Configured to set the plunging force

1. A friction stir spot welding device which welds an object to be welded, by softening the object by friction heat and by stirring the object, the friction stir spot welding device comprising:

a tool with a cylindrical shape, the tool being rotatable around an axis thereof, advanceable along a direction of the axis, and retractable along the direction of the axis;

a temperature detector that detects a temperature of the tool; and

a controller configured to weld a plurality of serial spot welds on the object, to compare i) a detected temperature of the tool (Tn), which is detected by the temperature detector in a time period which passes from end of a first weld of the plurality of serial spot welds until a start of a next second weld of the plurality of serial spot welds, to (ii) predetermined reference temperatures, and to set a plunging force (Fn) applied by the tool to press the object in the next second weld using the detected temperature of the tool (Tn) according to the following formula:

Fn=F 1−(( Tn−T 1)/( T 2− T 1))× F 2, wherein:

the predetermined reference temperature includes a first predetermined reference temperature and a second predetermined reference temperature, which is higher than the first predetermined reference temperature, and

the first predetermined reference temperature is designated by (T1), the second predetermined reference temperature is designated by (T2), a plunging force applied by the tool to press the object at the first predetermined reference temperature is designated by (F1), and a plunging force applied by the tool to press the object at the second predetermined reference temperature is designated by (F2).

2. The friction stir spot welding device according to claim 1 , wherein the controller is configured to compare the detected temperature of the tool (Tn) at the start of the first weld to the predetermined reference temperatures.

3. The friction stir spot welding device according to claim 1 , wherein the controller is configured to compare the detected temperature of the tool (Tn) in the time period to the predetermined reference temperatures to set a rotational speed of the tool for the next second weld.

4. The friction stir spot welding device according to claim 3 , wherein the controller is configured to reduce the rotational speed of the tool so that the rotational speed is lower than a rotational speed at the first predetermined reference temperature (T1) when the detected temperature of the tool (Tn) is higher than the first predetermined reference temperature (T1).

5. The friction stir spot welding device according to claim 1 , wherein the controller is configured to compare the detected temperature of the tool (Tn) in the time period to the predetermined reference temperatures to set an actuation time of the tool in the next second weld.

6. The friction stir spot welding device according to claim 5 , wherein the controller is configured to reduce the actuation time of the tool so that the set actuation time is shorter than the an actuation time at the first predetermined reference temperature (T1) when the detected temperature of the tool (Tn) is higher than the first predetermined reference temperature (T1).

7. The friction stir spot welding device according to claim 1 , further comprising:

a clamp member that is advanceable and retractable in the direction of the axis,

wherein the temperature detector detects a temperature of the clamp member as the temperature of the tool.