Friction stir processing for surface properties
A surface of an article is heated and subjected to pressure by operation of a friction stir tool. The pressure of the face of the tool on the article surface, and the speed of rotation of the tool and of its progression along the surface of the article, are controlled to progressively hot-work a specified surface area. The face of the tool may be round and have a profiled, axially extending, smaller diameter pin for higher concentration of frictional heat and load and for plasticizing a surface layer of the workpiece. Such hot work may be applied for reducing surface porosity or for producing another change in the surface microstructure, or for effecting another change in surface material properties.
1. A method of hot working a selected surface area of an internal cylindrical surface of an aluminum alloy cylinder block casting or a magnesium alloy cylinder block casting to produce a desired change in microstructure to a predetermined depth to increase the wear resistance of cylinder bores against piston wear, the internal cylindrical surface having an axis and a circumference, the method comprising:
heating and hot working the internal cylinder surface by radially pressing a face of a rotating tool against a selected portion of the cylindrical surface while controlling the speed of rotation of the tool and pressure applied by the tool face to the portion of the cylindrical surface to thermo-mechanically deform the article surface to the predetermined depth; and
traversing the rotating tool in a combination of circumferential and axial paths over successive portions of the surface area to thus hot work the selected surface area.
2. A method of hot working a selected surface area of an internal cylindrical surface of an aluminum alloy cylinder block casting or a magnesium alloy cylinder block casting as recited in claim 1 in which the heating and hot working of the surface area is accomplished using a rotating tool with an axis of rotation and having a circular face centered on the axis of rotation, the face having a diameter for treating the portion of the surface area, the face having an axially protruding pin with a cross-section smaller than the diameter of the face, and the length of the protruding pin being determined for altering the article surface to the predetermined depth.
3. A method of hot working a selected surface area of an internal cylindrical surface of an aluminum alloy cylinder block casting or a magnesium alloy cylinder block casting as recited in claim 2 in which the rotating tool is driven by a rotatable shaft that is aligned with the axis of the internal cylindrical surface.