Friction Stir Welding of Copper Canisters Using Power and Temperature Control

This thesis presents the development to reliably seal 50 mm thick copper canisters containing the Swedish nuclear waste using friction stir welding. To avoid defects and welding tool fractures, it is important to control the tool temperature within a process window of approximately 790 to 910°C. The welding procedure requires variable power input throughout the 45 minute long weld cycle to keep the tool temperature within its process window. This is due to variable thermal boundary conditions throughout the weld cycle. The tool rotation rate is the input parameter used to control the power input and tool temperature, since studies have shown that it is the most influential parameter, which makes sense since the product of tool rotation rate and spindle torque is power input. In addition to the derived control method, the reliability of the welding procedure was optimized by other improvements. The weld cycle starts in the lid above the joint line between the lid and the canister to be able to abort a weld during the initial phase without rejecting the canister. The tool shoulder geometry was modified to a convex scroll design that has shown a self-stabilizing effect on the power input. The use of argon shielding gas reduced power input fluctuations i.e. process disturbances, and the tool probe was strengthened against fracture by adding surface treatment and reducing stress concentrations through geometry adjustments. In the study, a clear relationship was shown between power input and tool temperature. This relationship can be used to more accurately control the process within the process window, not only for this application but for other applications where a slow responding tool temperature needs to be kept within a specified range. Similarly, the potential of the convex scroll shoulder geometry in force-controlled welding mode for use in applications with other metals and thicknesses is evident. The variable thermal boundary conditions throughout the weld cycle, together with the risk of fast disturbances in the spindle torque, requires control of both the power input and the tool temperature to achieve a stable, robust and repeatable process. A cascade controller is used to efficiently suppress fast power input disturbances reducing their impact on the tool temperature. The controller is tuned using a recently presented method for robust PID control. Results show that the controller keeps the temperature within ±10°C of the desired value during the 360º long joint line sequence. Apart from the cascaded control structure, good process knowledge and control strategies adapted to different weld sequences i.e. different thermal boundary conditions have contributed to the successful results

Reliability study of friction stir welded copper canisters containing Sweden's nuclear waste

The plan for the final disposal of spent fuel front Swedish nuclear power plants is to place it in copper canisters that are sealed and stored in a deep repository. The canisters will be sealed by friction stir welding (FSW) and the reliability of this process has now been evaluated. The reliability study was performed in three steps: first an optimization experiment to identify optimal process settings and establish the process window; then a demonstration experiment with welding tinder production-like conditions; and finally a postdemonstration series to evaluate an adjustment in the welding equipment. A process window was defined around the optimal process setting, i.e., the limits within which the welding variables must lie in order for the process to produce the desired result. In the demonstration experiment, a series of 20 scaling welds was carried out under production-like conditions. The maximum discontinuity in each weld-detected by non-destructive testing-was fitted to a generalized extreme value distribution. The 95% confidence interval for the maximum discontinuity in a production series of 4500 canisters was estimated at 4.5-7.7mm. The best estimate from the postdemonstration series suggests that the maximum size of a discontinuity will not exceed 2.3 mm. A main conclusion is that the FSW process produces reliable results, fulfilling the predetermined requirements for minimum copper thickness by a very good margin

Software for PID design: benefits and pitfalls

The most common PID design methods in industry are based on formulas. This article will present some major advantages of instead using the power of computer based softwares for PID controller design. The Matlab based software used in this work was developed in 2007 and derives robust, IAE minimizing, PID controllers. The experiences of using this software are collected in this article and include control signal activity limitation due to measurement noise, controller design on an industrial Friction Stir Welding process and fast controller design for large batches of processes. It is shown that the properties of the software make it suitable for design of PID controllers and in PID research. There are, however, some possible design pitfalls that the user needs to be aware of. Some of these are presented as well

