EP1081243A1

EP1081243A1 - Member formed from magnesium alloy

Info

Publication number: EP1081243A1
Application number: EP00117254A
Authority: EP
Inventors: Kazuo Sakamoto; Motoyasu Asakawa; Yukio Yamamoto
Original assignee: Mazda Motor Corp
Current assignee: Mazda Motor Corp
Priority date: 1999-09-06
Filing date: 2000-08-14
Publication date: 2001-03-07
Also published as: JP2001073059A; KR20010030267A

Abstract

The present invention relates to a member formed by magnesium alloy which has a solid phase fraction of more than 0% to 60% or less, contains 2.0 wt% or more to 6.5 wt% or less of aluminum, and has an internal defective ratio of 1% or less and a strain rate of 1 x 10²/s or more.

Description

The present invention relates to a member formed by magnesium alloy being excellent in high-speed deformation characteristics.
Alloy components molded by die casting or gravity casting using aluminum, magnesium, and the like as raw materials are used as automobile components such as instrument panels which are required to absorb energy at the time of collision.
Japanese Patent Laid-Open No. 9-272945 proposes a heat-resistant magnesium alloy, which contains 2 to 6 wt% of aluminum, 0.5 to 4 wt% of calcium, and the balance of magnesium and inevitable impurities, has a calcium to aluminum ratio of 0.8 or less, and is excellent particularly in molding and elongation while maintaining excellent creep resistance.
However, no high-speed deformation characteristics are considered for the above prior-art molded component. A component molded in a semi-solid state is poorer in high-speed deformation characteristics than a component molded in a perfect molten state due to internal defective ratio and elongation. When a component molded in a semi-solid state is applied to an automobile component, energy absorption at the time of collision may be unsatisfactory.
The present invention has been made in consideration of the above situation, and has as its object to provide a member formed by magnesium alloy, which is excellent in molding, tensile strength, and high-speed deformation characteristics and can assure a high elongation.
In order to solve the conventional problem described above and achieve the above object, according to the first aspect, there is provided a member formed by magnesium alloy, which is excellent in molding, tensile strength, and high-speed deformation characteristics and can assure a high elongation, comprising a portion which is excellent in high-speed deformation characteristics, has a solid phase fraction of more than 0% to 60% or less, contains 2.0 to 6.5 wt% of aluminum, and has a strain rate of not less than 100/s. The member formed by magnesium alloy can be used as, e.g., a collision energy absorption member for an automobile.
According to the second aspect, there is provided member formed by magnesium alloy, which is excellent in molding, tensile strength, and high-speed deformation characteristics and can assure a high elongation, comprising a portion which is excellent in high-speed deformation characteristics, has a solid phase fraction of more than 0% to 60% or less, contains 2.0 to 6.5 wt% of aluminum, and has a strain rate of not less than 100/s. The member formed by magnesium alloy can be used as, e.g., the collision energy absorption member of an automobile.
According to the third aspect, in member formed by magnesium alloy whose high-speed deformation characteristics are adversely affected by a surface defective portion, a molded surface is left to inhibit the defective portion from appearing on the surface of the member to assure excellent high-speed deformation characteristics, although the defective portion (pore) tends to form at the central portion of a molded product in relation to a cooling rate.
According to the fourth aspect, there is provided a member formed by magnesium alloy, which is excellent in molding, tensile strength, and high-speed deformation characteristics and can assure a high elongation, wherein a local internal defective ratio is not more than 1%. The member formed by magnesium alloy can be used as, e.g., a collision energy absorption member for an automobile.
According to the fifth aspect, there is provided a member formed by magnesium alloy, which is excellent in molding, tensile strength, and high-speed deformation characteristics and can assure a high elongation, wherein an overall internal defective ratio is not more than 1%. The member formed by magnesium alloy can be used as, e.g., a collision energy absorption member for an automobile.
According to the sixth aspect, the aluminum content is 3.0 to 6.5 wt% to improve the fluidity of the raw material in molding.
According to the seventh aspect, the solid phase fraction is more than 0% to 40% or less to improve the fluidity of the raw material in molding.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.
Fig. 1 is a schematic view showing the main part of a semi-solid injection molding machine according to an embodiment of the present invention;
Fig. 2 is a graph showing the relationship between the tensile strength and the cylinder temperature using the solid phase fraction as a parameter;
Fig. 3 is a graph showing the relationship between the elongation and the aluminum content;
Fig. 4 is a graph showing the relationship between the tensile strength and the internal defective ratio;
Fig. 5 is a graph showing the relationship between the tensile strength and the strain rate;
Fig. 6 is a view for explaining a method of reducing the internal defective ratio in a portion requiring a high tensile strength;
Fig. 7 is a view for explaining a tensile strength test method in high-speed deformation; and
Fig. 8 is a perspective view showing the outer appearance of member formed by magnesium alloy according to the embodiment of the present invention.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.
An embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[Arrangement of Semi-solid Injection Molding Machine]

