A method of welding a joint includes directing a first output from a high energy density heat source, such as a laser, against a first side of the joint. The method further includes directing a second output from an arc welding heat source, such as a gas metal arc welding torch, against a second side of the joint. The first output produces a keyhole surrounded by a molten metal pool which extends from the first side of the joint toward the second side of the joint. In some embodiments a third output from a second arc welding heat source may also be directed at the first side of the joint. A second molten metal pool produced by the arc welding heat source joins with the first molten metal pool and the third molten metal pool to form a common molten metal pool which solidifies to form the weld.