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Introduction

Earlier in our story of Aluminum, we provided background information and data concerning the production of aluminum, its alloys, its varying chemistry and uses. We are now ready to move onto its weldability which poses a number of things we need to be aware of prior to attempting to join. These are:

• Porosity. Hydrogen gas is very soluble in molten aluminum and can result in porosity in the solidifying weld pool

• The oxide layer on the surface has a very high melting point (approx. 2060 Deg C) which is much higher than the underlying base metal which melts around 660 Deg C. If the oxide gets into the weld pool it can cause lack of fusion and a reduction in strength

• Its high thermal conductivity which results in fast removal of heat and the possibility of distortion due to the need to weld with high heat inputs

Regarding porosity, the main cause in aluminium is hydrogen, which has very high solubility in molten aluminium but very low solubility in solid. Hydrogen gas is therefore evolved as the weld pool solidifies. If the cooling rate is too high, the gas remains in the metal in the form of porosity. Thus, any compound containing hydrogen and contaminating the filler wire or work surface can cause porosity.

Filler wires should ideally be kept in their packaging until needed; wire that is left out in open workshop conditions will absorb moisture into its oxide layer. It is advisable when GTAW welding aluminium to wipe each wire prior to use with a clean rag dipped in acetone. Oil, moisture or other contaminants may be present on the filler wire. In addition, the oxide layer of aluminium tends to get hydrated and improper cleaning of the oxide layer immediately preceding welding could be a cause for porosity. Ensuring that the plate is clean before welding and switching to clean, high-quality electrodes will reduce the likelihood of forming porosity.

When welding, the surface oxide film is problematical and the welding processes that help breaking up this film up are the Gas Metal Arc Process (GMAW) and the Gas Tungsten Arc process (GTAW). For this and, other reasons, these two processes offer the preferred way of joining aluminum and its alloys.

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The GMAW Process on Aluminum

Because of the deeper penetration that can be obtained with the GMAW arc and the high current densities employed, this process is particularly suited to welding aluminum alloys in a wide range of thickness. When welding aluminum, the GMAW process operates on direct current (DC) with the electrode positive as per Figure 1.

Electrons flow from the electrically negative workpiece to the positive electrode. At the same time there is a so called 鈥渋on flow鈥� or bombardment from the positive electrode onto the surface. This ion flow sputters the surface and removes, or cleans away, the problematic oxide. This cleaning action,听 called 鈥渃athodic cleaning鈥�, works best with the inert argon shielding gas.

Cathodic or 鈥渁rc鈥� cleaning is essential in GMA welding to facilitate the coalescence between the faces of the joints.

Figure 1. Electrode positive applied to GMA welding means that the filler wire is the positive electrode (DCEP)

Due to aluminums high thermal conductivity, the use of high energy modes of metal transfer are required and, consequently, spray and pulsed spray transfer are the two recommended GMAW modes of transfer for aluminum.听

With direct current electrode positive (DCEP), when the current density exceeds a certain transition value, the filler metal is transferred through the arc in a stream of fine droplets, defined as spray. The transition current density varies with electrode size and alloy but the arc stabilizes as higher currents and, therefore, higher current densities are achieved.

Pulsed current welding is a variation of the GMAW process in which metal transfer takes place periodically as controlled by pulsing the welding current. A low-level current is used to maintain the arc for electrode melting and parent metal cleaning. The high current pulses are superimposed to cause metal transfer. The result is a process in which metal transfer takes place in the spray mode, but at greatly reduced average welding currents. The advantage of this process for aluminum welding is a much softer arc in which penetration is more readily controlled and travel speeds are lower. This makes possible the welding of lighter gauge material than would be possible with normal spray transfer GMAW welding. Figure 2 summarizes the effects of pulse parameters.

Figure 2 Output current waveform of a pulsed current power supply showing the metal transfer sequence

The inert gases, argon and helium are the only gases in general use. Of these two, the lower-cost argon is most commonly used, but a mixture of argon with helium is sometimes justified for welding of thicker aluminum materials because of the greater heating effect of the helium gas.

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The GTAW Process on Aluminum

Gas tungsten arc welding of aluminum is usually performed with alternating current. It was originally used to weld a wide range of products and structures in all thickness ranges. GTAW has now found application mainly for the thinner gauges, for complicated joints, for repair welding and for autogenous welds.

Using AC power, modern day electronic power sources are able to correct and control the imbalance in the alternating wave, and in certain applications, can control the cleaning action (surface oxide breakdown) or the penetration by setting the imbalance at a specific acceptable level as shown in Figure 3. For example, with the control set for greater cleaning action, the electrons fly off the metal surface and break down the oxide film leading to a more stable arc.

Figure 3. AC balance control for desired weld features showing setting for increased cleaning action

Shielding gases used for GTA welding aluminum are completely inert. Inert gases commonly used are argon, helium and argon-helium mixtures at 99.9% purity or above.听

Argon is normally the shielding gas of choice because of its:听

• smoother and quieter arc听

• lower arc voltage for a given current and arc length, making it easier to weld thin materials without burn-through听

• greater cleaning action in welding aluminum using alternating current听

• easier arc starting听

• lower flow rates than helium for good shielding and better resistance to wind drafts听

• lower cost and greater availability听

Helium, for the same arc length, requires greater voltage than argon, leading to higher heat inputs. It is therefore, used advantageously for shielding when welding thicker aluminum joints.

The other area of concern when welding aluminum alloys is possible distortion caused by the necessary use of high heat inputs. This is a weld procedural item and can be controlled by judicious use of welding parameters, balanced welding and jigs and fixtures.

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In the next article in our continuing 鈥淪tory of Aluminum鈥� we will move onto the welding consumables that are available for use with the welding processes we have covered above.