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BS L — Overview of Aerospace Wrought Aluminium Alloy Material Standards (Al-Mg-Si-Cu-Cr Alloy Tube and Al-Zn-Mg-Cu-Cr Extruded Bars)
BS L Aerospace Aluminium Alloy Material Standards Overview (BS L 117 and BS L 170)
✈️ BS L is the letter-prefix standard series within the BSI Aerospace Series that specifically governs wrought aluminium alloy materials (tube, bar, section, sheet, and similar). It is overseen by the Aerospace Standards Policy Committee and drafted by Technical Committee ACE/24. This batch covers two single-file standards: BS L 117:1975 specifies requirements for tube of aluminium-magnesium-silicon-copper-chromium alloy (6061 type), and BS L 170:1989 specifies requirements for extruded bars and sections of aluminium-zinc-magnesium-copper-chromium alloy (7075 type). Both standards use “international designations” to correspond to internationally recognised alloys and are widely referenced in the UK aerospace material supply chain.
💡 The BS L series uses a two-layer structure of “material specification plus general inspection procedure”: each material standard (e.g., BS L 117, BS L 170) specifies the composition, condition, and properties of a particular alloy, while the general procedure BS L 100 (Procedure for inspection and testing of wrought aluminium and aluminium alloys) specifies the common inspection and testing requirements. The two are used together.
1. Series Overview 📌
Aerospace imposes extremely demanding requirements on structural materials: light weight and high strength, together with good corrosion resistance, machinability, and weldability. The BS L series was established to meet these needs as a system of aluminium alloy material standards, where each standard corresponds to a specific alloy composition and supply condition and is linked to the internationally recognised aluminium alloy designation (e.g., 6061, 7075).
The two standards in this batch represent two typical applications of aerospace aluminium alloys: BS L 117 is aimed at medium-strength, corrosion-resistant, weldable structural tube (6061 type, often used for fuselage internal members, hydraulic line support structures, and similar); BS L 170 is aimed at high-strength, high-specific-strength primary load-bearing structures (7075 type, often used for wing spars, fuselage frames, landing gear, and other critical load-bearing components). The two differ significantly in chemical composition and mechanical property requirements, reflecting the aerospace principle of selecting materials according to application.
⚠️ Both BS L 117 and BS L 170 are readable-text material standards. This article is based on the standard text (composition tables, heat treatment requirements, mechanical properties, and tolerances); specific values should be confirmed against the official BSI published edition.
2. Standard List and Technical Overview 📋
| Standard No. | Title | International Designation | Product Form |
|---|---|---|---|
| BS L 117:1975 (Confirmed 2011) | Tube of aluminium-magnesium-silicon-copper-chromium alloy — solution treated and artificially aged, not tested hydraulically, not exceeding 10 mm thickness | 6061 | Tube |
| BS L 170:1989 (Confirmed 1999) | Extruded bars and sections of aluminium-zinc-magnesium-copper-chromium alloy — solution treated, controlled stretched, and artificially aged, not exceeding 150 mm diameter or minor sectional dimension | 7075 | Extruded bars/sections |
Both standards are in “Confirmed” status, meaning they were reviewed and confirmed as still current after publication. BS L 117 was amended by Amendment No. 3827 issued in 1982; BS L 170 was amended by Amendment No. 6838 issued in 1991 and partially supersedes the DTD 5124 aviation material specification.
3. Key Technical Requirements 🔧
3.1 Chemical composition
BS L 117 specifies the chemical composition range for the Al-Mg-Si-Cu-Cr alloy (6061 type), with typical control values: silicon 0.40%–0.80%, iron 0.70% max, copper 0.15%–0.40%, manganese 0.15% max, magnesium 0.80%–1.20%, chromium 0.04%–0.35%, with upper limits on zinc (0.25% max), titanium (0.15% max), and other elements (0.05% max each, 0.15% total), remainder aluminium.
BS L 170 specifies the chemical composition for the Al-Zn-Mg-Cu-Cr alloy (7075 type), with typical control values: silicon 0.40% max, iron 0.50% max, copper 1.2%–2.0%, manganese 0.30% max, magnesium 2.1%–2.9%, chromium 0.18%–0.28%, zinc 5.1%–6.1%, titanium 0.20% max, titanium+zirconium 0.25% max, other elements 0.05% max each and 0.15% total, remainder aluminium.
3.2 Supply condition and heat treatment
BS L 117 material shall be supplied solution treated and artificially aged (T6 condition). Heat treatment requirements are: solution treatment by uniform heating at 510 °C–540 °C and quenching in water at a temperature not exceeding 40 °C; artificial ageing by uniform heating at 165 °C–195 °C for 3 h–12 h. The material is tube not exceeding 10 mm thickness, stated as “not tested hydraulically”; tube of similar composition, in the same condition and tested hydraulically, is covered by BS L 118.
BS L 170 material shall be supplied solution treated, controlled stretched, and artificially aged, for extruded bars and sections not exceeding 150 mm diameter or minor sectional dimension.
3.3 Mechanical properties and inspection
BS L 117 specifies minimum tensile test properties for the tube and an optional flattening test: if stated on the order, the specified number of tubes shall be subjected to the flattening test, and the distance between the inner sides of the test piece after flattening shall not exceed the specified values. Material quality is based on aluminium and alloying constituents produced at the manufacturer’s discretion.
BS L 170 presents all technical requirements in tabular form, covering chemical composition, heat treatment, mechanical properties, and inspection and testing procedures. Symbols and units in the tables shall be interpreted in accordance with BS 5555 (SI units), BS 5775 (quantities, units, and symbols), or BS EN 2500 (guidelines for metallic material standards).
