High Frequency welding is a process which uses an electric field to create the heat needed to melt a metal/metal joint? The field is created when the metal to be welded is passed through a copper coil i.e. both the metal to be jointed and the copper coil have inherent magnetic properties-ability to magnetise. Metals commonly welded using this process are stainless steel and aluminium. This is actually the preheat and heating stage of the tube similar to preheating of a flat weld piece before welding to make it molten/softer before actual welding.
Radiators in your car are made from aluminium components and the primary component in the radiator, which is the tube, is produced through this process. The process entails running a flat metal sheet (cut into width sizes from 32mm-128mm) through different welding stations which gradually fold the flat sheet into an oval shape or for b-type tubes into a rectangular shape.
After folding both ends of the sheet will meet forming the weld joint, ideally the joint must resemble a flat butt weld joint. But since sheets are cut using blades which are subject to wear and tear, the ends of the joint might have a V-like surface creating a V-joint instead.
Once the joint is centralized the folded tube passes through an impeder which opens the tube up creating an insertion angle called α; refer to Fig.1.
Fig.1: Exaggerated version of folded tube at welding.
- If you pull the impeder back then the angle α will decrease, resulting in higher surface area of tube joint contact. If the weld roller pressure is consistent then a gradual welding pressure will be exerted and a more even weld bead will result. Meaning a better quality welded tube will be created as your fusion zone (region where weld/tube joints meet) will be increased (through pressure exertion by the rollers and greater surface contact); and is evidenced by a large weld bead; refer Fig.2. However the aim is not to get too much weld bead formation but adjust the impeder so as to get a balanced weld bead that must not be >0.3mm and <0.1mm. Although a definite standard has not been set for the 0.1mm as a minimum and the maximum is often set by the customer ordering these types of tubes. An international standard is yet to be defined.
- If you push the impeder forward then the angle α will increase, resulting in a lowered surface area of joint contact. Assuming a consistent welding roller pressure exertion; the fusion zone will also be decreased resulting in a low weld bead. A phenomena like this is similar to spot welding, in actual fact, HF can referred too as a more advanced form of spot welding. The only thing is: if the impeder is moved too forward then there are greater chances of producing inconsistent welding (discontinuous welding) on the tube. This is evidenced by leaking tube before brazing (which we will explain in a separate article) or low burst pressure at production. Also with adjustment of this type a balance has to be reached based on set tolerance (customer related) and process parameters (production induced).
Welding Rollers
I need to elaborate further on welding rollers for the sake of greater understanding. The rollers as explained exert pressure on the tubes as it passes through them and depending on the extent of pressure exerted can either exert too much pressure or too little. Too much pressure leads to; too high a weld bead which can sometime exceed 0.3mm. However low pressure exertion will result in improper fusing of the weld joints leading to a weaker weld and subsequent bad quality tube/s.
Note: In this section quality is determined by the life cycle your cars radiator will have after tube manufacturing. So it stands to reason that tube which fail during production are a good example of extremely bad quality and those which pass the radiator tests without fault are a good indication of excellent quality. But that is not necessarily true as bad tubes can still pass all the standard tests during radiator production and still fail during service of the tubes. This is related to hidden faults that at this stage of the tube manufacturing technology are not a factor as yet; but might be in future.
The roller also determine the alignment of the weld joint, if one side of the roller is higher than the other then this will lead to misalignment a phenomena related to tooling adjustment at the rollers more than the rollers themselves.
Quenching and Cutting
After welding comes quenching of the welded tube, the water temperature can vary from cold to warn due to usage from the first tube run right through the daily run. This will mean a varied weld outcome from a more hardened weld fusion at cold temp to a less tough joint at warm temperature; refer to Fig.2.
