Joint product between steel product and aluminum material, spot welding method for the joint product, and electrode chip for use in the joint product

Abstract

To provide a joint product of a steel product and an aluminum material, and a spot welding method for the joint product, ensuring that spot welding with high bonding strength can be performed. In one embodiment, a steel product 1 having a sheet thickness t 1 of 0.3 to 3.0 mm and an aluminum material 2 having a sheet thickness t 2 of 0.5 to 4.0 mm are joined together by spot welding to form a joint product of a steel product and an aluminum produce. In this joint product, the nugget area in the joint part is from 20×t 2 0.5 to 100×t 2 0.5 mm 2 , the area of a portion where the thickness of the interface reaction layer is from 0.5 to 3 μm is 10×t 2 0.5 mm 2 or more, and the difference between the interface reaction layer thickness at the joint part center and the interface reaction layer thickness at a point distant from the joint part center by a distance of one-fourth of the joint diameter D c is 5 μm or less. According to this construction, there can be provided a dissimilar material joint product with excellent bonding strength, which can be formed by an existing spot welding apparatus at a low cost without newly using other materials such as clad material or without newly adding a separate step, and a spot welding method for the dissimilar material joint product.

1 . A joint product of a steel product and a pure aluminum or aluminum alloy material (which is hereinafter referred to as an aluminum material), said joint product being formed by joining a steel product having a sheet thickness t 1  of 0.3 to 3.0 mm with an aluminum material having a sheet thickness t 2  of 0.5 to 4.0 mm by spot welding, wherein a nugget area in a joint part is from 20×t 2   0.5  to 100×t 2   0.5  mm 2 , an area of a portion where a thickness of an interface reaction layer is from 0.5 to 3 μm is 10×t 2   0.5  mm 2  or more, and a difference between the thickness of the interface reaction layer at a joint part center and the thickness of the interface reaction layer at a point distant from the joint part center by a distance of one-fourth of a joint diameter is 5 μm or less. 
   
   
       2 . The joint product of a steel product and an aluminum material according to  claim 1 , wherein the interface reaction layer has a maximum thickness in a range of 0.5 to 10 μm. 
   
   
       3 . An electrode chip for use in spot welding for forming the joint product according to  claim 1 , which is brought into contact with a material to be joined at two or more points or in a linear or planar manner. 
   
   
       4 . The electrode chip according to  claim 3 , which is an electrode chip for use in spot welding, said electrode chip having a tip formed in a dome shape and having a recess which has a diameter of 2 mm or more and is formed in the center of the tip. 
   
   
       5 . A method for spot-welding a steel product and an aluminum material, which is a spot welding method for forming the joint product according to  claim 1 , said method comprising using an electrode chip which is brought into contact with the material to be joined at two or more points or in a linear or planar manner, for at least one of a pair of electrode chips for use in spot-welding for forming the joint product. 
   
   
       6 . A method for spot-welding a steel product and an aluminum material, which is a spot welding method for forming the joint product according to  claim 1 , said method comprising performing a spot welding while cooling at least one of the steel product and the aluminum material to 5° C. or less. 
   
   
       7 . A dissimilar material joint product of a steel product and an aluminum material, said dissimilar material joint product being formed by joining a steel product having a sheet thickness t 1  of 0.3 to 3.0 mm with an aluminum material having a sheet thickness t 2  of 0.5 to 4.0 mm by spot welding, wherein the spot welding is performed in such a state that a Zn, Zn alloy, Al or Al alloy film having a melting point of 350 to 1,000° C. and an average thickness of 3 to 19 μm and an organic resin adhesive film or phosphate film are previously provided between a joining surface of the steel product and the aluminum material to be joined together, and an area of a portion where a thickness of an interface reaction layer in a welded part after the spot welding is from 0.5 to 5 μm is 10×t 2   0.5  mm 2  or more. 
   
   
       8 . The dissimilar material joint product of a steel product and an aluminum material according to  claim 7 , wherein the area of the portion where the thickness of the interface reaction layer is from 0.5 to 5 μm is 50×t 2   0.5  mm 2  or more. 
   
