WO1992005293A1

WO1992005293A1 - Extrusion of reinforced composite material

Info

Publication number: WO1992005293A1
Application number: PCT/CA1991/000253
Authority: WO
Inventors: Paul W. Jeffrey; Robert Warren +Di Hains
Original assignee: Alcan International Limited; Hains, Sylvia, Margaret, Joyce +Hf
Priority date: 1990-09-14
Filing date: 1991-07-22
Publication date: 1992-04-02
Also published as: CA2025428A1; AU8097991A

Abstract

A process is described for extruding metal matrix composite materials, e.g. aluminum matrix composites. The problem of inconsistent extrudate surfaces is solved by controlling the extrusion speed such that it is maintained above a lower speed limit below which low speed tearing of the extrudate surface occurs and below an upper speed limit above which incipient melting and consequent speed cracking of the extrudate surface occurs.

Description

Extrusion of Reinforced Composite Material Technical Field

This invention relates to the extrusion of metal matrix composite materials and, more particularly, to the extrusion of particulate reinforced aluminum matrix composites.

Background Art

Metal matrix composite materials have gained increasing acceptance as structural materials. These composites are typically composed of reinforcing particles such as fibres, grit, powder or the like that are embedded in a metallic matrix. The reinforcement imparts strength, stiffness and other desirable properties to the composite, while the matrix protects the particles and transfers the load within the composite. The two components, matrix and reinforcement, thus cooperate to achieve results improved over what either could provide on its own.

There are now many commercial applications for such materials and it would be highly desirable to be able to produce shaped products of such composite materials by conventional direct extrusion with shear-face dies. However, when particulate reinforced aluminum matrix composites were directly extruded through a die, there was a problem of inconsistency of product. Thus, in some instances there was tearing of the extruded surface and, in other instances, there was cracking of the extruded surface. It is the object of the present invention to find a solution to the above inconsistency in product surface. Disclosure of the Invention

According to the present invention, it has been discovered that in the extrusion of particulate reinforced metal matrix composites there exists an extrusion speed window within which an extruded product with a smooth bright surface can be obtained. The actual extrusion speed used and the magnitude of the speed window vary depending on such factors as the shape of the extrusion. the extrusion ratio, the extrusion temperature and the matrix composition. However, once it has been recognized that such a speed window exists, the actual permissible speeds in each circumstance can readily be determined. Thus, in accordance with the present invention, it has been determined that there are transition speeds between which an acceptable extrusion speed window exists. These two transitional speeds comprise a "lower speed limit" which represents the transition from stick-slip (causing tearing of extruded surface) to operational

(smooth extruded surface) and an "upper speed limit" which is the transition from operational to incipient melting (and consequent speed cracking of the extruded surface) . Composition of Metal Matrix Composites The matrix metal or metal alloy may be defined as (a) soft matrix, (b) medium matrix or (c) hard matrix. Using Aluminum Association designations, a typical soft matrix may be AA-1060 (chemically pure aluminum of a minimum purity of 99.60%) or AA1100 (chemically pure aluminum containing 0.1% Cu) . A typical medium matrix is AA-6061 containing 0.15-0.40% Cu, 0.8-1.2% Mg, 0.4-0.8% Si or AA-6063 containing 0.2-0.6% Si, 0.45-0.9% Mg. A typical hard matrix is AA-2014 containing 3.9-5.0% Cu, 0.4-1.2% Mn and 0.2-0.8% Mg. The above matrices typically have the following approximate Brinell Hardness in the "0" condition (fully annealed) :

