Glenn J. Grant

Study of friction stir joining of thin aluminium sheets in lap joint configuration

Abstract Thinning in friction stir lap joints and its relation with the process variables was investigated. Friction stir welds were made on 1 mm thick AA6111 aluminium alloy sheets in order to study the effects of rotation rate, traverse speed, plunge depth, tilt angle and pin height on faying surface defects. Sheet thinning on the advancing and retreating sides was quantified and the lap shear strength of the joints was evaluated. A decrease in the pitch of the runs at constant rotation rate increased the sheet thinning and reduced the joint strength in a linear manner. Process pitch and pin height were found to be the most critical factors in determining the faying surface lift up. A pin of the same height as the sheet thickness resulted in maximum lap shear strength. Tool tilt did not show a significant effect on the sheet thinning.

Friction stir spot welding of DP780 carbon steel

Abstract Friction stir spot welds were made in uncoated and galvannealed DP780 sheets using polycrystalline boron nitride stir tools. The tools were plunged at either a single continuous rate or in two segments consisting of a relatively high rate followed by a slower rate of shorter depth. Welding times ranged from 1 to 10 s. Increasing tool rotation speed from 800 to 1600 rev min–1 increased strength values. The 2-segment welding procedures also produced higher strength joints. Average lap shear strengths exceeding 10·3 kN were consistently obtained in 4 s on both the uncoated and the galvannealed DP780. The likelihood of diffusion and mechanical interlocking contributing to bond formation was supported by metallographic examinations. A cost analysis based on spot welding in automobile assembly showed that for friction stir spot welding to be economically competitive with resistance spot welding the cost of stir tools must approach that of resistance spot welding electrode tips.

Modeling of Friction Stir Welding (FSW) Process with Smooth Particle Hydrodynamics (SPH)

We present a novel modeling approach to simulate FSW process that may have significant advantages over current traditional finite element or finite difference based methods. The proposed model is based on Smoothed Particle Hydrodynamics (SPH) method, a fully Lagrangian particle method that can simulate the dynamics of interfaces, large material deformations, void formations and materials strain and temperature history without employing complex tracking schemes. Two- and three-dimensional simulations for different tool designs are presented. Preliminary numerical results are in good qualitative agreement with experimental observations.

Development of Friction Stir Processing for Repair of Nuclear Dry Cask Storage System Canisters

The Nuclear Regulatory Commission has identified chloride-induced stress corrosion cracking (CISCC) of austenitic stainless steel dry cask storage systems (DCSS) as an area of great concern. Friction stir processing (FSP) was used to repair laboratory-generated stress corrosion cracking (SCC) in representative stainless steel 304 coupons. Results of this study show FSP is a viable method for repair and mitigation CISCC. This paper highlights lessons learned and techniques developed to apply FSP to crack repair in sensitized thick section stainless steel 304. These include: development of process parameters, welding at low spindle speed, use of weld power and temperature control and optimization of these controls. NDE and destructive analysis are also presented to demonstrate effectiveness of the developed methods for SCC crack repair.

Friction Stir Processing of Degraded Austenitic Stainless Steel Nuclear Fuel Dry Cask Storage System Canisters

Chloride-induced stress corrosion cracking (CISCC) of austenitic stainless steel dry cask storage system (DCSS) canisters has been identified as an industry concern. Typical DCSS canisters are constructed from Types 304 or 316 stainless steel or their variants via conventional fusion welding processes. The presence of residual tensile stress and Cr-carbide precipitation within the weld heat affected zone (HAZ) places canisters near salt-bearing environments at an elevated risk for CISCC. The current study evaluates the suitability of friction stir processing (FSP) to repair stress corrosion cracking (SCC) and remediate sensitized fusion weld HAZs. FSP was applied to furnace sensitized Type 304 specimens containing laboratory-generated SCC and evaluated using liquid penetrant inspection, phased array ultrasonic inspection, and optical microscopy. In addition, fusion welded Type 304L specimens were fabricated, subjected to FSP, and destructively analyzed via ASTM A262 and optical microscopy. Results demonstrate that FSP is a viable option for SCC repair and sensitization remediation.

Optimization of the Friction Stir Welding Process for Superplastic Forming and Improved Surface Texture for Titanium Aerospace Structures

In friction stir welding (FSW), the semi-circular shaped FSW pin tool feed marks that are left behind varied in depth and shape which are detrimental not only in fatigue performance but also in further processing such as superplastic forming (SPF). Experimental investigation was conducted to determine the effects of changes to the FSW process parameters on the surface roughness of the weld of fine grain 2 mm thick titanium alloy, Ti- 6Al-4V. In addition to optimizing the surface texture of the welds, the superplastic performance of the weld nugget was made to be equal to the superplasticity of the parent metal by altering the spindle speed and feed rate used during FSW to identify the quality in terms of cold weld or hot weld. FSW process conditions of spindle speed of 500 RPM and a feed rate of 150 mm/min was found to produce a uniform deformation in both weld and parent metal when the joint was superplastc formed.

