After ordering 7075 aluminium from a motorsport supplier, you are sent a PDF document that contains a material certificate, which contains numbers, codes, and technical jargon. Building safety-critical parts is one of the fundamentals of engineering, and understanding material certificates is the only proof you have that what you received is what you ordered. Understanding the certificates has a huge impact, for example, as avoiding guesswork and expenses and getting certified components.
A material certificate is also known as a mill certificate or material test report. It documents the grade of materials provided by a manufacturer. It also certifies that the materials conform to the relevant specifications and the documents that particular piece of metal to a certain point in the production process. It shows the batch, the production time, and what its mechanical characteristics are.
The importance of certification levels is subtle but vital. A standard motorsport certificate is 3.1. This means that the material has gone through manufacturer testing and has been verified by a third-party inspection body. This means that the test results come from actual samples of the batch instead of just presumptive estimates based on that grade. A 2.2 certificate means manufacturer testing, but no third-party verification. This means the manufacturer testing could be faulty, and you could end up with shoddy materials. For anything structural or safety-critical, you want 3.1 certification. It’s a bit more expensive, but you do get more pieces of paper.
Typical certificates have the material grade near the top, but you have to learn to read them. For aluminum, you may see “EN AW-7075” or “AA 7075-T6” and others. The EN prefix means it is up to European standards, and AA means it is also up to North American standards through the Aluminum Association. The T6 means it is solution heat treated and artificially age hardened to full strength. If you get pieces of material that do not have the heat treatment code, it means that you may get the right alloy on paper, but you will get the wrong mechanical properties.The section on chemical composition is perhaps the most dry and mechanical section to engage with, looking like an elementary school chemistry exam with the percentages of the different alloying elements (zinc, magnesium, copper, etc.), and numbers that must fall within a specified range of grades. For example, on a 7075 aluminium, you’re looking at zinc from 5.1 to 6.1%, magnesium from 2.1 to 2.9%, and copper from 1.2 to 2.0%. If these numbers do not fall within these ranges, you are not getting 7075 aluminium, regardless of what the label says. This is important, since the alloying elements determine the strength and other important materials properties of the material.
The mechanical properties section is the other main part of a material cert. Here you see tensile strength (Rm), yield strength (Rp0.2), and elongation values. For a 7075-T6 plate, you expect tensile strength to be around 540 to 570 MPa and yield strength to be (at minimum) 470 MPa. These are tested values from specific batches and not handbook values. If a cert shows yield strength is below the specification, that material is not fit for the purpose, regardless of what grade the material is meant to be.
The percentage of elongation indicates ductility, i.e. how much a material will stretch before failing. For 7075-T6, this is commonly in the 6-8% zone and lower % numbers are typically indicative of material brittleness, which is a tendency to fail suddenly under load. This is especially important for elements that are intended to withstand impact loads or endure fatigue cycling.
The batch number/heat number provides traceability. This alphanumeric number tells you which production batch your material is from. If you ever have a recall or discover a quality issue with a batch, you can use this number to see if it impacts your material. For liability purposes, it is important to keep records of heat numbers for safety critical components such as rollcage tubes or suspension parts.
Dimensional tolerances show up for plate, sheet, and bar stock. A certificate might say “6mm plate”, but the actual measured thickness could be 6.15mm. This is important for component design with exact thickness requirements, or for strength estimations that rely on cross-sectional measurements.
Surface finish specifications use “mill finish”, which means as rolled surfaces. Other designations mean polished or brushed finishes. This affects how the material machines and how it needs to be prepared for welding or coating.
Standard testing occurs around 20°C, but the test temperature line exists because properties change with temperature. If your material will operate at high temperature, the standard test results will likely differ from what you will see in service.
Vendor supplied documentation means that you will be supplied a traceable certificate for the materials ordered. Components that have certifiable, traceable construction are considered more dependable. Components like un-load bearing structural brackets, forged, and un-certified brackets do not give you traceable construction.
These components will inform you what construction processes were done to the components, like what heat treatments, and what grain flows were tested. Components like these are important when strength design factors are pertinent to the designs.
These offerings are costly because of the traceable nature of the construction. These certificates are important for construction process documentation, and for proving due-diligence for rules, standards, and compliance during construction. These paper trails, while looking like a lot of bureaucratic, un-organized numbers, prove that valuable time and resources were put into the construction of components.
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