Friction stir welding (FSW)
The usual method for joining wide floor extrusions side by side. Solid-state, low distortion, very flat panels and leak-tight joints with less strength loss than fusion welding.
Electric vehicle materials guide
Aluminium for EV Battery Trays and Battery Enclosures
Most electric-vehicle battery trays are built from 6xxx aluminium extrusions welded into a frame, closed with an extruded or sheet floor and an aluminium, steel or composite lid. Aluminium gives the stiffness, crash energy absorption and corrosion resistance the pack needs at a fraction of the mass of steel. This guide explains which alloys go where, how they are joined and sealed, and what the tray must survive. Metline supplies the extrusions, sheet and plate, cut and machined on request.
- Frame extrusions
- 6005A, 6082, 6061 (T6)
- Floor and covers
- 5754, 5182, 6xxx sheet
- Joining
- FSW, MIG, laser, CMT, rivets
- Pack tests
- UN ECE R100 · GB 38031 · IP67
What a battery tray must do
The battery tray (or lower housing) carries the cells or modules, bolts the pack into the vehicle body and is usually part of the car’s structure. It has to stay stiff under the weight of several hundred kilograms of cells, protect them from side-pole impacts and from objects striking the underbody, keep water and dust out for the life of the car, carry the cooling system, and contain a cell fire long enough for occupants to leave.
Aluminium meets these demands with low density (2.70 g/cm³, about one third of steel), extrusions that put material exactly where it is needed, good energy absorption when hollow sections fold in a crash, natural corrosion resistance against road salt, and high thermal conductivity that helps spread heat to the cooling plates. Because the pack is one of the heaviest components in an EV, saving mass in the tray directly increases range or allows a smaller battery for the same range.
Most volume trays today use a hybrid design: a welded frame of extruded side rails and cross members, a floor made of welded multi-hollow extrusions or a sheet pan, and a top cover. High-pressure die castings are also used for some trays and for parts of others, and steel trays remain common where cost matters most.
Which alloy goes where in a battery tray
| Component | Typical product form | Typical alloys and tempers | Why |
|---|---|---|---|
| Side rails / perimeter frame | Multi-cavity hollow extrusions | 6082-T6, 6005A-T6, 6061-T6 | High strength and energy absorption in side impact; good weldability |
| Cross members | Hollow or open extrusions | 6005A-T6, 6063-T6, 6082-T6 | Carry module mass and stiffen the frame; 6005A extrudes thin walls well |
| Floor (extruded) | Wide multi-hollow extrusions welded side by side | 6005A-T6, 6063-T6, 6061-T6 | Stiff, flat floor with optional integrated cooling channels |
| Floor (sheet) and underbody shield | Sheet or plate | 5754-H22 / O, 5182-O, 6061-T6 | Formable, weldable and impact-resistant; 5xxx resists corrosion without heat treatment |
| Top cover / lid | Stamped sheet | 5182-O, 5754-O, 6016-T4 | Deep-drawable, lightweight; steel or composite lids are also common |
| Brackets and mounting blocks | Plate, bar or forgings, machined | 6061-T6, 6082-T6 | Machined interfaces for bolting the pack to the body |
| Cooling plates | Brazing sheet, multi-port extrusions | 3003 and clad 4343 / 3003 brazing sheet, 6063 | High thermal conductivity, brazeable, leak-tight |
Alloy choice varies by vehicle maker; the table shows common practice, not a single standard.
6082 is the stronger choice for crash-critical rails and is standard in Europe; 6005A offers a better balance of strength and extrudability for large, thin-walled multi-hollow sections, so it dominates wide floor profiles; 6061 is common in North America and Asia, and 6063 is used where strength demands are lower and surface or thin-wall extrusion matters more.