Fig. 1 is a schematic view showing the main part of a semi-solid injection molding machine according to this embodiment.
An outline of the screw type semi-solid injection molding machine used in this embodiment will be described with reference to Fig. 1.
Referring to Fig. 1, in a screw type molding machine 1, a screw 2 is rotated to feed a raw material 3 to a heating cylinder 4. The screw 3 heats the raw material 3 to a semi-solid state while sufficiently stirring and kneading the raw material 3. As the semi-solid raw material 3 is fed in front of the screw 2, the screw 2 moves backward by the pressure of the semi-solid raw material 3. Note that the screw may be forcibly moved backward at an arbitrary speed in another method. When the screw 2 moves backward by a predetermined length, a high-speed injection mechanism 5 detects this and stops rotating the screw. At the same time, backward movement of the screw 2 stops. The raw material 3 is metered by setting the backward movement distance of the screw 3. The high-speed injection mechanism 5 moves the screw 2 forward to inject the semi-solid raw material 3 into a die 6. The raw material 3 is granulated magnesium pellets which are fed from a hopper 8 to the cylinder 4. A path 7 from the hopper 8 to the cylinder 4 is filled with argon gas. The raw material 3 (e.g., magnesium pellets) is exposed to the argon atmosphere to prevent its oxidation.
As described above, in the screw molding machine 1, the raw material 3 can be uniformly heated in a heating zone 1 in the heating cylinder 4 while being sufficiently stirred and kneaded with the screw 2.

[Member formed by Magnesium Alloy]

The member formed by magnesium alloy of this embodiment will now be described below.
Fig. 2 is a graph showing the relationship between the tensile strength and the cylinder temperature using the solid phase fraction as a parameter. Fig. 3 is a graph showing the relationship between the elongation and the aluminum content. Fig. 4 is a graph showing the relationship between the tensile strength and the internal defective ratio. Fig. 5 is a graph showing the relationship between the tensile strength and the strain rate. Table 1 shows the chemical compositions of members formed by magnesium alloy of this embodiment.
Referring to Fig. 2, the tensile strength at the strain rate of 2 x 10³/s exhibits the test result in high-speed deformation, while the tensile strength at the strain rate of 4 x 10^-3 exhibits the static tensile strength test result in low-speed deformation.
The tensile strength of alloy I in Table 1 extremely decreases when the solid phase fraction is 0%. When the solid phase fraction exceeds 60%, it is difficult to obtain stable members formed by magnesium alloy by continuous injection molding.
As shown in Fig. 3, alloy II having an aluminum content of 9.0 wt% or more exhibits the same elongation in both high-speed deformation and low-speed deformation. Alloy I having an aluminum content of 6.5 wt% or less exhibits excellent elongation but alloy I having an aluminum content of 6.5 wt% or more exhibits an elongation lower than the tensile strength (JIS reference value) in low-speed deformation. Alloy I cannot obtain a sufficient elongation in high-speed deformation at a strain rate of 1.8 x 10³/s. When the aluminum content is less than 2.0 wt%, it is difficult to supply the semi-solid raw material to the heating cylinder and hence perform injection molding. Fig. 3 shows as reference values the static tensile test results of alloys I and II based on the JIS (Japanese Industrial Standards).
As shown in Fig. 4, when the internal defective ratio of alloy I exceeds 1%, the tensile strength extremely decreases in high-speed deformation.
The solid phase fraction is the volume ratio of a solid phase present in the semi-solid state. The internal defective ratio is a value calculated by (1 - density of cast member/theoretical density) x 100 (%) and represents the degree at which an air gap (e.g., a void and gas hole) is present in local areas of the product, which particularly need strength.
Based on the above test results, according to this embodiment, a member formed by magnesium alloy is made to have a solid phase fraction of 0% (exclusive) to 60% (inclusive), an aluminum content of 2.0 wt% (inclusive) to 6.5 wt% (inclusive), and a strain rate of 1 x 10²/s or more corresponding to an internal defective ratio of 1% or less, thereby achieving tensile strength and elongation excellent in high-speed deformation.

