Friction stir welding (FSW) equipment for tank structures

Why an FSW system is more than a welding machine

Welding a large tank structure requires accurate tool movement, stable workpiece positioning and repeatable joint geometry simultaneously. The system therefore includes a CNC machine and welding unit alongside rotary tables, supports, external bands, internal bracing, steady rests and specialised fixtures.

Layout follows weld type. Longitudinal welds need one arrangement of locating and clamping; circumferential welds require controlled shell rotation; meridional or circular welds on large tanks impose additional spatial positioning requirements. The process task thus determines the entire system’s kinematics.

Three configurations for different weld types

The Progress FSW system includes three machine variants for different tank structure operations. All provide pre-weld edge milling, thickness calibration, FSW up to 30 mm and automated ultrasonic inspection. Their main differences are workpiece positioning and fixtures.

This avoids forcing fundamentally different geometric tasks into one universal arrangement. In series production, dedicated mechanics reduce changeovers and improve locating repeatability.

Machine rigidity, axial force and motion accuracy

Structural rigidity is critical to FSW: the tool applies substantial axial load, and positional deviation affects plunge depth and joint formation. The presented machine has a main structure mass of approximately 180 tonnes and tool axial force of up to 10 tonnes.

The design uses electromechanical linear and rotary axes, servo drives, optical and magnetic measuring systems, hydraulic counterbalancing and centralised lubrication. Five-axis kinematics support spatial machining. These operate as one system: high power alone cannot compensate for inadequate rigidity, measurement accuracy or fixture stability.

Foundations and load distribution are particularly important for large equipment. Gantry, table and workpiece masses must be considered when designing the production site.

Welding unit and controlled tool axes

The system documentation shows a two-axis rotary head with continuously controlled axes. Axis A rotates from −45° to +90°, while axis B covers approximately ±10°. This arrangement maintains the required tool orientation relative to complex surfaces and weld paths.

The motor spindle has an integrated synchronous motor with nominal torque of 840 N·m and maximum torque of 1,690 N·m. Linear hydraulic axes moving the spindle and pin each provide a working force of up to 10 tonnes.

Engineers must consider coordinated performance rather than isolated maximum values: speed range, torque, axial force, positioning accuracy, cooling and process recording must suit the material, thickness and tool type.

Automated ultrasonic inspection in the process cycle

Quality inspection is integrated into production. The automated phased-array ultrasonic system specifies at least 16 elements, an operating frequency of at least 5 MHz, connections for at least two phased arrays, and A-, B-, C- and S-scan displays.

Automation offers more than inspection speed. Coordinating probe position with machine movement links inspection results to specific weld locations and allows comparison with recorded welding parameters.

The ultrasonic procedure, acceptance criteria and sensitivity are configured for the material, thickness, joint geometry and project requirements. Inspection equipment capabilities must therefore be considered alongside the product’s manufacturing process.

Tools, bobbin tools and exit-hole closure

The tooling set includes fixed tools, adjustable-pin tools and bobbin tools. Tool design variations adapt the process to material thickness, underside access and weld support requirements.

Completing the weld is a separate operation. The documentation shows one exit-hole closure method: preparing the hole in the shell, inserting a compatible material plug and friction stir spot welding it. This requires dedicated fixtures and a process procedure.

Tool selection and weld completion must follow a qualified procedure. Pin and shoulder geometry, rotation and axial load directly determine heat input and material mixing.

CNC and integrated control

The system uses HNC closed-loop CNC control. It manages axis positions and load synchronisation between drives, with five simultaneously controlled axes and RTCP tool centre point control during multi-axis motion.

This is especially important for spatial FSW paths: the controller must move the gantry, maintain tool orientation relative to the surface, synchronise axes and keep welding conditions stable.

Open control architecture also facilitates integration of parameter logging, diagnostics, process cycles and data exchange with plant production systems.

Specifying a new FSW system

Design starts by defining the product, not spindle power: material, thickness range, weld types and lengths, maximum dimensions and mass, required throughput, underside access and inspection methods.

Next come positioning, fixtures, travel and load ranges, measuring systems, CNC requirements and inspection automation. Large tanks also require dedicated foundation, workpiece logistics and maintenance-zone calculations.

An integrated approach treats the machine, rotary tables, welding unit, fixtures and ultrasonic inspection as one production line. Coordination between these subsystems determines repeatable results in series production.