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Heat Treatment, Mechanical Properties and Certification: What OEM Buyers Should Demand on Structural Aluminium Castings
2026-09-18
A Valve Body Passed Pressure Test and Then Cracked in Service
An industrial OEM buyer in the pump sector approved a valve body that passed every dimensional check and a hydrostatic test at the factory. Four months after commissioning, three units cracked at a bolt boss during a routine service and the end customer opened a claim. The investigation found the casting had been shipped in the as-cast condition without the specified T6 treatment, so the tensile strength sat roughly thirty percent below the drawing requirement while the pressure test still passed. The buyer had never required a Heat Treatment chart or a test coupon, and the certificate supplied with the shipment listed only chemical composition.
Why Composition Alone Proves Almost Nothing
A chemical analysis certificate confirms the alloy, not the strength, and this is the gap most buyers leave open. Mechanical properties in aluminium are created by the thermal cycle, not by the melt chemistry alone, so the purchase order must name the temper as explicitly as the alloy. Write the requirement as alloy plus temper plus the property values, for example minimum tensile strength, minimum yield strength and minimum elongation. Ask for the governing standard and revision year as well, because a supplier quoting to a withdrawn standard is quoting to nothing. Accepting a composition-only certificate is the most common shortcut that ends in a field failure.
The Three Heat Treatment Documents to Request
Heat treatment is a process, and a process can only be verified through records. Request three documents with every production batch. The first is the furnace chart showing time and temperature for the solution treatment, the second is the quench record with media temperature and transfer time, and the third is the ageing chart covering the full artificial ageing cycle. Require the batch number on all three to match the casting lot marking. A chart without a traceable batch number is decoration, and the presence of these three records separates a real foundry from a broker faster than any audit question.
Transfer time from furnace to quench is the detail buyers most often skip. A slow transfer drops the achievable strength even when the chart looks perfect, so specify a maximum quench delay in seconds in the technical agreement. This matters most on aluminium alloy gravity castings used for pressure-containing pump and valve bodies, where the wall thickness varies and the quench severity is uneven.
Test Coupons: Separately Cast or Cut From the Part
The strength number has to come from somewhere, and where the coupon comes from changes what the number means. A separately cast coupon is poured alongside the part and cooled faster, so it routinely reports higher properties than the casting itself. A coupon cut from a real production casting tells the truth about the section you care about, and it is the option buyers should insist on for safety-critical or pressure-retaining components. State the coupon location on the drawing so both sides test the same material. Comparing a separately cast coupon against a design minimum is not a valid acceptance.
Matching the Process to the Property Requirement
Not every process can reach every property level, and asking a high-Pressure Die Casting to meet a T6 structural spec is a common mismatch. Gas porosity in die castings blisters during solution treatment, so high-pressure parts are usually supplied in the as-cast or T5 condition at lower elongation. Low pressure casting and Gravity Casting both accept full T6 and deliver the elongation a structural part needs. Fix the process at the same time as the mechanical spec, because changing the process after tooling is cut costs the full tooling budget again.
For thin-wall cosmetic or high-volume parts, a aluminium alloy high pressure die casting is the right economic choice provided the drawing does not demand T6 elongation. Write the real service requirement, not a copied standard value, so the foundry can propose the honest process.
Hardness Screening and When It Is Not Enough
Hardness testing is a fast and cheap incoming screen for heat treatment consistency, and every buyer should run it. It is not a substitute for tensile testing, because hardness correlates but does not equal strength. Use hardness to catch a missed or incomplete ageing cycle across a batch, and reserve destructive tensile testing for first article and periodic audit lots. Record the hardness values per batch and watch the trend, since a drifting average is an early warning that the furnace calibration is slipping.
Where Zinc Still Wins
When the part is small, intricate and not requiring a structural heat treatment, a zinc high pressure die casting often delivers better dimensional stability and longer tool life than aluminium. Challenge the default aluminium assumption on small housings and the total part cost usually falls.
The Bottom Line
Specify alloy, temper and property values together, then demand the furnace, quench and ageing charts with a matching batch number. A certificate that shows only chemistry proves the melt, not the part, and the OEM buyer who accepts one is carrying the field risk personally.
