Metal founding – Process – Shaping liquid metal against a forming surface
Patent
1992-07-02
1994-05-31
Bradley, Paula A.
Metal founding
Process
Shaping liquid metal against a forming surface
164125, 164126, 164135, B22D 2104, B22D 2506, B22D 2704
Patent
active
053160703
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an improved process for the production of articles by permanent mould casting of hypereutectic Al-Si alloys. The invention is applicable to the casting of articles by use of gravity and pressure fed permanent and semi-permanent moulds (hereinafter collectively referred to as "permanent moulds").
In recent times, there has been substantial interest in the use of hypereutectic Al-Si alloys, in particular in automotive applications such as for the production of engine blocks and cylinder heads. Examples of alloys considered include our 3HA alloy as disclosed in our Australian patent specification 536976 (and corresponding patents and applications in other countries), and versions of modified 3HA alloy as disclosed in our International patent specifications PCT/AU89/00054 and PCT/AU90/00341. The interest in alloys of the general class encompassing 3HA and modified 3HA alloys is attributable to their wear resistance and, in the specific case of 3HA and modified 3HA alloys, also their improved machinability.
With hypereutectic Al-Si alloys in general, the presence of primary Si particles reduces the machinability of cast articles. Known wear resistant alloys of that general type have been proposed to overcome this problem and to achieve superior wear resistance. Our 3HA and modified 3HA alloys are considered to provide further substantial advances in terms of wear resistance, as well as machinability and control of primary Si formation.
In permanent mould casting of hypereutectic Al-Si alloys, we have found that a particular problem can occur, at least in some instances. For example, while our 3HA and modified 3HA alloys can be used for the production of complex articles by a wide variety of casting techniques, some difficulty still can be encountered with some articles produced by permanent mould casting. Where this is the case, the casting can be found to be characterised by a microstructure which varies between regions of the article, with the variation being considerable in some instances. Thus, with engine blocks produced by conventional low pressure casting techniques, the microstructure in regions above the gates can be unmodified and contain many primary Si particles grading through to regions remote from the gates in which the microstructure is modified and contains few if any primary Si particles.
The variation in microstructure is found not to be able to be eliminated by normal variation in metal temperature, mould or core preheat temperature or mould fill rate, while little if any improvement is achieved with variation in section thickness. However a slight, but insufficient, improvement is found with lower metal temperatures and also lower mould or core preheat temperature.
We have found that the problem of microstructure variation can be overcome by the present invention which entails modification of the casting operation. Also, while the problem encountered with use of our 3HA and modified 3HA alloys can be overcome by the process of the present invention, we have found that the invention also can be used with some benefit in producing articles from other hypereutectic Al-Si alloys.
Our research has shown that the problem addressed by the present invention is in part attributable to an accumulation of heat, in what is herein referred to as the control region of the mould, a region above and extending upwardly from the gate, during an operating cycle of a permanent mould casting operation. As a consequence of such heat accumulation, the temperature of the mould in the control region progressively increases and approaches the temperature of the melt. Due to this, the solidification rate of alloy above and extending from the gate, within the control region of the mould cavity, is insufficient to enable attainment of a microstructure which is substantially the same as that of the remainder of the casting. The heat accumulation in the control region may not be detrimental in an initial operating cycle or initial few cycles but, if this is the case, it is found to become progre
REFERENCES:
patent: 4875518 (1989-10-01), Imura et al.
patent: 4976305 (1990-12-01), Tanaka et al.
Heathcock Christopher J.
Rogers Kevin P.
Bradley Paula A.
Comalco Aluminum Limited
Puknys Erik R.
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