Friction Stir Welding Explained: Why Solid-State Joining Wins in EV Battery Trays and Liquid Cold Plates

Invented by The Welding Institute (TWI) in the UK in 1991, friction stir welding is often described as the most revolutionary joining technology of the 20th century. Yet outside the welding community, surprisingly few engineers understand why it has quietly become the default process for EV battery trays, liquid cold plates, and other safety-critical aluminum structures.

It welds metal without melting it. A rapidly rotating tool plunges into the joint line and travels along it. Friction between the tool and the workpiece generates heat that plastically softens the surrounding metal — but never melts it. The softened metal is stirred and forged behind the tool, consolidating into a solid-state bond. The result is a dense, wrought-like microstructure with fine equiaxed grains, rather than the cast structure left by fusion welding.

Why this matters for real products:

● Low distortion. Because heat input is far lower than in arc welding, thin-walled and large-format parts — exactly what battery trays and cold plates are — stay flat. Flatness of the cooling interface is a pass/fail requirement, not a cosmetic one.

● No porosity, no hot cracking. Fusion welding of aluminum is notorious for hydrogen porosity and solidification cracking. FSW sidesteps both because there is no molten pool.

● Sealing performance. A solid-state weld is inherently leak-tight, which is why FSW has become the standard sealing method for liquid cold plates.

● Environment-independent. The process is not affected by ambient temperature or humidity, and produces no arc light, spatter, or fume — cleaner, safer, and more energy-efficient.

● Dissimilar-friendly. Aluminum to copper, aluminum to steel, aluminum to titanium — combinations that are nearly impossible by fusion welding — can be joined reliably.

What it takes to do FSW at production scale. A capable lab weld is one thing; a production line welding hundreds of thousands of parts a year is another. Three elements separate the two:

● Tooling. The stirring tool is the core of the process — the single biggest factor in joint performance and efficiency. Deyanfu operates a dedicated tooling division that has developed more than 1,000 tool designs, backed by an accumulated welding-parameter database.

● Equipment and intelligence. Deyanfu's friction stir welding robot workstations feature intelligent visual guidance that precisely locates the weld seam, automatic process monitoring with early warning of defective products, all-position welding, and complex-trajectory capability. Mass production reaches welding speeds of 3 m/min with a maximum arm span of 2,826 mm, and the process leaves no burrs — eliminating downstream grinding operations.

● Verification. Weld quality must be proven, not assumed. Automated phased-array ultrasonic testing, dimensional measurement on coordinate measuring machines, and helium leak detection close the loop.

The capability behind it. Deyanfu Machinery relies on the State Key Laboratory of Precision Welding & Joining of Materials and Structures of Harbin Institute of Technology (HIT), and covers the complete FSW process portfolio: bobbin-tool FSW, stationary-shoulder FSW, 3D FSW, refill friction stir spot welding (RFSSW), unequal-thickness welding, and dissimilar-metal welding. Parts up to 3,000 × 2,000 mm in aluminum and copper can be joined in both 2D and 3D paths, and more than 93% of manufacturing processes are completed in-house.

The proof is in the field. Deyanfu was the first — and so far the only — mass-production supplier of friction stir welding robots in China. Its welded circular products alone have reached a cumulative shipment of 150,000 pieces with zero leakage, and its cast-aluminum electronic control housings have exceeded 400,000 units with 100% factory inspection.

If your design involves large aluminum weldments that must stay flat, stay sealed, or join dissimilar metals, friction stir welding deserves a hard look before defaulting to MIG or TIG.