An EvAluAtion of QuAlity PArAmEtErs for HigH PrEssurE DiE cAstings
R. Lumley, N. Deeva and M. Gershenzon CSIRO Future Manufacturing Flagship, Victoria, Australia
Copyright © 2011 American Foundry Society Abstract
Several techniques for examining casting “quality” as it relates to high pressure diecast alloy A380 have been evaluated in the as-cast condition. The roles of three simple parameters were considered: a) metal velocity at the gate, b) the effect of increased Cu or Zn content, and c) the effect of rotary degassing on a recycled melt. It was shown that tensile failure in high pressure die casting (HPDC) specimens is influenced by complex defect clusters and the interaction of a variety of casting defects. The two major defect cluster types identified in the current work were comprised of a dispersed foam-like shrinkage defect, and/or large oxide films present on the fracture surfaces. The removal of hydrogen had little effect on average tensile properties which was a surprising
measures of comparative “Quality” of Al castings statistical techniques
One significant advantage of the HPDC manufacturing route is the very high production rates with which parts may be produced. With regards to laboratory testing, this means that large numbers of nominally identical samples can be produced without substantial variations in prop- erties, composition, microstructure or dimensions. Since HPDC tensile test bars require no machining, and their gage dimensions are identical within a very small mar- gin of variation, rapid determination of large numbers of tensile results is possible. This means an examination of several different techniques for casting quality may be quantitatively tested.
± 3 sigma Analysis
It is a common practice for buyers of aluminium castings to specify values of -3σ for the products they purchase from metalcasting facilities. That is, the lower bounds are speci- fied as the mean minus three times the standard deviation of a statistically relevant number of test samples. This tech- nique has excellent utility in that it is easy to use and in the- ory, only 0.27% of products manufactured will fall outside the 3σ values. (i.e., 0.135% above or below the +3σ or -3σ values respectively) In the context of aluminium castings,
International Journal of Metalcasting/Summer 2011
result, but rotary degassing did appear to remove a portion of the oxides present in the melt, thereby improving casting quality. It is shown that of the different analyses conducted, all could differentiate a degree of casting quality, but some techniques (i.e., Weibull statistics combined with flow curve derivations based on the Ludwik-Holloman equation) are particularly useful. It is proposed that complex strain localization and failure occurs in HPDC specimens, which results in a proportionately large fraction of defects appearing on the fracture surface.
Keywords: high pressure diecasting, HPDC, casting quality, A380 aluminium
another advantage is that an adverse change in quality may be quickly detected by comparing results against a known baseline value of -3σ. This procedure also allows for a rapid comparison of the same products manufactured by different facilities, or even different machine operators. The disad- vantage of this technique is that it assumes a normal distribu- tion of data exists, which is often not true for a statistically large number of castings.
Weibull Analysis
It has been proposed that examination of the Weibull dis- tribution is a better test of quality in aluminium castings1,2 than -3σ. Weibull statistics were first developed for brittle ceramic materials, and have the major advantage in that adjustments can be made based on the size and scale of the product being made. Another benefit is that the Weibull modulus, “m,” is a useful means by which to compare components made by different processes. As the value of m increases, the narrower the range of tensile strengths (and ductility) which will exist, so reliability is improved. This approach has many merits for aluminium high pres- sure diecastings. As with other brittle materials, the tensile strength data of castings in practice exhibits a wide amount of scatter, and the Weibull distribution provides a simple method to determine accurately the probability of failure at any given stress. A practical method for using Weibull analysis is summarized from Davies:3
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