Case Study: Friction Stir Welding ADC12 Cast Aluminum to 6061 Extrusions in a Bus Battery Tray

Modern battery trays are a metallurgy puzzle wearing a structural role. The tray must be light, crash-stiff, leak-manageable, and cheap — so designers mix material forms: die-cast nodes and bosses for complex geometry, extruded profiles for rails and frame members, and rolled plate for covers. Each form is easy to make alone. Joining them is the problem, and a recent bus battery tray program shows how one manufacturer solved it.

The part. A commercial bus battery tray for a major Chinese bus OEM, joining ADC12 die-cast components with 6061 aluminum extrusion, using a combination of butt and lap joints — all welded by friction stir welding (FSW).

Why fusion welding struggles here. ADC12 is a near-eutectic Al-Si die-casting alloy (roughly 9–12% silicon). It casts beautifully and welds terribly: die castings trap gas porosity that expands in a molten weld pool, silicon segregation embrittles the joint, and the cast skin's fine structure is destroyed where it melts. 6061-T6 extrusion has its own sensitivity — hot cracking in the weld and softening in the heat-affected zone. Join the two together with an arc and the result is typically porosity-laced, under-strength welds with erratic quality from batch to batch. Many tray programs route around the problem with bolts and adhesives, at the cost of weight, assembly labor, and stiffness.

The solid-state route. Friction stir welding never melts the joint. A rotating tool plasticizes and forges the material, so die-cast porosity isn't boiled out — it's simply never given the chance to expand, because peak temperature stays below melting. Heat input is low, the heat-affected zone is narrow, and distortion stays controllable across a tray-scale part. The weld nugget forms a dense, wrought-like microstructure. For dissimilar pairings like ADC12-to-6061, solid-state stirring disperses rather than concentrates the problematic phases.

Holding flatness at tray scale. A battery tray is a large, thin-walled structure where post-weld flatness determines whether cells, seals, and vehicle mounts align. That makes fixturing and weld-sequence engineering as important as the welding itself: pre-weld machining and assembly fix the datum structure, welding follows sequenced passes that balance distortion, and post-weld precision machining restores mounting faces and sealing surfaces. Deyanfu machines these faces on its own machining centers — 185+ vertical, horizontal, and gantry machines — with positioning accuracy at the ±0.01 mm level, which is what lets a welded tray behave like a machined one.

Verification. Structural welds of this class are checked by automated ultrasonic inspection of the weld seam, dimensional measurement on Hexagon coordinate measuring machines (the largest covering 2,200 × 3,300 × 1,500 mm — big enough for full tray programs), and helium leak detection where the tray also serves as part of the enclosure. The company's friction stir welded circular products have reached 150,000 pieces shipped with zero leakage, and its cast-aluminum electronic control housings have passed 400,000 units with 100% factory inspection — a quality regime applied to every new program.

What the buyer should take away. Cast-to-extrusion joining is where friction stir welding pays for itself fastest: it converts an unreliable fusion problem into a controllable forging process. When evaluating suppliers for welded battery structures, ask for dissimilar-joint strength data, post-weld flatness capability, in-house NDT — and whether they machine the mating faces themselves, because that single question separates assemblers from manufacturers.