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

A method for spot-welding a steel product and an aluminum material, said method comprising:

• forming a joint product of the steel product and the aluminum material by spot welding a steel product having a sheet thickness t1 of 0.3 to 3.0 mm with an aluminum material having a sheet thickness t2 of 0.5 to 4.0 mm,
• wherein a nugget area in a joint part is from 20×t20.5 to 100×t20.5 mm2,
• wherein an area of a portion where a thickness of an interface reaction layer is from 0.5 to 3 μm is 10×t20.5 mm2 or more,
• wherein 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,
• wherein the aluminum material is either a pure aluminum or an aluminum alloy material, and
• wherein the spot welding is performed while cooling at least one of the steel product and the aluminum material to 5° C. or less.