Aluminum alloy die casting method

Details Aluminum alloy die casting method Aluminum alloy die casting method

1999-08-04

2002-03-05

Lin, Kuang Y. (Department: 1722)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S113000

Reexamination Certificate

active

06352099

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aluminum alloy die casting method which allows for the manufacture of a weldable casting.
2. Description of the Related Art
Casting a molten aluminum alloy in a die in atmospheric air using only gravity as a force is called a “gravity die casting method” or simply a “die casting method”. Such a casting method has been effectively used to manufacture aluminum alloy castings for use as parts of a two-wheeled or lightweight vehicle body, or for engine parts.
However, the gravity die casting has problems, as in a sand mold casting method, such that its long cycle of casting limits productivity, the produced castings have poor dimensional accuracy, and a post heat-treatment is required to improve the strength of the castings.
In an attempt to solve these problems, it became necessary to consider adopting a die casting method which provides improved dimensional accuracy and has an extremely short cycle of casting. The principle of this die casting method resides in press-injecting molten metal into a cavity of a die at a high speed and pressure. In this die casting method, since molten metal is injected at a high speed, air is entrapped in the molten metal and remains as bubbles in a casting, with the result that the bubbles produce blisters on the surface of the casting upon heating of the latter. On the other hand, this die casting method is advantageous in that since molten metal is injected at a high pressure, a die-cast product obtained by the method has a dense structure and a flat cast surface, which lead to increased strength of the product and thus eliminate the necessity for giving a post heat-treatment to the product.
For manufacturing a three-dimensional structure such as a two-wheeled vehicle body, it becomes necessary to weld castings together. A casting obtained by the above-described gravity die casting is weldable but not a casting obtained by the latter die casting method.
Various improved die casting methods have been proposed for the manufacture of weldable die-cast products. An example is Japanese Patent Laid-Open Publication No. HEI-4-172166 entitled “METHOD OF MANUFACTURING ALUMINUM CAST PARTS FOR BRAZING”. According to this method, as shown in the drawing figures of the publication, a flow rate of molten metal at a gate (gate velocity) is switched stepwise between a low flow rate ranging from 0.3 m/sec to 0.6 m/sec for a first half of processing and a high flow rate ranging from 10 m/sec to 30 m/sec for a latter half of the processing.
However, in an associated die casting machine, an expensive control mechanism and a highly advanced control technique are required to switch the moving speed of a piston in the course of forward movement thereof. Further, the die casting machine is required to have increased rigidity so that it can withstand a large accelerating or decelerating force generated due to the change in moving speed of the piston. There has also been known a die casting machine having two cylinders that can be selectively used for effecting the change in moving speed of a piston. Although such a die casting machine facilitates the control of the movement speed of the piston to some extent, it becomes large in size.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an aluminum alloy die casting method which enables the manufacture of a weldable die-cast product without requiring expensive modifications to an existing die casting machine and a highly advanced control technique.
To achieve the above object, according to the present invention, there is provided an aluminum alloy die casting method comprising the steps of providing a die casting machine having a gate for allowing passage of molten aluminum alloy, setting a flow rate of the molten aluminum alloy at the gate to be in a range of 5 m/sec to 15 m/sec, and press-injecting the molten aluminum alloy into a cavity of a die.
With this arrangement, it becomes possible to obtain a weldable casting with no entrapment of air. For example, an aluminum alloy for a vehicular part, which is formed of a die cast product manufactured by the die casting method of the present invention, is weldable and also dense in structure. As a result, these vehicular parts formed of the die-cast products are manufactured on a large scale at a low cost.
As will be described in detail with reference to
FIG. 2
, the amount of gas entrapped in a casting gradually increases with the increase in the flow rate of molten metal passing through the gate. It significantly increases particularly when the flow rate exceeds 15 m/sec. Also, as will be described in detail with reference to
FIG. 3
, the yield strength of the casting is maximized when the flow rate of molten metal passing through the gate is in a range of 7 m/sec to 9 m/sec. The yield strength becomes small when the flow rate is reduced to 5 m/sec, although the amount of gas entrapped in the casting is small at such a flow rate, 5 m/sec. This is believably because molten metal gets cooled and loses some of its fluidity during filling into a mold cavity at a flow rate less than 5 m/sec, thereby causing incomplete filling. In other words, at a flow rate less than 5 m/sec, the die casting cannot fully perform its operation of molten metal filling at a high speed and pressure. In aluminum alloy die casting, it is thus necessary to keep the flow rate at a pouring gate in a range of 5 m/sec to 15 m/sec.
It may be readily appreciated by a skilled artisan that the shape and size of a casting reflects upon the die casting. For example, a time required to fill up a mold cavity becomes larger with the increase in size of a casting. When producing a large-sized casting, there may occur an inconvenience such that an initially injected part of molten metal is solidified before the cavity is completely filled up, or a portion having a thin thickness is solidified in a far shorter time compared to other portions of the casting.
Although the inventive molten metal flow rate at the pouring gate is determined irrespective of the shape and size of a casting, it would be more practical if the shape and size of the casting are taken into consideration.
To this end, by using F. C. Bennett's equation which is based on the thought that filling should be completed within a time of 70% of the time required for complete solidification of molten metal, the present inventors have decided to establish a simplified equation for determining a sectional area of the gate with the flow rate of molten metal passing through the gate, the specific heat of molten metal, the temperature of molten metal, the thickness of a casting, and the like taken into consideration.
The following equation (1) is given by multiplying F. C. Bennett's equation by a modification factor a. A relationship of t=0.808T
2
is obtained by substituting numerical values in variables of the equation, for example, 0.23 and 650 into “c” and “Tm”, respectively.
The density (2.35 g/cm
3
) of molten metal becomes smaller than the density (2.7 g/cm
3
) of the metal in solid state at room temperature due to thermal expansion of the metal.
t
=
α
·
c
⁡
(
Tm
-
Ts
)
+
Ga
4
⁢
λ
⁡
(
Tm
-
Td
)
·
ρ
·
T
2
(
1
)
where
t (filling time): (sec)
&agr; (modification factor): 1.5
c (specific heat of molten metal): 0.23 (cal/g·° C.)
Tm (temperature of molten metal): 650 (° C.)
Ts (temperature of solid line): 598 (° C.)
Td (surface temperature of die): 300 (° C.)
Ga (latent heat of molten metal): 94 (cal/g)
&rgr; (density of molten metal): 2.35 (g/cm
3
)
&lgr; (heat conductivity): 0.33 (cal/cm·s·° C.)
T (thickness of casting): (cm)
The following equation (2) (W=&ggr;·100vn·t·S) is obtained for a casting having a weight W which is manufactured by filling a cavity with molten metal through a gate having a sectional area S at a flow rate v1 or v2 of the molten metal passing through the gate for a filling time t. Numeral 100 on the right side of the equation is

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