Unit: wt%

Al Zn Mn Fe Ni Cu Mg

Alloy I 6.1 0.8 0.23 0.003 0.0007 0.002 balance

Alloy II 9.1 0.8 0.21 0.003 0.0008 0.001 balance
As shown in Fig. 3, it is preferable that the aluminum content be 3.0 wt% or more to obtain a member with more stable quality by continuous injection molding and hence increase the elongation. Therefore, the aluminum content is preferably set in the range of 3.0 wt% (inclusive) and 6.5 wt% (inclusive).
As shown in Fig. 2, when the solid phase fraction is set in the range of 0% (exclusive) to 40% (inclusive), stable fluidity can be obtained. The heating cylinder can be filled with the material for a relatively large member, and a high tensile strength can be maintained.
As shown in Fig. 5, a member whose molded surface is left on one surface is more excellent in tensile strength in high-speed deformation than a member whose molded surface is perfectly removed. A solid phase fraction of 0% (i.e., perfect molten molding) exhibits a high tensile strength in high-speed deformation if the molded surface is left. The solid phase fraction may be set in the range of 0% (inclusive) to 60% (inclusive), over which it is difficult to obtain members with stabler quality by continuous molding, while the aluminum content and strain rate remain in the above ranges.
The molded surface of a portion in the member formed by magnesium alloy of this embodiment, which has a high tensile strength in high-speed deformation is completely left without machining. In particular, the molded surface is left on a portion which experiences a high stress value in the event of a collision.
Fig. 8 is a perspective view showing the outer appearance of member formed by magnesium alloy according to the embodiment of the present invention.
The member formed by magnesium alloy of this embodiment is effectively applied to an instrument panel, seat frame (Fig. 8), or steering wheel, which is required to have a high energy absorption (high elongation) in collision (high-speed deformation).
Note that the member formed by magnesium alloy of this embodiment may locally or entirely form a portion which requires high tensile strength and elongation in high-speed deformation.

[Method for Setting Internal Defective Ratio of 1% or less]

Fig. 6 is a view for explaining a method of reducing an internal defective ratio at a portion which requires a high tensile strength.
The internal defective ratio of a portion which requires a high tensile strength can be reduced by forcibly applying a pressure to that portion during solidification or by using a local pressurizing process.
For example, as shown in Fig. 6, a local pressurizing pin 10 is disposed in a portion (e.g., a thick-walled portion of a cast member) which tends to have an internal defect, and a pressure is forcibly applied to the portion P1 during solidification, thereby reducing the internal defective ratio.
That is, the portion of the member formed by magnesium alloy of this embodiment, which requires a high tensile strength and elongation in high-speed deformation may locally or entirely have an internal defective ratio of 1% or less.

[Tensile Strength Test Method in High-Speed Deformation]

Fig. 7 is a view for explaining the tensile strength test method in high-speed deformation.
As shown in Fig. 7, this embodiment employs the Hopkinson bar method of indirectly determining the dynamic load acting on a test piece 23 and its strain using the strain gauges of an input bar 21 and output bar 22 in accordance with the one-dimensional theory of elastic wave propagation.
More specifically, the test piece 23 is sandwiched between the input bar 21 and output bar 22, and an impact load is applied on the input bar 21.
Let ε_i be the strain of an elastic wave propagating through the input bar 21, ε_r be the strain produced when the elastic wave is reflected by the interface between the input bar 21 and test piece 23 and returns to the input bar 21, and ε_t be the strain produced when the elastic wave having passed through the test piece 23 passes through the output bar 22. The relation between a propagation speed C₀ and displacement u of the elastic wave at the input and output bars 21 and 22 is defined as: u = 0 t ∂u ∂t dt = 0 t Vdt = 0 t C 0εdt where V is the particle velocity.
Using equation (1), displacements u₁ and u₂ at the left and right interfaces of the test piece 23 are given by: u1 = 0 t C0(εi - εr)dt u2 = 0 t C0εtdt
An average strain ε_s and strain rate dε_s/dt of the test piece 23 are: ε s = u 1-u 2 L = C 0 L 0 t (ε i -ε r -ε t )dt dε s dt = C 0 L (ε i -ε r -ε t ) where L is the length of the test piece 23.
An average stress σ_s acting on the test piece 23 is expressed by: σ s = EA 2As (ε i +ε r +ε t ) where A_s is the sectional area of the test piece, A is the sectional area of the input and output bars, and E is the Yong's modulus of the input and output bars.
Substitutions of measured ε_i, ε_r, and ε_t into equations (4) and (6) yield the relationship between the dynamic stress and strain.
The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention.

Claims

A member formed by magnesium alloy characterized by comprising a portion which is excellent in high-speed deformation characteristics, has a solid phase fraction of 0% or more to 60% or less, contains 2.0 to 6.5 wt % of aluminum, and has a strain rate of not less than 100/s, and characterized in that a molded surface of said portion is left unprocessed, if the solid phase fraction is 0%.
The member according to claim 1, characterized in that said portion which is excellent in high-speed deformation has a solid phase fraction of more than 0% to 60% or less, contains 2.0 to 6.5 wt% of aluminum, and has a strain rate of not less than 100/s.
The member according to claim 1 or 2, characterized in that a molded surface is left on said portion which is excellent in high-speed deformation.
The member according to claim 2 or 3, characterized in that a local internal defective ratio is not more than 1%.
The member according to claim 2 or 3, characterized in that an overall internal defective ratio is not more than 1%.
The member according to any one of claims 1 to 5, characterized in that the aluminum content is 3.0 wt% to 6.5 wt%.
The member according to claim 2, 4, or 5, characterized in that the solid state fraction is more than 0% to 40% or less.