Soft alloy <25

Medium hard alloy 25-40 Hard alloy 40-60 The hardness of the matrix controls the extent to which the matrix metal bonds to the die surface. A soft matrix readily yields in a subcutaneous manner which limits the shear stresses which can be developed at the surface to remove deposited material. Such bonding to the die face is emphasized with metal matrix composites due to the cleaning action of the particulate which removes any intermediate films between the matrix alloy and the die bearing which would inhibit bonding. In addition, the particulate throughout the matrix alloy acts to nucleate subcutaneous fracture more readily than with conventional material. In the extrusion of such composites, the soft matrix moves both the lower speed limit and the upper speed limit in an upward direction. On the other hand, the hard matrix causes a lowering of both the upper speed limit and the lower speed limit. Thus, with the hard matrix, tearing in the lower speed zone is less severe and there is an increased liability to speed cracking because more eutectic is present. The acceptable operating window between the upper and lower speed limits reduces with increasing matrix hardness. The particulate material used in the composite may be selected from a wide range of known reinforcing particulates. These include particles of alumina (A1₂0₃)₂, silicon carbide (SiC) , boron carbide (B₄C) , aluminum nitride (A1N) , silicon nitride (Si₃N₄) , titanium diboride (TiB₂) , titanium carbide (Tie) and MgAl₂0 (spinel) . Of these, silicon carbide is the hardest, while MgAl₂0₄ is the softest and alumina is moderate in its hardness. It has been found that the particulates in the composite tend to clean the die face of adhering deposits. At slow speeds, sticking action between the matrix alloy and the die dominates, and the deposits grow. However, at a transition speed, the cleaning action of the particulate is sufficient to maintain the die surface free of deposits. It is believed that this transition speed is due to the decreased time for localised bonding in the interparticle regions before the arrival of the next particle which acts to remove the deposit.

The surface of the extrudate produced within the intermediate speed zone can be extremely specular due to the reduction in heat generation resulting from lower friction if relatively hard dies which visually polish are employed. The presence of reinforcing particles has an effect on the extrusion speeds as compared with unreinforced alloys, and the type of particulate used has an effect on the extent to which the die becomes polished. A wide range of particulate loading may be used, e.g. up to 35% by volume or more, preferably up to 20%. Increased particulate loading lowers both the lower speed limit and the upper speed limit of the extrusion speed window, and reduces the acceptable operating window between the two limits.

Extrusion Temperature

In general terms, low preheat temperatures move both the lower speed limit and the upper speed limit in a downward direction, while high preheat temperatures move both the lower speed and the upper speed limit in an upward direction. Generally the extrusion is practised at extrusion ingot temperatures in the range of 350-525^βC. There are preferred limits depending upon the matrix and, for instance, AA-1060 or 1100 is preferably extruded at an extrusion ingot temperature in the range of 350-400*C, AA- 6061 is preferably extruded at an extrusion ingot temperature of 450-500"C and AA-2014 is preferably extruded at an extrusion ingot temperature of 350-400"C.

Prolonged preheating of the ingot homogenizes the as- cast microstructure. This is a known phenomenon in conventional aluminum alloy extrusion practice, and refers to the dissolution of coarse non-uniformly distributed micro-constituents present in the alloy and their partial re-precipitation on subsequent cooling as fine uniform dispersions. The speed cracking which occurs above the upper speed limit is due to incipient melting and is more prevalent with unhomogenized eutectic-containing ingot. In addition, due to redistribution upon solidification the ceramic reinforcement is not uniformly distributed but tends to be concentrated at the intercellular regions and may accentuate the tendency to fracture. It is desirable to keep the homogenization time/temperature to a minimum in Mg-containing composites reinforced with alumina due to the tendency to spinel reaction which results in depletion of the available Mg, and which may adversely affect thhe strength of the particulate to matrix bond. Die Geometry

The length of die bearing is preferably in the range of about 2 to 50 mm and within this range there is little effect on the extrusion speed window. However, the shape and size of the extruded section is significant and, for instance, a decrease in the perimeter of the extruded section causes an increase in the upper speed limit. Thus, the tendency to speed cracking is decreased and the extrusion speed can be increased roughly in proportion to the decrease in perimeter. As the perimeter of the extruded section becomes larger in size, it has been found that the lower speed limit actually decreases.

Extrusion Ratio

The extrusion ratio can vary quite widely within the range 5:1 to 500:1, preferably in the range 16:1 to 425:1. However, in commercial operations it is usually less than 100:1.

Brief Description of the Drawings

Certain preferred embodiments of the invention are illustrated by the attached drawings in which: Figure 1 represents plots of strength and frictional forces as a function of extrusion speed;

Figure 2 is a photograph of a torn extrudate surface from a too slow extrusion speed;

Figure 3 is a photograph of a bright surface from a correct extrusion speed, and

Figure 4 is a photograph of a speed cracked surface from a too fast extrusion speed.

Best Modes for Carrying Out the Invention

Further specific embodiments of the invention are illustrated by the following non-limiting examples.

