An apparatus, comprising: a first piece of metal; a second piece of metal, wherein the first piece of metal and the second piece of metal are joined to form a butt joint; a shim attached to the butt joint; a friction stir welding machine comprising an anvil, a pin tool, characterised by: a cooling element, and by a computer system, wherein the computer system is programmed for achieving the following steps: entering a number of parameters for a friction stir welding process, entering a number of parameters for the shim, calculating then an elongation ratio and a pseudo heat ratio to identify a desired spindle speed and a desired feed rate based on a desired thickness ratio and desired pseudo heat ratio, identifying based on the desired spindle speed and the desired feed rate a spindle feed and a feed rate for the friction stir welding process, wherein the pseudo heat ratio is N 2 /fr with N being the pin speed in rotation per minute and fr being the feed rate in millimeter per second of the movement perpendicular to the axis of the pin, wherein the pin tool is rotated about an axis, is moved in a direction perpendicular to the axis, and exerts a downward force on the shim and the butt joint to weld the two pieces (802, 804) of metal together to form a weld nugget in which the weld nugget has a desired thickness ratio between about 1.1 to about 1.25 after a superplastic forming process, and wherein the first piece of metal, the second piece of metal, and the weld nugget has a number of characteristics that are substantially the same after the superplastic forming process. a first piece of metal; a second piece of metal, wherein the first piece of metal and the second piece of metal are joined to form a butt joint; a shim attached to the butt joint; a friction stir welding machine comprising an anvil, a pin tool, characterised by: a cooling element, and by a cooling element, and by a computer system, wherein the computer system is programmed for achieving the following steps: entering a number of parameters for a friction stir welding process, entering a number of parameters for the shim, calculating then an elongation ratio and a pseudo heat ratio to identify a desired spindle speed and a desired feed rate based on a desired thickness ratio and desired pseudo heat ratio, identifying based on the desired spindle speed and the desired feed rate a spindle feed and a feed rate for the friction stir welding process, wherein the pseudo heat ratio is N 2 /fr with N being the pin speed in rotation per minute and fr being the feed rate in millimeter per second of the movement perpendicular to the axis of the pin, wherein the pin tool is rotated about an axis, is moved in a direction perpendicular to the axis, and exerts a downward force on the shim and the butt joint to weld the two pieces (802, 804) of metal together to form a weld nugget in which the weld nugget has a desired thickness ratio between about 1.1 to about 1.25 after a superplastic forming process, and wherein the first piece of metal, the second piece of metal, and the weld nugget has a number of characteristics that are substantially the same after the superplastic forming process. entering a number of parameters for a friction stir welding process, entering a number of parameters for the shim, calculating then an elongation ratio and a pseudo heat ratio to identify a desired spindle speed and a desired feed rate based on a desired thickness ratio and desired pseudo heat ratio, identifying based on the desired spindle speed and the desired feed rate a spindle feed and a feed rate for the friction stir welding process, wherein the pseudo heat ratio is N 2 /fr with N being the pin speed in rotation per minute and fr being the feed rate in millimeter per second of the movement perpendicular to the axis of the pin, wherein the pin tool is rotated about an axis, is moved in a direction perpendicular to the axis, and exerts a downward force on the shim and the butt joint to weld the two pieces (802, 804) of metal together to form a weld nugget in which the weld nugget has a desired thickness ratio between about 1.1 to about 1.25 after a superplastic forming process, and wherein the first piece of metal, the second piece of metal, and the weld nugget has a number of characteristics that are substantially the same after the superplastic forming process.