Friction stirred introduction of particles into a metallic substrate for surface durability treatment

An axially elongated cylindrical tool has a protrusion at a lower end thereof which is inserted into a metallic work piece substrate under pressure during rotation of the tool and displacement thereof in one direction. Mixing of a matrix of metal and particles by the protrusion occurs within a stir zone underlying the substrate surface which is thereby treated by dispersion of the particles therein in response to rotation of the tool. Influx of the hard particles into the work piece substrate is effected during said mixing by the rotating tool.

The present invention relates generally to the production of a metal work piece with improved surface durability.

STATEMENT OF GOVERNMENT INTEREST

BACKGROUND OF THE INVENTION

Work piece components are often subject to wear and abrasion during use or operation in a variety of environments. Currently various methods are provided for improving surface wear durability by nitriding, carburizing, quenching, surface coating, cladding and centrifugal casting with hard carbides injected into the melt. Such methods are limited by reaction kinetics, diffusion kinetics, formation of undesirable phase, inhomogeneity due to concentration gradients and thermodynamic stability of the phases of interest at high treatment temperatures. Workpiece distortion is a typical consequence of the aforesaid high treatment temperatures. It is therefore an important object of the present invention to provide for surface treatment of work piece components which avoids the problems and disadvantages heretofore associated therewith.

SUMMARY OF THE INVENTION

Pursuant to the present invention, the durability of the surface of a metallic work piece substrate, subject to wear and corrosion, is improved by influx of hard particles into the substrate within which dispersion of the particles is effected by rotation of a friction stir welding type tool under axial pressure having a protrusion inserted into the substrate to effect mixing. Various different methods may be utilized for influx of the hard particles. The hard particles may be transferred from a layer deposited onto the substrate surface or may be injected from some external source. The hard particles could also be derived and delivered by ablation of the friction stir tool during use. A layer of the hard particles may be laminated with layers of base metal before being mixed by the tool to transfer the particles into the surfaces. Finally the hard particles may be injected from an external source through a central hole in the tool extending along its axis.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the drawing in detail,FIG. 1illustrates a friction-stir welding tool10positioned in spaced relation above a metal substrate work piece component12to be surface treated pursuant to one embodiment of the present invention. The tool10has an axially elongated cylindrical body14with a lower axial end from which a downwardly convergent protrusion16extends. A lower axial end section18of the tool body14, as diagrammed inFIG. 1, is constructed of a material such as ceramics or layered steel that is softer and more easily ablated than an upper section20made of harder and ablation resistant layered steel material. The tool10undergoes rotation22, as diagrammed inFIG. 1, about its axis24while axial pressure26is being applied thereto just before the tool protrusion16is inserted into a surface28of the metal substrate12as shown inFIG. 2. A metal matrix with hard particles therein is deposited as a layer32onto the surface28from which the hard particles enters into a stir zone30established within the work piece12as shown inFIGS. 2 and 3. Stirring of such matrix is effected below the melting temperature of the work piece12by the aforesaid rotation22of the tool10under the axial pressure26so to mix the metal matrix with the hard particles received from the layer32together with any residual alloys in the layer32.

The hard particles within the deposited layer32is composed of a chemically selected material which will substantially improve properties of the metallic surface28, such as wear resistance, heat resistance and corrosion resistance, as a result of the aforesaid stir mixing within the zone30. The stirring tool10undergoes shift in a feed direction34as denoted inFIG. 2so as to apply surface treatment along the entire surface28on the metal substrate component12to thereby render it more durable by enhanced resistance to wear, heat and corrosion.

According to another embodiment of the present invention as diagrammed inFIG. 4, the hard particles are introduced from an external source36through an injector38into the stir zone30within the metal substrate12for dispersion therein by rotation of the tool10with its axis24at an inclination angle27. Surface deposit of such hard particles from the surface layer32is thereby avoided.

The construction of the stirring tool10as hereinbefore described insures adequate performance of the surface treatment process by adequate dispersion of the hard particles introduced into the metal substrate component12either by transfer from the deposited layer32or injection from the external source36wherein the disintegration of the particles occurs with or without ablation and erosion. Only after such dispersion has occurred, so as to effect an increase in durability and an improvement in other physical properties of the metal substrate surface28, the tool14may be replaced.

Referring now toFIG. 5, yet another embodiment of the present invention involves initial preparation of the work piece component12into alternating layers40of base metal sheets and layers42of dispersed particles that are loosely or weakly consolidated with a binder. The layers40and42are intermixed by rotation of the stir tool protrusion16, resulting in a dispersion44of the hard particles within the metal work piece component12. The degree of such mixing by the protrusion16may be controlled by providing threads or surface steps along the tapered outer surface of the protrusion16. Also the layers40and42may be stacked 90° from the orientation thereof shown inFIG. 5.

FIG. 6illustrates yet another embodiment of the present invention, wherein influx of the hard particles from the external source36is effected during rotation of the tool10through a hole46formed therein. The hard particles accordingly emerge from the hole46at the lower end of the protrusion16into the stir zone30.

Obviously, other modifications and variations of the present invention may be possible in light of the foregoing teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.