What type of weld aligns two metal pieces approximately in the same plane?

The butt joint is specifically designed to align two metal pieces edge-to-edge in the same plane. This type of joint allows for strong connections where the pieces are butted together, making it suitable for applications where strength and continuity are critical. In a butt joint, the edges of the materials are precisely aligned, facilitating efficient welding and resulting in a smooth surface that is often required in structural applications.

Other types of joints, such as the T joint, lap joint, and fillet joint, serve different purposes. A T joint is created when two pieces are positioned at a right angle to each other, resembling the letter "T." A lap joint involves one piece overlapping another, which can result in a stronger connection but does not maintain the same flat alignment in the weld area. A fillet joint is used to join two pieces at an angle to each other, which does not satisfy the requirement of aligning them in the same plane. Thus, the butt joint is the appropriate answer for aligning two metal pieces in the same plane.