Using an Argon Helium mix will also aid in rapid penetration...
The first thing one needs to know is what type & grade of cast Aluminum is being welded together as a repair or fabrication...
For instance
Preheating of the casting is not necessary before making a weld repair except in case it is required to reduce the size of gas porosity in the weld.
However preheating to 300 to 400 degrees F may be necessary to prevent cracking or distortion.
Heat treatable alloy castings are often repaired or welded with a filler rod of the same composition as the base metal. If needed, filler material can be cast in rod-shaped molds in the same casting facility. In this case post weld heat treatment will develop the same properties in the weld as in the casting.
In most cases however, the filler metal is not heat treatable or only mildly heat treatable. This selection is adopted for ease of welding, if hot short weld cracking is feared.
However, to obtain at least some response to post weld heat treatment, one should strive to keep weld composition around a ratio of 70% filler alloy to 30% base alloy.
Different filler metals may be adequate for a given application but only one may give the optimal response to specific performance requirements.
The following factors must be considered:
Ease of welding, freedom from cracking, strength of weld, ductility, properties at service temperature, resistance to corrosion,
color match of weld and base metal after anodizing...
The most popular filler metal alloys for casting repair are usually selected from the following list:
2319, 4008, 4009, 4010, 4011 (wrought alloys similar to cast C355.0, A356.0, A357.0), 4043, 4047, 4145, 5183, 5356, 5554, 5556.
Castings that require heat treatment should be heat treated after welding. Welding heat treated castings will reduce locally the tensile properties.
Postweld heat treatment, even if limited to aging alone, will restore some of their properties if the proper filler alloy has been used in welding.
With TIG welding, an alternating current (AC) power source is generally used. Aluminum forms an oxide layer on the surface, which can contaminate the weld.
An AC power source switches the polarity of the arc between positive and negative. When the polarity of the arc is positive, current flows from the work surface towards the electrode producing a cleaning effect that blasts the oxide away from the surface. When the arc switches back to negative, energy flows from the electrode to the work surface producing heat that melts the metal.
By increasing or decreasing the duration of the positive or negative phase of the AC cycle, the cleaning and welding characteristics of the arc can be modified.
Some TIG welders have a "squarewave" control feature that allows the electrode negative (EN) portion of the AC cycle to be increased up to 90 percent.
By increasing the negative phase the AC cycle provides greater heat penetration, faster welding and narrows the weld bead.
It also allows the use of a smaller diameter tungsten electrode to more precisely direct the heat in a confined area.
Reducing the negative phase of the cycle produces greater cleaning action to remove heavier oxidation, and reduces heat penetration when welding thin castings.
Most of the significant GTA welding advances focus on new ways of manipulating the AC wave form. These advances largely resulted from using "inverter-based" power sources, which permit controlling the arc in ways never before possible, i.e., extending the balance control, adjusting output frequency and independently controlling current in each half of the AC cycle.
There are no hard rules about setting balance control, but the typical error involves over-balancing the cycle. Too much cleaning action (electrode positive or EP) causes excess heat build-up on the tungsten. This creates a very large ball on the end of the tungsten electrode.
Subsequently, the arc loses stability and the operator loses the ability to control the direction of the arc and the weld puddle.
Arc starts begin to degrade as well.
On the other hand, too much electrode negative results in too much penetration and a scummy weld puddle. If the puddle looks like it has black pepper flakes floating on it, add more cleaning action (EP) to remove oxides and other impurities.
Inverter-based welders let operators adjust the welding output frequency from 20 to 250 Hz. Conventional welders have a fixed output of 60 Hz.
Decreasing frequency produces a broader arc cone, which widens the weld bead profile and better removes impurities from the surface of the metal.
It also transfers the maximum amount of energy to the work piece, which speeds up applications requiring heavy metal deposition (as when filling a large crack or void in a casting).
Increasing frequency produces a tight, focused arc cone. This narrows the weld bead, which helps when welding in tight spots. The operator can direct the arc precisely at the crack and not have the arc dance from side to side. And, very importantly, operators can use a pointed tungsten, which further improves arc control and bead shape.
