An EvAluAtion of QuAlity PArAmEtErs for HigH PrEssurE DiEcAstings PArt ii: HEAt trEAtmEnt
R. Lumley and N. Deeva CSIRO Future Manufacturing Flagship, Victoria, Australia Copyright © 2011 American Foundry Society Abstract
In our earlier paper, comparative quality of as-cast high pressure diecastings (HPDC’s) was evaluated using Weibull statistics combined with flow curve and defect equivalence derivations based on the Ludwik-Holloman equation.1
Here, we extend this work to examine the effect
of heat treatment on one A380 composition. Samples were examined in as-solution treated (solution treated, quenched, then immediately tested), T4 (solution treated, quenched then naturally aged) and T6 (solution treated, quenched and artificially aged) tempers, with comparisons made against the as-cast condition. Solution treatment appeared to remediate, or otherwise render innocuous, a
Heat treatment of High Pressure Diecastings
Successful heat treatment of conventionally produced high pressure diecastings (HPDC’s) has been shown to be possi- ble by the use of severely truncated solution treatment times at lower than normal temperatures (e.g. 15 minutes at 430- 490C [806-914F]).2-8
This procedure avoids the well-known
problems associated with formation of surface blisters and dimensional instability, both of which occur during the con- ventional heat treatment of HPDC’s.
Following solution treatment and quenching, HPDC’s may be heat treated to a wide variety of different tempers (e.g. T4, T6, T7), which vary in the time and temperature of the procedures utilized. In the solution treated condition (with no age harden- ing), the 0.2% proof stress of a HPDC alloy component de- creases by around 30%, the tensile strength is little changed, but the ductility is increased.3
If the material is then aged to
a T4 temper by exposure at ambient temperatures (e.g. 25C [77F]), the properties continue to rise slowly until they sta- bilize in around four days (See Figure 1). For a T4 temper in A380 alloy, the 0.2% proof stress is around 25% higher than as-cast; increases to the average tensile strength and ductility (above the as-cast condition) are also observed.3
Alternately,
if the alloy is artificially aged to a T6 temper at an intermedi- ate temperature, (e.g. 150-180C [302-356F]), the properties rise more quickly to high levels of hardness and strength (Fig- ure 1).3
may be doubled, and the tensile strength increased, but usually only a slight reduction in elongation at failure (Ef
For example, in a T6 temper, the 0.2% proof stress ) is observed
International Journal of Metalcasting/Fall 2011
Figure 1. A hardness-time curve showing the T4 (at 25C [77F]) or T6 (at 150C [302 F]) response to heat treatment of A380 alloy following solution treatment at 490C (914F) for 15 minutes and water quenching.3
portion of the complex defect arrangement present in the die-casting through microstructural changes, resulting in an improvement in casting quality. Similar to the as-cast condition, the lower limits of quality for solution treated and T4 treated alloy were related to oxide films or flakes present on the fracture surfaces. The T6 treated material did not noticeably exhibit these films or flakes, and the equivalent fraction of defects present on the fracture surface was reduced.
Keywords: high pressure diecasting, quality index, A380 aluminium, heat treatment
when compared to the as-cast condition. For the T4 temper in an A380 alloy, the precipitates that give rise to strengthening within the aluminium grains are typically very fine clusters or GP zones. In the T6 (and T7) temper, the precipitates present within the aluminium grains are mostly θ' (Figure 2). For al- loys of different compositional specifications, different pre- cipitates may form in peak aged conditions, such as the L, Q', or β" phases, for example.8
Alloy
composition was Al-9Si-3.1Cu-0.1Mg-0.53Zn-0.86Fe- 0.16Mn-<0.2(others total).
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