comparative Quality of Aluminium High Pressure Diecastings
One of the primary advantages of the high pressure diecast- ing manufacturing route is the very high production rates. For laboratory testing, large numbers of nominally identi- cal samples can be produced quickly and without substantial variation in geometry, composition or mechanical proper- ties. Samples also do not require machining prior to testing. The above factors allow for statistically large numbers of cast specimens to be readily evaluated.
od of examining casting quality, but the earlier work also re- vealed that it was most useful when used together with other techniques. (Here we use the notation σ(sd)
For as-cast diecastings,1 mean and σ(sd)
is the standard deviation) provided a rapid meth- the values of µ-3σ(sd) to differentiate from
stress, σ). Weibull analysis was a particularly powerful tool for examining casting quality in large numbers of test samples, pro- viding a robust evaluation of the flaw size distribution.1
Casting
quality determined in parallel using the methods of Cáceres9,10 which are based on the flow curve described by the Ludwik- Holloman equation, was also extremely useful. Further to these procedures, the Cáceres-Selling model,11
which relates the
elongation during a tensile test to the equivalent defect frac- tion present on the fracture surface, provides a complimentary assessment of the relative quality of castings. Two key defects identified in the earlier work1
for the as-cast condition that led
to lower levels of tensile strength and elongation in HPDC test bars were found to be a) dispersed, foam-like shrinkage defects (which were particularly prominent on the fracture surfaces of samples cast at 26 m/s), and b) large oxide flakes (present in all conditions tested). Both of these defect types were present and identified on the fracture surfaces of samples following tensile
Weibull statistics are an excellent means of evaluating cast aluminium materials.12
Weibull analysis works on the prin-
ciple that, as the value of the Weibull modulus “m” gets larger, the narrower is the range of properties (e.g. strength) which exists since the flaw size distribution is reduced.13 Therefore, high values of m mean superior reliability is achieved, and this also means quality is improved. Weibull analysis has added usefulness, since it can be scaled to dif- ferent (component) sizes and different loading conditions. If a large enough number of samples are used in deriving the data set (e.g.>30),12
a means to describe the probability of failure in even larger numbers of castings, for example 105
extreme value statistics may be used as , or 106
cal production run volumes of HPDC castings. , similar to typi- (where µ is the
testing. In the current paper, the above mentioned techniques for evaluating quality are again used, here to specifically ex- amine the role of heat treatment on casting quality. Overviews of these techniques were provided in our earlier paper1
be quickly detected by comparing results against a known base- line value of µ-3σ(sd)
the data falls below µ-3σ(sd) . The major disadvantage of this technique
is that it assumes a normal distribution of data exists, which is often not true for a statistically large number of castings.
Weibull Analysis
HPDC’s. This approach has excellent utility because µ±1σ(sd) represents 68.2% of the data, µ±2σ(sd) data, and µ±3σ(sd)
summarized in the following sections. ±3 Sigma Analysis The values of µ-3σ(sd)
and are
are commonly used to evaluate quality in represents 95.4% of the
represents 99.7% of the data. Only 0.135% of . An adverse change in quality may
(a)
(b) Figure 2. TEM Images showing a) aluminium grains and Si phase present in T6 treated A380 alloy.6
the aluminium grains, showing two variants of the θ’ phase, Al2 of the alloy is the same as for Figure 1.
48 Also shown within
the aluminium grains and on grain boundaries are insoluble intermetallic dispersoids, less than 0.5 µm in size. The grain located centrally in the image is close to the [001]α
orientation. Image b) shows the precipitate structure within Cu viewed from the [001]α
orientation. The composition International Journal of Metalcasting/Fall 2011
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