Minimum mechanical properties of common tray alloys
| Alloy and temper | Form | Tensile strength Rm (min) | Yield Rp0.2 (min) | Elongation A (min) |
|---|---|---|---|---|
| 6082-T6 | Extruded profile, wall 5 – 25 mm | 310 MPa | 260 MPa | 8 – 10 % |
| 6082-T6 | Extruded profile, wall ≤ 5 mm | 290 MPa | 250 MPa | 6 – 8 % |
| 6005A-T6 | Extruded profile, wall ≤ 5 mm | 270 MPa | 225 MPa | 6 – 8 % |
| 6061-T6 | Extruded profile | 260 MPa | 240 MPa | 8 – 10 % |
| 6063-T6 | Extruded profile, wall ≤ 10 mm | 215 MPa | 170 MPa | 6 – 8 % |
| 5754-H22 | Sheet | 220 MPa | 130 MPa | By thickness |
| 5182-O | Sheet | 255 MPa | 110 MPa | By thickness |
Minimum values summarised from EN 755-2 (extrusions) and EN 485-2 (sheet). Elongation and some limits depend on wall thickness and on whether the profile is open or hollow; the certificate for each lot shows the actual values.
Two property issues dominate tray design. First, welding softens 6xxx alloys: the heat-affected zone of a fusion weld in 6082-T6 can lose roughly a third to a half of its yield strength, so designers place welds in low-stress areas or use friction stir welding, which runs cooler. Second, crash behaviour depends on ductility, not just strength. Rails must fold progressively without cracking, so vehicle makers often specify crush tests on the extrusion and control quench rate and ageing to achieve it.
Joining, sealing and machining the tray
MIG and CMT welding
Used for frame corners, cross-member joints and brackets. Filler 4043 or 5356 depending on joint design; low-heat-input processes such as cold metal transfer limit distortion.
Laser welding
High speed and low heat input for thin sheet and lid flanges; requires good fit-up.
Mechanical joining and bonding
Self-piercing rivets, flow-drill screws and structural adhesive, often combined, especially for joining aluminium to steel or composite.
Sealing
Lid-to-tray seals use foam gaskets or formed-in-place sealant on a machined flange; flatness of the flange and leak testing of every pack are critical.
Machining
Frames are CNC machined after welding to give accurate mounting faces, sealing flanges and module seats; long-bed five-axis machines are typical.
Distortion is the constant enemy. A tray two metres long assembled from many welded extrusions can move several millimetres if weld sequence and fixturing are not controlled, and the sealing flange and module seats must then be machined flat. Specifying extrusions with tight straightness and twist tolerances reduces the work needed after welding.
Designing extrusions for trays
- Multi-cavity sections: internal webs stiffen rails and floors and give crash load paths; keep wall thickness as uniform as possible for consistent extrusion
- Wide integrated floors: fewer, wider profiles mean fewer welds, less distortion and fewer leak paths, but need larger presses and careful control of flatness
- Integrated cooling: coolant channels can be extruded into the floor, avoiding separate cooling plates, though designs with separate brazed plates are easier to repair and modify
- Weld preparation: design tongue-and-groove or backing features into the section to locate profiles for friction stir welding
- Tolerances: agree straightness, twist, flatness and wall tolerances to EN 755-9 or EN 12020-2 (for 6060/6063 precision profiles) or tighter where the part will be machined
- Temper sequence: profiles are often bent or formed in T4 and aged to T6 afterwards; define the delivery temper with the forming plan
Standards and tests the pack must pass
| Requirement | Typical standard | What it means for the tray |
|---|---|---|
| Battery system safety (REESS) | UN ECE Regulation No. 100 | Vibration, thermal shock, mechanical shock and crush, fire resistance and other tests at pack level |
| China traction battery safety | GB 38031 | Includes bottom impact and thermal propagation requirements |
| Electric vehicle safety | ISO 6469-1 | Safety specification for on-board energy storage |
| Ingress protection | IEC 60529 (IP67), ISO 20653 (IP6K9K) | Seals and flange design; leak testing of every pack |
| Transport of lithium batteries | UN Manual of Tests and Criteria, section 38.3 | Applies to cells and packs shipped separately |
| Material | EN 755-2, EN 485-2, ASTM B221, ASTM B209 | Mechanical properties and tolerances for extrusions and sheet |
Aluminium melts at around 600 – 650 °C, below the temperatures reached in a cell thermal runaway, so a bare aluminium lid can burn through over a failing cell. Designs counter this with mica, ceramic-fibre or intumescent layers under the lid, venting paths that direct hot gases away from the cabin, and steel or composite lids in some packs. The tray frame and floor are usually less exposed but are included in the fire resistance test.
