Friction stir welding of metal matrix composites

A method of making a fiber-reinforced component includes: providing a first composite member having a metal matrix with reinforcing fibers distributed therein; providing a second composite member having a metal matrix with reinforcing fibers distributed therein; and joining the first member to the second member by friction stir welding along a predetermined joint path, such that an average volume fraction of the reinforcing fibers within the joint path is substantially the same as an average volume fraction thereof in the composite members before joining. A fiber-reinforced component includes: first and second members each having a metal matrix with reinforcing fibers distributed therein. The first member is bonded to the second member by a solid state bond along a predetermined joint path, such that an average volume fraction of the reinforcing fibers within the joint path is substantially the same as an average volume fraction thereof in remainder of the members.

BACKGROUND OF THE INVENTION

This invention relates generally to metal matrix components and more particularly bonding of such components by friction stir welding.

It is known in the prior art to construct composite materials using a metallic matrix with reinforcing fibers, hereinafter referred to as “metal matrix composites”. These materials combine light weight and good strength. Typically, the reinforcing fibers are relatively short in length and are oriented randomly so that the component will have isotropic properties. Non-limiting examples of turbine engine components which may be constructed from such composites include rotating fan blades and other kinds of airfoils, rotating shafts and disks, static structures.

Metal matrix composites can be molded to desired shapes or can be bonded through means such as heat welding. Unfortunately, the fluid flow that occurs during the welding process disturbs this intended orientation and therefore undesirably creates an area along the joint in which only the matrix carries any loads placed on the component.

Accordingly, there is a need for joining metal matrix composites while maintaining their mechanical properties.

BRIEF SUMMARY OF THE INVENTION

The above-mentioned need is met by the present invention, which according to one aspect provides a method of making a fiber-reinforced component, including; providing a first composite member comprising a metal matrix with reinforcing fibers distributed therein in a selected orientation; providing a second composite member comprising a metal matrix with reinforcing fibers distributed therein in a selected orientation; and joining the first member to the second member by friction stir welding along a predetermined joint path, such that an average volume fraction of the reinforcing fibers within the joint path is substantially the same as an average volume fraction thereof in the composite members before joining.

According to another aspect of the invention, a fiber-reinforced component includes: a first composite member comprising a metal matrix with reinforcing fibers distributed therein in a selected orientation; and a second composite member comprising a metal matrix with reinforcing fibers distributed therein in a selected orientation; wherein the first member is bonded to the second member by a solid state bond along a predetermined joint path, such that an average volume fraction of the reinforcing fibers within the joint path is substantially the same as an average volume fraction thereof in remainder of the members.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,FIG. 1depicts an exemplary prior art reinforced metal matrix component10comprising first and second members12and14bonded together along a joint path16. The members12and14are both made from a composite material comprising a metal polymeric matrix18with reinforcing fibers20disposed therein. In the illustrated example the reinforcing fibers20have a random three-dimensional orientation to impart isotropic structural properties to the members12and14. It should be noted that the fibers20are depicted in highly exaggerated scale for the purpose of illustration. The members12and14are bonded together using a conventional process such as thermal welding wherein the matrix18of each member12and14is heated along the joint, to a temperature above its solidus point, so that it will melt and flow together. The members12and14are then allowed to cool to form the unitary component10. Unfortunately, the thermal welding process creates a heat-affected zone22within which the reinforcing fibers20are absent or disturbed in their density of distribution, or orientation. Under these circumstances, the area around the joint path16lacks the full strength that would be expected from the use of the reinforcing fibers20.

FIGS. 2-4depict a process of bonding reinforced metal matrix components together in a butt joint using friction stir welding. In this example, first and second members112and114are bonded together along a joint path116to form a completed component110. The illustrated members112and114are simple plate-type elements with a constant thickness. However, these are merely representative examples, and the process described herein may be used to join any type of component which is amenable to friction stir welding. Examples of turbine engine components which may be constructed from fiber reinforced metals include rotating fan blades, outlet guide vanes, reverser cascades, and various other static structures. Furthermore, the present method is applicable to joint configurations other than the illustrated butt joint.

Each of the first and second members112and114comprise a metal matrix118with reinforcing fibers120disposed therein. In the illustrated example the reinforcing fibers120have a random three-dimensional orientation to impart isotropic structural properties to the members112and114, but other orientations could be used to achieve desired properties. The reinforcing fibers120are essentially uniformly distributed throughout the volume of each of the members112and114. This distribution can be described as an average volume fraction of fibers for a unit volume of the matrix118, i.e. a value of 0.0 would represent a total lack of reinforcing fibers120within the matrix118, and a value of 1.0 would represent a solid mass of reinforcing fibers120.

