifier for tool condition is also evaluated.

In-Process Detection of Faying Surface Sealant Application Flaws in Friction Stir Welding

In this study, a process for detecting faying surface sealant application flaws in friction stir welded (FSW) lap joints is developed. The process uses a technique shown previously to enable the detection of machined gaps in the same joint type. This technique involves computing the frequency spectra of the process forces and reducing the dimensions of the data using well-known methods for discrimination purposes. Aluminum alloys 2024-T3 and 7075-T6 in 0.063-in.- (1.6-mm-) thick sheets were welded with a variety of PR-1432-GP sealant configurations, including in both the cured and uncured state, and applied in the tool path and adjacent to the tool path. It is shown that sealant flaws such as gaps or thin spots can indeed be discriminated from control welds with proper sealant application, and the success of this technique depends directly on the input force signal, the sealant configuration, and the dimensional reduction method. Factors affecting the real-time implementation of this technology in aerospa...

Evaluation of torque as a means of in-process sensing of tool wear in friction stir welding of metal matrix composites

Purpose The purpose of this paper is to evaluate the tool experiences using torque during welding as a means of in-process sensing for tool wear. Metal matrix composites (MMCs) are materials with immense potential for aerospace structural applications. The major barrier to implementation of these materials is manufacturability, specifically joining MMCs to themselves or other materials using fusion welding. Friction stir welding (FSW) is an excellent candidate process for joining MMCs, as it occurs below the melting point of the material, thus precluding the formation of degradative intermetallics’ phases present in fusion welded joints. The limiting factor for use of FSW in this application is wear of the tool. The abrasive particles which give MMCs their enhanced properties progressively erode the tool features that facilitate vertical mixing and consolidation of material during welding, resulting in joints with porosity. While wear can be mitigated by careful selection of process parameters and/or the use of harder tool materials, these approaches have significant complexities and limitations. Design/methodology/approach This study evaluates using the torque the tool experiences during welding as a means of in-process sensing for tool wear. Process signals were collected during linear FSW of Al 359/SiC/20p and correlated with wear of the tool probe. The results of these experiments demonstrate that there is a correlation between torque and wear, and the torque process signal can potentially be exploited to monitor and control tool wear during welding. Findings Radial deterioration of the probe during joining of MMCs by FSW corresponds to a decrease in the torque experienced by the tool. Experimentally observed relationship between torque and wear opens the door to the development of in-process sensing, as the decay in the torque signal can be correlated to the amount of volume lost by the probe. The decay function for tool wear in FSW of a particular MMC can be determined experimentally using the methodology presented here. The decay of the torque signal as the tool loses volume presents a potential method for control of the wear process. Originality/value This work has near-term commercial applications, as a means of monitoring and controlling wear in process could serve to grow commercial use of MMCs and expand the design space for these materials beyond net or near-net-shape parts.

Applied torque control of friction stir welding using motor current as feedback

When controlling friction stir welding, effort must be given to maintaining proper tool shoulder contact with the workpiece in order to achieve consolidation of the parent materials. Axial force control has been used prior with some success. The research presented in this article examines the controlling of welding torque as an alternative to force control. A mathematical model of welding torque was enhanced for the design and study of convex shoulder profiles. Focus was placed on linearizing the response between plunge depth and torque. The model predicted that a spherical profiled shoulder was preferred for a more linear response. In conjunction with the spherical shoulder, a closed-loop torque controller was implemented and its performance evaluated. Welding torque for feedback control was sensed indirectly through the spindle motor current using a commercially available clamp-on current meter. The system produced 1/4 in (6 mm) bead-on-plate welds that were 10 ft (3 m) in length. Over the course of the welds, the torque controller responded to workpiece elevation changes of 1/8 in (3 mm) and 1/4 in (6 mm). Results show that the tool maintained a near constant plunge depth into the workpiece as the tool traversed along the workpiece. It was concluded that the presented method of torque control is a reliable and less complex alternative to axial force control of friction stir welding.

Automatic Tracking of Blind Sealant Paths in Friction Stir Lap Joining

An automatic joint-tracking technique that employs an extremum-seeking controller is evaluated as a method for automatically tracking sealant paths that have been applied in dissimilar friction stir lap joints of 2024 and 7075 aluminum alloys. Sealants are commonly used to prevent the ingress of corrosion at the faying surfaces of lap joints, and in this study, an attempt is made to exploit sealant presence to reduce necessary robotic path planning procedures. Controller parameters are tuned, and baseline tracking performance is established with milled channels, which are used to replicate the force signature of sealant, and the tracking technique is then evaluated with Pelseal® 2077 sealant applied in a prescribed fashion in both cured and uncured states. Mechanical testing is conducted to determine the implications for weld strength when welding parameters are selected primarily for successful tracking. Results are promising and demonstrate a new level of interaction between sealants and robotic control...