A good starting point for general welding, would be 80 to 120 Hz. These frequencies will be comfortable to work with, increase control of the arc direction and boost travel speed. For a fillet weld application with full penetration in the weld without putting too much amperage in the metal, increase the frequency to 225 to 250 Hz. For build-up work, start at 60 Hz and adjust lower from there.
Independent amperage control of the EN and EP portions of the AC cycle, allows the operator to direct more or less energy into the work piece, as well as take heat off the tungsten. For example, when welding a thick piece of aluminum, the operator can put 250 amps of EN into the work and only 60 amps of EP into the tungsten.
This provides faster travel speeds, faster feed of filler rods, deeper penetration, and the potential to eliminate preheating.
Some companies have cut production time by up to two-thirds using this technology.
Independently increasing EN amperage while maintaining or reducing EP amperage also provides the following: narrows the arc cone, nearly eliminating the etched zone at the toes of welds... It also lets the operator use a smaller diameter electrode to make narrower welds, example; using a 3/32˝ diameter to weld at 280 amps EN; may allow the use of straight Argon in place of Argon/Helium, a costly mix which produces more heat.
With the new technology directing heat into the work, not the tungsten, straight Argon alone may suffice.
Independently increasing EP amperage while maintaining or reducing EN amperage produces a wider arc cone, wider bead and shallower penetration.
No strict guidelines for setting independent EN and EP current values have been established because it is such new technology.
However, like balance control, don’t start out with huge variations.
Start with practice pieces and experiment to find the values that work best for a particular application.
Frequency control is another area where advances have been made. The AC arc in a traditional, rectifier-based TIG welder is prone to stumbling, wandering and outages, all of which lead to poorer weld quality. These symptoms usually occur during the EN to EP transition because the welder does not have enough voltage to drive through the zero amp range and then reestablish the arc at the electrode, or because the welder cannot transition through the zero amp range quickly enough.
To improve arc starts and arc stability, a traditional GTAW machine superimposes high frequency (HF) on the AC sine wave to form a path for the arc to follow as it crosses the zero amp range. As a result, one control switch on the front panel typically lets the operator select between "HF off," "HF start only" or "HF continuous."
Unfortunately, welders generate HF at 1.2 MHz, a frequency close to that of AM radio. As a result, HF interferes with electronics such as that used in CNC machines, computers and other electronically controlled equipment that may be in the shop.
Using an inverter-based AC TIG welder, which uses microprocessor controls to quickly switch between EP and EN, can minimize such problems.
An inverter eliminates the need for continuous high frequency to maintain the arc, as well as and the cost of purchasing a separate high-frequency module.
It can also minimize or even eliminate the need for HF on start-up.
Welding variables
A basic, professional quality TIG welder with AC output lets the operator adjust four variables: amperage, balance control, shielding gas pre-flow time and gas post-flow time.
The welder will have either potentiometer-type control knobs or touch panels to control these variables.
"Pre-flowing" the shielding gas serves two functions. It purges the immediate weld area from contaminants in the surrounding atmosphere, and it aids with arc initiation.
For non-critical applications, says setting the pre-flow control timer at one second should sufficiently purge the weld area. For critical applications, increase pre-flow time to six seconds or more (a typical TIG welder offers a 0 to 10 second range).
"Post-flowing" the shielding gas protects the weld puddle, at the end of the weld, while it cools through temperature ranges where the weld becomes more susceptible to oxidation, cracking and contamination. As a rule of thumb, use one second of post-flow time for every 10 amps of weld output (0 to 50 seconds is an average TIG machine’s capability).
The larger the puddle, the longer the required post-flow time. The post-flow also cools the tungsten and protects it from oxidization.
Some TIG welders have "sequencer controls" which provide added control over the start current, start time, crater time and final current.
The start controls (current over time) allow the operator to have a hotter or cooler start in comparison to the welding amperage.
Thick aluminum castings, which act as a giant heat sink, benefit from hot starts because the extra amperage on start-up helps form a puddle more quickly.
Thin sections, at risk from melting or warping, benefit from a cool start.
