Friction stir welding tool and process for welding dissimilar materials

Abstract

A friction stir welding tool and process for lap welding dissimilar materials are detailed. The invention includes a cutter scribe that penetrates and extrudes a first material of a lap weld stack to a preselected depth and further cuts a second material to provide a beneficial geometry defined by a plurality of mechanically interlocking features. The tool backfills the interlocking features generating a lap weld across the length of the interface between the dissimilar materials that enhances the shear strength of the lap weld.

1 . A friction stir welding tool, characterized by:
 a scribe cutter that extends from a radially offset position on a terminal end surface of a pin, the pin is operatively positioned between a shank and the scribe cutter having a preselected height that prevents contact by a piece of material to the shoulder of the shank when the scribe cutter cuts the material.   
     
     
         2 . The friction stir welding tool of  claim 1 , wherein the scribe cutter comprises tungsten carbide. 
     
     
         3 . The friction stir welding tool of  claim 1 , wherein the scribe cutter includes a member selected from the group consisting of: nickel, titanium, tungsten, steel, carbide steel, polycrystalline cubic boron nitride, silicon nitride, rhenium, boron, and combinations thereof. 
     
     
         4 . The friction stir welding tool of  claim 1 , wherein the scribe cutter extends a distance from the surface of the pin selected in the range from about 0.1 mm to about 1.0 mm. 
     
     
         5 . The friction stir welding tool of  claim 1 , wherein the scribe cutter includes a radial offset distance measured from the center of the surface that is at least about one quarter of the diameter of the base of the pin. 
     
     
         6 . The friction stir welding tool of  claim 1 , wherein the scribe cutter includes a rotational velocity of between about 100 rpm and 1000 rpm. 
     
     
         7 . The friction stir welding tool of  claim 1 , wherein the scribe cutter is coupled to the pin that includes a taper angle greater than or equal to about 90 degrees. 
     
     
         8 . The friction stir welding tool of  claim 1 , wherein the scribe cutter provides a plunge depth in the second material that is less than or equal to the length of the scribe cutter. 
     
     
         9 . The friction stir welding tool of  claim 1 , wherein the scribe cutter generates a weld interface with a width that is at least about two times the radial offset distance of the scribe cutter. 
     
     
         10 . The friction stir welding tool of  claim 1 , wherein the scribe cutter provides the first material extruded by same such that it backfills the mechanical interlocking features in the second material along the length of the weld interface forming the lap weld joint. 
     
     
         11 . The friction stir welding tool of  claim 1 , wherein the scribe cutter extrudes the first material at below the melting temperature thereof such that the first material maintains a local shear stress characteristic of the solid state that allows it to fill the mechanical interlocking features in the second material forming the lap weld joint. 
     
     
         12 . The friction stir welding tool of  claim 1 , wherein the scribe cutter extrudes the first material such that the extruded first material fills the mechanical interlocking features at a substantially uniform hydrostatic pressure. 
     
     
         13 . The friction stir welding tool of  claim 1 , wherein the scribe cutter is angled at between 0 and 90 degrees with respect to the vertical direction. 
     
     
         14 . The friction stir welding tool of  claim 1 , wherein the scribe cutter maintains an operating temperature for the second material that is below the melting temperature of the first material. 
     
     
         15 . The friction stir welding tool of  claim 1 , wherein the cutting scribe yields a lap weld with increased shear strength and lower statistical deviation compared to a lap weld generated absent the cutting scribe. 
     
     
         16 . A method for forming a lap weld between two dissimilar materials, the method comprising the steps of:
 stacking a first material that is dissimilar from a second material atop the other in a lap weld stack with an overlap therebetween sufficient to form a weld interface of a preselected width along the length between the materials being joined;   penetrating through the first material of the lap weld stack with a scribe cutter that extends from a radially offset position on a terminal end surface of said pin, extruding same to a preselected depth and cutting a surface of the second material to form a plurality of mechanically interlocking features therein; and   backfilling the mechanically interlocking features along the length of the interface with extruded first material to form a lap weld between the two dissimilar materials with an enhanced shear strength.   
     
     
         17 . The method of  claim 16 , wherein the stacking step includes a first material that is a metal selected from aluminum, magnesium, titanium, or alloys thereof. 
     
     
         18 . The method of  claim 16 , wherein the second material is steel or a steel alloy. 
     
     
         19 . The method of  claim 16 , wherein the first dissimilar material is a metal selected from: aluminum, magnesium, titanium, or an alloy thereof; and the second dissimilar material is steel or a steel alloy. 
     
     
         20 . The method of  claim 16 , wherein the first dissimilar material is a ceramic and the second dissimilar material is steel or a steel alloy. 
     
     
         21 . The method of  claim 16 , wherein the first dissimilar material and second dissimilar material have a melting temperature that is different from the other by at least about 20%. 
     
     
         22 . The method of  claim 16 , wherein the first dissimilar material and second dissimilar material have a density that is different from the other by at least about 10%. 
     
     
         23 . The method of  claim 16 , wherein the penetrating step introduces cuts in the second material that are cross-sectional cuts. 
     
     
         24 . The method of  claim 16 , wherein the penetrating step includes extruding the first material at a temperature below its melting temperature. 
     
     
         25 . The method of  claim 16 , wherein the lap weld has a shear strength that is at least about 80% of the strength of the lower melting material therein.