✅ Both standards emphasize that “inspection and testing procedures shall be in accordance with the relevant sections of BS L 100,” meaning material inspection must follow the common procedures for sampling, specimen preparation, and test methods, ensuring comparability of results across different material standards.
4. Representative Standard Technical Interpretation 🔍
BS L 117:1975 (6061-type alloy tube)
Scope: Applies to tube of Al-Mg-Si-Cu-Cr alloy (6061), solution treated and artificially aged (T6), not exceeding 10 mm thickness, not tested hydraulically.
Key points: The 6061-type alloy belongs to the heat-treatable Al-Mg-Si family, offering medium strength, excellent corrosion resistance, good weldability, and cold workability. BS L 117 controls chromium (0.04%–0.35%) to inhibit intergranular corrosion and refine grain size, thereby improving corrosion resistance; a strict solution and ageing schedule produces a stable T6 condition. The standard also specifies thickness tolerances and tolerances on outside/inside diameters of drawn round tube (Tables 1 and 2), ensuring dimensional suitability for aircraft assembly.
BS L 170:1989 (7075-type alloy extruded bars and sections)
Scope: Applies to extruded bars and sections of Al-Zn-Mg-Cu-Cr alloy (7075), solution treated, controlled stretched, and artificially aged, not exceeding 150 mm diameter or minor sectional dimension.
Key points: The 7075-type alloy belongs to the Al-Zn-Mg-Cu ultra-high-strength family and is among the most widely used high-strength structural materials in aerospace. Its high strength comes from the combined ageing strengthening of zinc, magnesium, and copper, with a small chromium addition to improve stress-corrosion resistance. BS L 170 supplies the material in the “controlled stretched” condition to relieve extrusion residual stress and reduce part distortion during machining, which is especially important for large structural components. The standard presents all technical requirements in tabular form for convenient reference by material suppliers and inspectors.
5. Engineering Practice and Application 🛠️
In aerospace material management, BS L 117 and BS L 170 are used in the following stages:
- Procurement: Purchase documents state the material standard number, condition, dimensions, and inspection grade; suppliers manufacture to the standard and issue material certificates.
- Incoming inspection: Sampling and testing follow BS L 100 general procedures, verifying chemical composition, mechanical properties, and dimensional tolerances, with supplementary tests such as the flattening test where specified.
- Process control: For the 7075-type high-strength alloy, note its stress-corrosion sensitivity; avoid excessive residual tensile stress during machining and assembly, and apply appropriate surface protection.
- Design selection: Designers weigh medium-strength corrosion-resistant (6061 type) versus high-strength (7075 type) options based on loading and environment, and determine product size within the standard’s specified dimensions.
⚠️ The 7075-type alloy has poor weldability and is generally not recommended for structures requiring welding; the 6061 type has good weldability. Selection should consider the product form and service conditions specified in the standards.
6. FAQ ❓
What is the difference between BS L 117 and BS L 118?
Both have the same chemical composition (6061 type). The main difference is that BS L 117 tube is “not tested hydraulically,” while BS L 118 tube is tested hydraulically. The standard note states that tube of similar composition, in the same heat treatment condition and tested hydraulically, is covered by BS L 118.
Both have the same chemical composition (6061 type). The main difference is that BS L 117 tube is “not tested hydraulically,” while BS L 118 tube is tested hydraulically. The standard note states that tube of similar composition, in the same heat treatment condition and tested hydraulically, is covered by BS L 118.
Why is the 7075-type alloy supplied in the “controlled stretched” condition?
Extruded bars and sections carry significant residual stress after extrusion; machining such material directly would cause part distortion. Controlled stretching (applying a small plastic stretch after solution treatment and before ageing) significantly reduces residual stress and improves dimensional stability, which is important for large precision structural components.
Extruded bars and sections carry significant residual stress after extrusion; machining such material directly would cause part distortion. Controlled stretching (applying a small plastic stretch after solution treatment and before ageing) significantly reduces residual stress and improves dimensional stability, which is important for large precision structural components.
Why must BS L series material standards be used with BS L 100?
BS L 100 specifies the general procedures for inspection and testing of wrought aluminium and aluminium alloys (sampling, specimen preparation, test methods, and acceptance criteria). Each material standard specifies only the composition, condition, and properties of a particular alloy; together with the general procedure, they form a complete inspection and acceptance system.
BS L 100 specifies the general procedures for inspection and testing of wrought aluminium and aluminium alloys (sampling, specimen preparation, test methods, and acceptance criteria). Each material standard specifies only the composition, condition, and properties of a particular alloy; together with the general procedure, they form a complete inspection and acceptance system.
Have these two standards been withdrawn?
Both are in “Confirmed” status, meaning BSI has reviewed and confirmed them as still current. BS L 117 was confirmed in 2011 and BS L 170 in 1999. Verify the latest confirmation status and amendment records before use.
Both are in “Confirmed” status, meaning BSI has reviewed and confirmed them as still current. BS L 117 was confirmed in 2011 and BS L 170 in 1999. Verify the latest confirmation status and amendment records before use.
How do BS L designations correspond to international designations?
The chemical composition of BS L 117 corresponds to International Designation 6061, and BS L 170 corresponds to International Designation 7075. Both establish correspondence with the international aluminium alloy designation system.
The chemical composition of BS L 117 corresponds to International Designation 6061, and BS L 170 corresponds to International Designation 7075. Both establish correspondence with the international aluminium alloy designation system.