Metal transfer in welding describes how molten metal moves from the electrode or wire to the weld pool. This fundamental process is central to welding, directly influencing how filler material is deposited to create a joint. Understanding metal transfer is important for achieving desired weld characteristics and overall structural integrity.
Understanding Metal Transfer
Metal transfer involves physical forces that cause molten metal droplets to form at the electrode end and detach, traveling across an electric arc to the workpiece. Surface tension holds the molten metal to the electrode tip, while gravity pulls the droplet downwards. Electromagnetic forces constrict the molten metal at the electrode tip, helping to detach droplets. Gas drag, caused by the flow of shielding gas, also influences droplet movement across the arc gap. Consistent movement of these molten droplets is important for creating strong, reliable welds, ensuring proper fusion and minimal defects.
Common Modes of Metal Transfer
There are several primary modes of metal transfer commonly used in welding, each with distinct characteristics.
Short Circuit Transfer
Short circuit transfer occurs when the electrode wire repeatedly touches the molten weld pool, creating a short circuit that melts and transfers metal. Short circuit transfer operates at lower current and voltage settings, making it suitable for thinner materials and all welding positions.
Globular Transfer
Globular transfer involves larger, irregularly shaped droplets of molten metal detaching from the electrode and falling into the weld pool. This mode uses higher current and voltage than short circuit transfer, resulting in a hotter and more fluid weld pool. Globular transfer produces more spatter and a less smooth bead appearance, and it is generally restricted to flat and horizontal welding positions due to the fluid nature of the weld pool.
Spray Transfer
Spray transfer is characterized by a continuous stream of very fine molten metal droplets spraying across the arc to the weld pool. This mode requires higher current and voltage settings and an argon-rich shielding gas. The arc is stable, producing very little spatter and deep penetration, making it suitable for thick materials and high deposition rates, though generally limited to flat and horizontal positions due to the high heat input.
Pulsed Spray Transfer
Pulsed spray transfer is a variation of spray transfer that cycles between a high peak current, which detaches one droplet per pulse, and a lower background current, which maintains the arc. This pulsing action provides better control over heat input and droplet transfer. Pulsed spray transfer minimizes spatter and allows for out-of-position welding while still offering good penetration and deposition rates. It is versatile for various material thicknesses and positions.
Controlling Metal Transfer
Welders manipulate several key parameters to achieve a desired mode of metal transfer. Welding current influences the heat generated and the rate at which the electrode melts. Increasing the current generally leads to higher deposition rates and can transition metal transfer from short circuit to globular and then to spray mode as the current rises sufficiently. Arc voltage also plays a role in controlling the arc length and the stability of the metal transfer. Voltage adjustments affect droplet size and transfer smoothness, with higher voltages often promoting spray transfer.
Shielding gas composition significantly impacts the arc characteristics and metal transfer. Different gas mixtures are used for various transfer modes, such as argon-rich mixtures for stable spray transfer. The electrode wire diameter and type also influence metal transfer. These parameters collectively dictate the forces acting on the molten metal, thereby controlling how droplets form and transfer to the weld pool.
Effects on Weld Quality
The chosen metal transfer mode has practical implications for weld quality. Short circuit transfer, with its lower heat input, is effective for joining thin materials and minimizing distortion, but it can be prone to spatter and the risk of cold lap. Globular transfer generally produces more spatter due to larger, less controlled droplet detachment, and it can result in a less aesthetically pleasing weld bead. While it offers good penetration, the spatter often requires more post-weld cleanup.
Spray transfer yields very low spatter, a smooth bead appearance, and deep weld penetration, making it suitable for structural applications requiring high integrity. However, its high heat input can lead to burn-through on thinner materials and limits its use to flat and horizontal positions. Pulsed spray transfer offers a balance, reducing spatter compared to globular and improving bead appearance over conventional spray transfer. It also allows for good penetration and fusion while providing better control over heat input, which helps prevent defects and enables out-of-position welding on thicker sections. Selecting the correct metal transfer mode is important for achieving the desired weld properties for a given application.