Decentralized Friction Stir Welding Control on Canisters for Spent Nuclear Fuel

The Swedish nuclear waste will be stored in copper canisters and kept isolated deep under ground for at least 100,000 years. To ensure reliable sealing of the canisters, friction stir welding is utilized. To repetitively produce high quality welds, it is vital to use automatic control of the process. A decentralized solution is designed based on an already existing temperature controller and a proposed linear plunge depth controller. The plunge depth control is challenging mainly because of deection in the machine, thermal expansion and cross couplings in the process. The decentralized controller has been implemented and evaluated on the real system with good results, keeping the plunge depth within the necessary 0:1 mm of its setpoint at the same time as the temperature specications are met

Simulation based Evaluation of a Nonlinear Model Predictive Controller for Friction Stir Welding of Nuclear Waste Canisters

The Swedish nuclear waste will be stored in copper canisters and kept isolated deep under ground for more than 100,000 years. To ensure reliable sealing of the canisters, friction stir welding is used. To repetitively produce high quality welds, it is vital to use automatic control of the process. This paper introduces a nonlinear model predictive controller for regulating both plunge depth and stir zone temperature, which has not been presented in literature before. Further, a nonlinear process model has been developed and used to evaluate the controller in simulations of the closed loop system. The controller is compared to a decentralized solution, and simulation results indicate that it is possible to achieve higher control performance using the nonlinear model predictive controller

Simulation based Evaluation of a Nonlinear Model Predictive Controller for Friction Stir Welding of Nuclear Waste Canisters

The Swedish nuclear waste will be stored in copper canisters and kept isolated deep under ground for more than 100,000 years. To ensure reliable sealing of the canisters, friction stir welding is used. To repetitively produce high quality welds, it is vital to use automatic control of the process. This paper introduces a nonlinear model predictive controller for regulating both plunge depth and stir zone temperature, which has not been presented in literature before. Further, a nonlinear process model has been developed and used to evaluate the controller in simulations of the closed loop system. The controller is compared to a decentralized solution, and simulation results indicate that it is possible to achieve higher control performance using the nonlinear model predictive controller

Decentralized Friction Stir Welding Control on Canisters for Spent Nuclear Fuel

The Swedish nuclear waste will be stored in copper canisters and kept isolated deep under ground for at least 100,000 years. To ensure reliable sealing of the canisters, friction stir welding is utilized. To repetitively produce high quality welds, it is vital to use automatic control of the process. A decentralized solution is designed based on an already existing temperature controller and a proposed linear plunge depth controller. The plunge depth control is challenging mainly because of deection in the machine, thermal expansion and cross couplings in the process. The decentralized controller has been implemented and evaluated on the real system with good results, keeping the plunge depth within the necessary 0:1 mm of its setpoint at the same time as the temperature specications are met

Cascade control of the friction stir welding process to seal canisters for spent nuclear fuel

This article presents the development to reliably seal copper canisters containing the Swedish nuclear waste, using friction stir welding. To avoid defects and welding tool fractures, it is important to control the welding temperature within a span of 790–910 degrees C. A cascade controller is used to efficiently suppress fast power input disturbances reducing their impact on the temperature. The controller is tuned using a recently presented method for robust PID control. Results show that the controller keeps the temperature within 710 degrees C during the 40 min long joint line sequences. Apart from the cascaded control structure, good process knowledge and control strategies adapted to different weld sequences have contributed to the successful results

Cascaded control of power input and welding temperature during sealing of spent nuclear fuel canisters

The friction stir welding procedure to seal copper canisters requires variable power input throughout the 45 minute long weld cycle to keep the welding temperature within the process window. This is due to variable thermal boundary conditions throughout the weld cycle which, together with fast disturbances in the spindle torque, requires control of both the power input and the welding temperature to achieve a stable and robust process. By using a cascaded loop that determines the power input requirement, the regulator will not be dependent on repeatability in the necessary power input between weld cycles. As a result, a more accurate and reliable closed-loop control of the welding temperature is acquired