Corrosion protection and surface treatment
Under the car, the tray sees salt spray, stone chipping and standing water. 6xxx and 5xxx alloys resist this well on their own, but trays usually receive a conversion coating and paint, cathodic e-coat or a stone-chip protective coating on the underside. Where steel fasteners or steel body parts touch the aluminium, isolating washers, coated fasteners and sealants prevent galvanic corrosion.
Coolant leaks are another concern. Glycol-water coolants are compatible with aluminium when inhibited correctly, but brazed or welded cooling channels must be leak-tested and the coolant chemistry maintained, since uninhibited or contaminated coolant can pit aluminium from the inside.
Supplying material for a tray project
- Share the drawings2D and 3D drawings of extrusion sections and components, or a BOM for the whole tray.
- Confirm materialsAlloy, temper and material standard (EN 755-2, ASTM B221, OEM specification) and any crush, bend or microstructure requirements.
- Define tolerancesSection dimensions, straightness, twist, flatness and critical-to-quality features.
- Choose the delivery conditionMill-length extrusions, cut-to-length, or drilled, milled and tapped semi-finished parts on request; sheet and plate cut to size.
- Plan the programmePrototype, pre-series or volume, with expected annual volume and inspection documents (EN 10204 3.1 certificate, dimensional reports).
- ShipExport packing that protects long profiles from bending and surface damage, on EXW, FOB, CIF or DAP terms.
Metline supplies the aluminium extrusions, sheet, plate and bar for battery tray programmes, with cutting and machining on request. It does not design or assemble complete battery packs or trays; the tray maker remains responsible for welding, assembly, sealing and validation testing.
References and standards
- UN ECE Regulation No. 100 – Uniform provisions concerning the approval of vehicles with regard to specific requirements for the electric power train
- GB 38031 – Electric vehicles traction battery safety requirements
- EN 755-2 – Extruded rod/bar, tube and profiles: mechanical properties
- EN 485-2 – Sheet, strip and plate: mechanical properties
- IEC 60529 – Degrees of protection provided by enclosures (IP Code)
- Oak Ridge National Laboratory – Stress characterization of friction stir welded electric vehicle battery trays
Aluminium (aluminum) for EV battery trays: FAQ
Which aluminium alloy is used for EV battery trays?
Frames and floors are mostly 6xxx extrusions: 6082-T6 for the strongest crash rails, 6005A-T6 for wide thin-walled floor profiles, and 6061-T6 and 6063-T6 in many designs. Sheet floors and lids use 5754 or 5182, and machined mounts use 6061 or 6082 plate and bar.
Why use aluminium instead of steel for battery enclosures?
Aluminium gives a substantially lighter tray for the same stiffness, absorbs crash energy well in extruded hollow sections, resists road-salt corrosion and conducts heat. Steel is cheaper and more fire-resistant, so some packs use steel lids or steel trays where cost or fire protection dominate.
How are aluminium battery trays welded?
Wide floor extrusions are usually friction stir welded, which gives flat, leak-tight joints with limited strength loss. Frame corners and brackets are MIG or CMT welded, lids are often laser welded or bolted, and mixed-material joints use rivets, flow-drill screws and adhesive.
Does welding weaken 6xxx alloys in a tray?
Yes. Fusion welding softens the heat-affected zone of T6 material significantly, so welds are placed in lower-stress areas and the design uses reduced HAZ strength. Friction stir welding reduces, but does not remove, this softening.
Can an aluminium battery tray contain a thermal runaway?
Not by itself, since aluminium melts at about 600 – 650 °C. Packs use mica, ceramic or intumescent barriers, venting paths and sometimes steel or composite lids to contain or direct the heat and gases during the time required by regulations.
Can Metline supply a complete battery tray?
No. Metline supplies the aluminium materials: custom extrusions, sheet, plate and bar, with cutting and machining on request and EN 10204 3.1 certificates. Welding, assembly and pack validation remain with the tray manufacturer.
Sourcing aluminium for a battery tray programme?
Send extrusion drawings, alloys, tempers and volumes. We reply with supply options, certification and a delivered price.
