Carter Hamilton

A Coupled Thermal/Material Flow Model of Friction Stir Welding Applied to Sc‐Modified Aluminum Alloys

A coupled thermal/material flow model of friction stir welding was developed and applied to the joining of Sc-modified aluminum alloy (7042-T6) extrusions. The model reveals that surface material is pulled from the retreating side into the weld zone where it is interleaved with in situ material. Due to frictional contact with the shoulder, the surface material is hotter than the in situ material, so that the final weld microstructure is composed of bands of material with different temperature histories. For this alloy and the associated FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. Therefore, depending on the surface and in situ material temperatures in relation to these transformation temperatures, disparate precipitate distributions can develop in the bands of material comprising the weld nugget. Based on the numerical simulation and on thermal analysis data from differential scanning calorimetry, a mechanism for the formation of onion rings within the weld zone is presented.

Friction stir welding of aluminium 7136-T76511 extrusions

Abstract This research programme evaluates the as welded properties of Al 7136-T76511 extrusions joined through friction stir welding (FSW). Microstructural characterisation and mechanical testing were performed on the baseline material and on panels friction stir welded at 250 and 350 rev min–1 (all other weld parameters held constant). Transmission electron microscopy revealed the microstructural features in each of the unique weld regions and demonstrated that the precipitate density and morphology in these regions correlates with the temperature profile produced by the FSW process. A thermal model of FSW is developed that utilises an energy based scaling factor to account for tool slip. The slip factor is derived from an empirical relationship between the ratio of the maximum welding temperature to the solidus temperature and energy per unit length of weld. The thermal model successfully predicts the maximum welding temperatures and profiles over a range of energy levels. The mechanical behaviour after welding is correlated to the temperature distribution predicted by the model and to the observed microstructural characteristics. As welded mechanical properties of the alloy trended positively with the energy per unit length of weld, i.e. the highest joint efficiency was achieved at the highest welding temperature.

Characterization of Friction Modified Processing – A Novel Tool for Enhancing Surface Properties in Cast Aluminium Alloys

Friction Modified Processing (FMP) is a novel solid state processing technique which can be used for microstructural modification of surface layers in metallic materials. The paper deals with the investigation of the influence of process parameters on the microstructure in the surface layer of a cast aluminum alloy. The FMP was conducted on a constructed welding machine equipped with appropriate devices (LOWSTIR and TermSTIR). The measurements of temperature in the stir zone were compared with a numerical model. Another model was developed to determine the quantitative relationships between mass of modified material and processing speeds over a wide experimental range. An exponential formula has been found to describe the relationship between mass of modified material and rotational speed. The evaluation of the traveling speed affecting the mass of the modified material was successfully approximated by linear functions.

Developing predictive tools for friction stir weld quality assessment

Abstract This research programme explores predictive tools that assess friction stir weld quality in aluminium alloys through dynamic characterisation. The study focuses on the correlations between dynamic interrogations measures of friction stir welded panels with the weld energy, as welded mechanical properties and the microstructure. 7136-T76 aluminium extrusions were joined at unique weld energies, and to characterise and identify the friction stir welds through non-destructive techniques, theoretical modelling and lab scale dynamic testing were conducted to establish the correlation between the weld energy and the associated spectral characteristics of the beam (natural frequencies/mode shapes). In this non-destructive evaluation study, the modal parameters were measured and were correlated with the friction stir weld microstructure and the physical parameters of the welded components, such as axial and flexural rigidities. The viability of weld parameter identification and weld quality assessment of friction stir welding beams using dynamic interrogation techniques is demonstrated.

Microstructural and flow characteristics of friction stir welded aluminium 6061-T6 extrusions

Abstract Tin plated 6061-T6 Al extrusions were friction stir welded in a 90° butt weld configuration. A banded microstructure of interleaved layers of particle rich and particle poor material comprised the weld nugget. Transmission electron microscopy revealed the strong presence of tin within the particle rich bands, but TEM foils taken from other microstructural regions showed no indication of Sn containing phases. Since tin is limited to the surface of the preweld extrusions, surface material flowed into the nugget region to form the particle rich bands, while the particle poor bands originated from material within the extrusion thickness. The morphology of the principle strengthening phases indicated the relative temperature profile across the welded joint, and hardness profiles of as welded specimens consistently displayed the lowest hardness on the retreating side. Specimens that were solution heat treated and aged after welding revealed a normalised hardness profile across the weld.