The metal matrix118will vary depending on the requirements of the specific application. One non-limiting example of a metal known to be suitable for structural components is titanium and alloys thereof.

The reinforcing fibers120will also vary according to the specific application. The fibers beneficially will have a tensile strength greater than that of the matrix118in order to form a synergistic structural combination with the matrix118. Non-limiting examples of materials useful for reinforcing fibers120include silicon coated carbon, silicon carbide, tungsten, glass, other kinds of carbon fibers, and metals. In the illustrated example, the reinforcing fibers120have a diameter of about 1 micrometer (40 microinches) to about 25 micrometers (980 microinches), with aspect ratios of about 100 to about 15,000 with resultant lengths of about 1 mm (0.004 in.) to about 38 cm (14.7 in.)

The member112is joined to the member114using a friction stir welding process. The welding process is carried out using friction stir welding machinery and fixtures of a known type (not shown). As shown inFIG. 2, a cylindrical, shouldered, wear-resistant pin “P” having a tip “R” is rotated and forced against the joint path116. The friction between the pin P and the members112and114causes the material to soften and flow without melting. Thus, friction stir welding is a type of solid state bonding process. In the illustrated example the pin P has a shoulder diameter “D” of about 10.7 mm (0.420 in.), and the tip R has a length “l” of about 2.8 mm (0.110 in.) from the shoulder to its distal end and a diameter “d” of about 6.4 mm (0.250 in.), and has a left-hand thread formed thereon. The following exemplary parameters have been found to produce an acceptable friction stir welded bond: pin speed about 700 to about 900 RPM; traversing speed about 10 cm/min. (4 in/min.) to about 15.2 cm/min. (6 in/min.); and force on the pin P about 499 kg (1100 lbs.) to about 635 kg (1400 lbs.). The pin P is traversed along the joint path116, straddling the member112and114, leaving the members112and114bonded together behind it.

As the pin P is traversed along the joint line, the heat generated is conducted away from the pin P and to the surface of the members112and114, which results in a decreasing temperature gradient. Along this gradient, various zones can be identified according to the effect on the members112and114. A stir zone “S” is created which has a width slightly greater than the width of the tip R, for example about 0.25 mm (0.010 in.) from the edge of the tip R on each side. A thermomechanically altered zone “T” extends outward from the edge of the stir zone “S”, for example about 0.25 mm (0.010 in.) on each side. A heat affected zone “H” extends outward from the edge of the thermomechanically affected zone T, for example about 0.76 mm (0.030 in.) on each side. The width of each of these zones will be affected by the thermal properties of the members112and114, as well as their shape and dimensions.

Within the stir zone S, a vortex spiral circular flow of the matrix118is generated around the tip R. Because the matrix118is in a fluid state, the reinforcing fibers120are free to move with this flow. They are carried around the periphery of the tip R (seeFIG. 3). It has been found that reinforcing fibers120will tend to align their longitudinal axes parallel to the shear gradient in the material. Thus, they will tend to remain tangent to the vortex flow as they are carried around it. Within the thermomechanically altered zone T, there is reduced transport of the reinforcing fibers120, but they tend to orient themselves parallel to a moving shear plane normal to the joint path116. In contrast to prior art types of thermal bonding, the reinforcing fibers120will tend to remain within in the vicinity of the joint path116.

As the probe P traverses the joint line, the stir zone S cools and solidifies, resulting in consolidation between the member112and the member114. Individual fibers120will remain in the locations and orientations where the matrix118“traps” them during solidification. The friction stir welding parameters can be modified to influence the final orientation of the reinforcing fibers120. For example, the traversing speed can be increased or decreased relative to the pin speed. A relatively rapid traverse rate will tend to result in reduced transport of the reinforcing fibers120across the joint path116, while a relatively higher traverse rate will result in increased transport of the reinforcing fibers120across the joint path116. Furthermore, higher pin speed or pressure will increase the size of the stir zone S and the thermomechanically altered zone T, tending to increase the amount of transport.FIG. 4shows the component110bonded in such a way that the reinforcing fibers120are substantially randomly oriented along the joint path116.FIG. 5shows a similar component110′ made from two members112′ and114′ bonded along a joint path116′. In this example, the reinforcing fibers120′ are more likely to be oriented transverse to the joint path116′. Such an orientation might be expected from using a relatively low traverse rate.

The completed weld leaves a smooth surface finish along the joint path which requires minimal processing to result in an acceptable finished product. In contrast to prior art thermal bonding methods, there will be a significant distribution of reinforcing fibers120within and across the joint path116, similar to the average fiber volume fraction before bonding. Accordingly, the structural properties of the members112and114are substantially preserved, and the component will not have a weakness along the joint path116.

The foregoing has described a process for bonding fiber reinforced metal composites using friction stir welding. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.