Forming Limits of Weld Metal in Aluminum Alloys and Advanced High-Strength Steels

This work characterizes the mechanical properties of DP600 laser welded TWBs (1 mm-1.5 mm) near and in the weld, as well as their limits of formability. The approach uses simple uniaxial experiments to measure the variability in the forming limits of the weld region, and uses a theoretical forming limit diagram calculation to establish a probabilistic distribution of weld region imperfection using an M-K method approach

Code Acceptance of a New Joining Technology for Storage Containments

One of the activities associated with cleanup throughout the Department of Energy (DOE) complex is packaging radioactive materials into storage containers. Much of this work will be performed in high-radiation environments requiring fully remote operations, for which existing, proven systems do not currently exist. These conditions require a process that is capable of producing acceptable (defect-free) welds on a consistent basis; the need to perform weld repair, under fully-remote operations, can be extremely costly and time consuming. Current closure-welding technologies (fusion welding) are not well suited for this application and will present risk to cleanup cost and schedule. To address this risk, Fluor and the Pacific Northwest National Laboratory (PNNL) are proposing that a new and emerging joining technology, Friction Stir Welding (FSW), be considered for this work. FSW technology has been demonstrated in other industries (aerospace and marine) to produce near flaw-free welds on a consistent basis. FSW is judged capable of providing the needed performance for fully-remote closure welding of containers for radioactive materials for the following reasons: FSW is a solid-state process; material is not melted. FSW does not produce the type of defects associated with fusion welding, e.g., solidification-induced porosity, cracking, and distortion due to weld shrinkage. In addition, because FSW is a low-heat input process, material properties (mechanical, corrosion and environmental) experience less degradation in the heat affected zones than do fusion welds. When compared to fusion processes, FSW produces extremely high weld quality. FSW is performed using machine-tool technology. The equipment is simple and robust and well-suited for high radiation, fully-remote operations compared to the relatively complex equipment associated with fusion-welding processes. Additionally, for standard wall thicknesses of radioactive materials containers, the FSW process can perform the final closure in a single pass (GTAW requires multiple passes) resulting in increased productivity. The performance characteristics of FSW, Le., high weld quality, simple machine-tool equipment and increased welding efficiency, suggest that this new technology should be considered for radioactive materials packaging campaigns. FSW technology will require some development, adaptation for this application, along with several activities needed for commercialization. One of these activities will be to obtain approval from the governing construction code to use the FSW technology. The American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME BP however, rules for the use of FSW are not currently addressed. A code case will be required, defining appropriate process variables within prescribed limits, and submitted to the Code for review/approval and incorporation.

Evaluation of the Mechanical Performance of Self-Piercing Rivets in Friction Stir Welded Structures

This paper presents the coupon performance data of friction stir welded tailor welded blanks (TWBs) joined to a monolithic aluminum sheet by self-piercing rivets (SPRs). Uniaxial tensile tests were performed to characterize the joint strength and the total energy absorption capability of the TWB/monolithic joint assemblies. Cyclic fatigue tests were also conducted to characterize the fatigue behavior and failure mechanisms of the jointed assemblies. It was found that the static and fatigue strength of the TWB/monolithic assembly was approximately 30 percent less in all loading configurations tested in comparison to a common monolithic sheet SPR assembly. The total energy absorbed by the TWB/monolithic sheet assemblies was also found to be 30 percent less than the monolithic sheet assemblies in cross tension loading. In lap shear loading, the total energy absorbed was comparable.

Simultaneous Independent Control of Tool Axial Force and Temperature in Friction Stir Processing

Maintaining consistent tool depth relative to the part surface is a critical requirement for many friction stir processing (FSP) applications. Force control is often used with the goal of obtaining a constant weld depth. When force control is used, if weld temperature decreases, flow stress increases and the tool is pushed up. If weld temperature increases, flow stress decreases and the tool dives. These variations in tool depth and weld temperature cause various types of weld defects. Robust temperature control for FSP maintains a commanded temperature through control of the spindle axis only. Robust temperature control and force control are completely decoupled in control logic and machine motion. This results in stable temperature, force and tool depth despite the presence of geometric and thermal disturbances. Performance of this control method is presented for various weld paths and alloy systems.