   
       9 . The dissimilar material joint product of a steel product and an aluminum material according to  claim 7 , wherein the Zn film is a plating film containing 88 mass % or more of Zn and being applied to a surface on the steel product side. 
   
   
       10 . A spot welding method for a dissimilar material joint product of a steel product having a sheet thickness t 1  of 0.3 to 3.0 mm and an aluminum material having a sheet thickness t 2  of 0.5 to 4.0 mm, said method comprising performing a spot welding in such a state that a Zn or Al metal or alloy film having a melting point of 350 to 1,000° C. and an average thickness of 3 to 19 μm and an organic resin adhesive film or phosphate film are previously provided between a joining surface of the steel product and the aluminum material to be joined together, wherein the spot welding is performed with a current pattern having a step of applying a welding pressure by electrode chips, with the electrode chip on the aluminum material side having a tip diameter of 7 mmφ or more, to satisfy a relationship of (R×W) 1/3 /R>0.05 between a tip curvature radius R mm and the welding pressure W kN while passing a current of 15×t 2   0.5  to 30×t 2   0.5  kA for 100×t 2   0.5  to 1,000×t 2   0.5  msec, and having no other step of passing a current higher than the current in said step. 
   
   
       11 . The spot welding method for a dissimilar material joint product according to  claim 10 , wherein the spot welding is performed with a current pattern having a step of passing a current of 1×t 2   0.5  to 10×t 2   0.5  kA for 100×t 2   0.5  to 1,000×t 2   0.5  msec after said step of passing a current of 15×t 2   0.5  to 30×t 2   0.5  kA for 100×t 2   0.5  to 1,000×t 2   0.5  msec. 
   
   
       12 . A dissimilar material joint product of a steel product and an aluminum material, said dissimilar material joint product being formed by joining a galvanized steel product which has a sheet thickness t 1  of 0.3 to 3.0 mm and includes a galvanization layer having an average thickness of 3 to 19 μm, with an aluminum material having a sheet thickness t 2  of 0.5 to 4.0 mm by spot welding,
 wherein, at a joint interface of the dissimilar material joint product,   the dissimilar material joint product has an Al 5 Fe 2 -based compound layer on the steel product side and an Al 3 Fe-based compound layer on the aluminum material side, and   a total area occupied in the plane direction at the joint interface on the aluminum material side by a portion where a total average thickness of said two layers in a nugget depth direction is from 0.5 to 10 μm accounts for 50% or more of an area occupied in the plane direction by a nugget at the joint interface on the aluminum material side; and   wherein, in the cross-sectional SEM observation of the joint interface portion where the total average thickness of the Al 5 Fe 2 -based compound layer and the Al 3 Fe-based compound layer in the nugget depth direction is from 0.5 to 10 μm,   a total area occupied in the cross-sectional direction by Zn—Fe-based compound layers contained respectively in said two layer accounts for 10% or less of the sum total of the area occupied in the cross-sectional direction by a portion where the total average thickness of said two layers is from 0.5 to 10 μm and the total area occupied in the cross-sectional direction by Zn—Fe-based compound layers.   
   
   
       13 . The dissimilar material joint product of a steel product and an aluminum material according to  claim 12 , wherein the nugget has an average diameter at the joint interface on the aluminum material side of 7 mm or more, and a total area occupied in the plane direction by a Zn layer at the joint interface in contact with the nugget is 30% or less of an area occupied in the plane direction by the nugget at the joint interface on the aluminum material side. 
   
   
       14 . The dissimilar material joint product of a steel product and an aluminum material according to  claim 12 , wherein a minimum residual sheet thickness of the aluminum material in the dissimilar material joint part is 50% or more of an original sheet thickness of the aluminum material. 
   
   
       15 . The dissimilar material joint product of a steel product and an aluminum material according to  claim 12 , wherein the sheet thickness ratio t 1 /t 2  of the steel product to the aluminum material is 1 or more.