The "start current" control knob lets the operator set the starting amperage at any point in the machine’s output range, while the "start time" control knob adjusts a timer, usually from 0 to 15 seconds. As soon as the operator triggers the amperage contactor/control device, i.e., foot pedal or fingertip control, the weld output equals the full preset start value (as opposed to manually ramping up to a value width). Once the timer times out, amperage control returns to the foot pedal or fingertip device.
"Crater time" control ramps down from weld current to final or minimum current over time; time is typically adjustable from 0 to 15 seconds.
"Final current" control lets the operator select the final amperage as a percentage (0 to 100) of the welding amperage.
These features help slowly cool the weld, which prevents crater cracking, a common problem with aluminum.
Activated by a contact switch on the torch, crater time and final current control simulate slowly letting off the foot control.
Welding guidelinesThough opinions may differ about specific details, there are 12 generalized steps to GTA welding Aluminum castings...
Following these guidelines should allow you to repair aluminum cylinder heads successfully:
1) Clean castings thoroughly. Some experts prefer thermal cleaning because it bakes all the oils and resins out of the casting.
But if you use thermal cleaning, don’t get the head too hot — see #5). Bead blasting may be needed after thermal cleaning to remove carbon and other residue.
A stainless steel brush can also be used to clean the surface (use the brush for aluminum only or you may contaminate the weld).
2) Pressure test the casting to check for cracks and porosity leaks. (if necessary)
3) If repairs are needed, remove any internal or external components that can easily burn or warp.
4) Fully grind out all cracks. Milling may be less messy than die grinding, where possible...
Clean the casting repair location with a safe, non toxic producing (Hydrogen chloride and/or phosgene gas) cleaner, such as acetone followed by alcohol and wait until all of the chemicals have evaporated.
5) Preheat the casting to 400° to 500° F for up to two hours before welding — unless the casting is a heat-treated alloy, in which case keep the temperature under 275° F. Overheating a heat-treated aluminum alloy can anneal (soften) the casting. The same caution applies when welding a heat-treated head, too.
Limit the weld time to about five minutes or less to avoid overheating the casting.
Once the casting is at temperature, it will retain heat for 15 to 20 minutes. Using a insulating type of blanket heat shielding around the weld area to reduce drafts and retain heat will improve heat retention and extend your welding time. If the heat’s temperature drops too low, put it back in the oven, or heat it up with a torch if possible and reheat it back to your working temperature.
6) Use an AC TIG welder with at least 250-amp capacity. A water-cooled torch and foot amp control are also recommended.
7) Use 15-20 cfm of argon shielding gas while welding, or a 25/75 mixture of argon/helium for faster welding.
8) Use a 9X to 11X welding lens to protect the operator’s eyes. Gold tint shields provide better visibility than green tint.
9) The type of electrode used will depend on the type of equipment. Pure tungsten, or "zirconiated wulfram" with a ball tip works well with most conventional GTA welders, but a sharp tipped 2% thorated tungsten electrode can be used with inverter-based equipment.
When welding cracks, use wave balancing to clean and float contaminants out of the weld puddle.
Do not allow the tungsten electrode to contact the base metal or filler rod. Start the arc using high frequency holding the electrode about 1/8˝ from the surface.
Maintain a consistent arc length equal to about one electrode diameter from the surface.
10) Use ER 4043, ER 5356 or other compatible alloy filler rod to add metal as needed. Store filler rods in a sealed container to minimize oxidation and possible weld contamination. When filling cracks, keep the filler rod in the weld zone so the shielding gas can protect it from oxidizing.
Hold the filler rod at a 15 to 20° angle from the workpiece, creating a 90° angle between the filler rod and tungsten electrode.
11) Put the casting back in the oven after welding, allow temperature to stabilize, then shut the oven off and allow the casting to slowly cool for several hours back to room temperature. This will minimize the risk of thermal stress causing new cracks to form.
12) finally, machine blend weld surface layer according to corresponding shape/texture of the adjacent surfaces if required, and inspect for any defects before declaring the repair completed.
Respectfully,
Henry