Using MATLAB to advance the robotics laboratory

The RV‐M1 Movemaster from Mitsubishi Electric is an excellent educational robot for students as they learn to program automated tasks and simulate manufacturing processes. Because it was publicly introduced in 1991, the RV‐M1 utilizes the DOS‐based QBASIC computer code as the primary interface language with the robotic drive unit. Todays students, however, often face great frustration when they work with the unfamiliar QBASIC language and the even more unfamiliar DOS operating system. To address these shortcomings, the Mechanical and Manufacturing Engineering Department at Miami University introduced MATLAB as an alternative Window‐based interface language to better reflect the current educational experience of students and to increase the functionality of the RV‐M1 robotic arms. MATLAB successfully overcomes the limitations of the QBASIC/DOS environment and augments the capability of the RV‐M1. Examples of the extended capability include the use of graphical user interfaces to facilitate student interaction with the robotic arm and the ability to move the robot along contoured paths. With MATLAB, instructors can develop projects that enhance the student experience with the RV‐M1 and give them greater insight into robotic applications.

Characterisation of friction stir welded 7042- T6 extrusions through differential scanning calorimetry

Abstract Extruded and T6 tempered plates of 7042, an Sc modified Al–Zn–Mg–Cu alloy, were joined by friction stir welding (FSW) at a constant weld velocity and various pin rotation speeds (PRSs). At a low PRS, hardness decreased from each edge of the weld to a local minimum at the weld centre, but at a high PRS, the hardness initially decreased from each edge of the weld, but then rose towards the weld centre. The transition in the hardness profile was related to different heating and cooling conditions during FSW. Differential scanning calorimetry of baseline and welded samples revealed that the volume fraction of strengthening particles within the weld regions strongly depended on the weld conditions. For FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. The maximum welding temperature during FSW increased from ∼250 to 450°C as the PRS increased from 175 to 400 rev min−1.

A Simulation of Friction-Stir Processing for Temperature and Material Flow

Utilizing a tool without a pin, cast AlSi9Mg aluminum alloy was modified by friction-stir processing. Since the tool design specifically targets the microstructure within the surface layers, the process is more appropriately termed friction-stir surfacing. A coupled numerical model of this special friction-stir process was developed to visualize the material flow patterns and temperature distribution. As the tool transports surface material from the leading edge toward the retreating side of the tool, the material follows the scroll of the tool shoulder toward the tool center with each tool rotation. At or near the tool center, the material flows into the workpiece thickness, forming the vortex of a process zone. Depending on the processing conditions, i.e., tool velocity and rotation speed, an upward material flow also develops within the process zone. Due to the flow of cooler, unprocessed material into the process zone, the temperature profile on the tool/workpiece interface is skewed toward the advancing side and leading edge with higher processing temperatures occurring in these locations. However, the process parameters influence the shape and magnitude of the temperature distribution on this surface.

Friction Stir Welding of Dissimilar Aluminum Alloys

Dissimilar aluminum alloy plates of 2017A-T451 and 7075-T651 with 6 mm thickness were friction stir butt welded. Numerous trials were conducted to determine the conditions that produce the highest weld quality. These parameters were found to be a welding speed of 112 mm/min, a rotation speed of 355 rev/min and a vertical force of 32,8 kN. The weldability and blending of the two materials were evaluated by using macro- and microstructural analysis as well as EDS mapping to show the distribution of main alloying elements within the weld. The effect of material locations, either on the advancing or retreating sides, on the microstructure and mechanical properties was also investigated. Hardness profiles differ substantially for different weld configurations. Regardless of the position of a particular alloy, the weld microstructure was composed of alternating layers of both materials. However, the layers of the 7075 alloy always exhibited smaller grain size and a larger number of secondary phase particles.

FRICTION STIR WELDING OF SC-MODIFIED AL-ZN-MG-CU ALLOY EXTRUSIONS

Small additions of scandium to Al-Zn-Mg-Cu 7000 series alloys can significantly improve mechanical properties and augment the strength retention at low and elevated temperatures. This research program evaluates the residual properties of Sc-modified Al-Zn-Mg-Cu alloy extrusions joined through friction stir welding (FSW). Mechanical and corrosion testing were performed on the baseline material and on panels friction stir welded at 175, 225, 250, 300, 350 and 400 RPM (all other weld parameters held constant). A thermal model of friction stir welding is developed that utilizes an energy-based scaling factor to account for tool slip. The proposed slip factor is derived from an observed, empirical relationship between the ratio of the maximum welding temperature to the solidus temperature and energy per unit length of weld. The thermal model successfully predicts the maximum welding temperatures over a range of energy levels, and the mechanical and corrosion behavior is correlated to the temperature distribution predicted by the model.Copyright