Example 1

A series of extrusions were produced by forcing billets of aluminum matrix composite through a die. The extrudate formed was a 9.5 mm diameter rod at an extrusion ratio of 49.5:1. The composite billets used, the extrusion temperature and the lower and upper speed limits determined are shown in Table 1 below:

Table 1

Example 2

A further test was carried out using an aluminum matrix composite comprising AA-1060 containing 10% A1₂0₃. A 6.3 mm diameter rod was extruded at an extrusion ratio of 166:1 and at an extrusion ingot temperature of 400°C. The results of extrusion speed tests are shown in Figure 1 and it will be seen that the transition representing the lower speed limit was approximately 40 m/min and the transition representing the upper speed limit was approximately 160 m/min. Examples of the torn, bright and speed cracked extrudates of Table 1 are shown in the photographs of Figures 2, 3 and 4 respectively. Example 3 Another test was carried out with AA1060/10% A1₂0₃. A 9.5 mm diameter solid rod was extruded at an extrusion ratio 430:1 and an extrusion ingot temperature of 400^βC. It was found that the lower speed limit was about 20 m/min, while the upper speed limit was about 90 m/min. Example 4

Further extrusion tests were carried out at variable composite compositions, variable die bearing lengths, variable billet temperatures and variable extrusion speeds. The composites used were AA-6061 with 10% A1₂0₃ and 15% A1₂0₃. Two different dies were used, both producing a 25 mm diameter solid extrudate rod and having die bearing lengths of 3.2 mm and 9.5 mm respectively. The billets were preheated to temperatures of 450°C and

525*C.

From the tests carried out, the operational speed range for bright extrudate was between 7.6 and 22.9 m/min.

Within that range, it was found that higher levels of reinforcement caused speed cracking to occur at lower speeds, particularly at the higher billet temperature.

The different die bearing lengths were not significant.

The lower billet preheat temperature (450^βC) allowed higher speeds before speed cracking. Example 5

A further extrusion trial was carried out with

AA-6061/20% A1₂0₃ to produce a seamless tube 74 mm diameter with a 2.5 mm thick wall. The billets were preheated to

500"C. At an extrusion ratio of 85:1, the lower speed limit was about 7 m/min, while the upper speed limit was about 9.5 m/min. In this case the effect is clearly apparent that increasing perimeter reduces the safe operating window.

The extruded products of this invention find industrial application where a combination of lightweight and high strength are required, e.g. for making automotive driveshafts.

Claims

Claims :

1. In a process for forming an extrusion of particle reinforced metal matrix composite material in which a heated billet of metal matrix composite is forced through a die, the improvement which comprises controlling the extrusion speed such that it is maintained above a lower speed limit below which low speed tearing of the extrudate surface occurs and below an upper speed limit above which incipient melting and consequent speed cracking of the extrudate surface occurs.

2. A process according to claim 1 wherein the metal matrix is aluminum or an alloy thereof.

3. A process according to claim 2 wherein the metal matrix composite billet is preheated to a temperature in the range of 300-525^βC.

4. A process according to claim 3 wherein the metal is a soft substantially pure aluminum, the ingot temperature is in the range of about 350-400"C, the lower extrusion speed limit is within the range of 20-45 m/min and the upper extrusion speed limit is within the range of 90-160 m/min.

5. A process according to claim 3 wherein the metal is an aluminum alloy of medium hardness containing Mg, Si and optionally Cu, the ingot temperature is within the range of about 300-525"C, the lower extrusion speed limit is about 3 m/min and the upper extrusion speed limit is about 55.

6. A process according to claim 3 wherein the metal is a relatively hard aluminum alloy containing Cu, Mn and

Mg, the extrusion temperature is in the range of 350- 400*C, the lower extrusion speed limit is less than 1 m/min and the upper extrusion speed limit is 4-5 m/min.

7. A process according to claim 3 wherein the reinforcing particles are selected from the group consisting of alumina, silicon carbide, boron carbide, aluminum nitride, silicon nitride, titanium diboride, titanium carbide and MgAl₂0₄.

8. A process according to claim 4,5 or 6 wherein the reinforcing particles are alumina.

9. A process according to claim 3 wherein the extrusion is at a ratio of 5:1 to 500:1.

10. A process according to claim 9 wherein the extrusion is at a ratio of 16:1 to 425:1.

11. A process according to claim 9 wherein the extrusion is at a ratio of less than 100:1.

12. A process according to claim 3 wherein the composite contains up to 20% by volume of reinforcing particles.

13. A process according to claim 12 wherein the composite contains up to 35% by volume of reinforcing particles.