Induction brazing is a high precision, high reliability process that has major advantages | By Mak Babi |
Induction brazing is widely used in highly critical manufacturing steps where good strength joints are desired without overheating the substrates, apart from the high degree of chemical cleanliness as it is a flux-free operation. Chemical residue left behind in conventional brazing can be a terrible germination for future trouble in a myriad ways. There are several ‘flux-free’ techniques of brazing that produce a clean brazed joint without leaving a corrosive residue. Induction brazing is fast, clean and highly reproducible when automated. The high-quality and high-reliability products in the aerospace industry and the highly demanding mix of physical properties in the cutting tools industry all depend on induction brazing.
Induction brazing is the joining of similar or dissimilar metal (parent materials) by using heat and braze material (filler material) whose melting temperature is normally above 5000 C and below the melting temperature of parent materials being joined. In induction brazing the temperature can be controlled from 5000 C to 18000 C for most of the brazing applications. It is a process where two or more workpieces such as pipes are joined together with a molten space filler metal using an induction heating coil which delivers a very high temperature, very fast.
Today, induction brazing is used in a broad range of manufacturing systems including refrigeration and air-conditioning, household utensils, tubular structures, automotive parts, tools and machinery, electrical components, nautical and aerospace equipment, farm implants, and business machines. All these can be attained by following proper brazing procedure, which can be read from the equipment suppliers’ instruction manuals. In this method temperatures are usually reached in a matter of seconds for higher manufacturing productivity. The method uses magnetism to induce an electrical current that heats up the base material. Induction heating in induction brazing has proven an important tool in these joining processes. It allows for rapid localised heating, joining high-strength components with minimum loss of strength.
An exact heat control allows the brazing process to be performed efficiently. Induction brazing alloys melt at high temperatures and provide high strength joints that can resist reasonably elevated temperatures without failure. Many of the alloys used for brazing are presented in the form of wire, strip, powder and also washers or rings for induction brazing process purposes. The design of the induction coil must be made to order, to suit the design and bulk of the workpiece.
When using induction brazing, special attention must also be given to the heating pattern; the method of pre-placing the joining alloy; the tolerances between mating parts; the thermal conductivity; and the expansion characteristics of the material to be joined. It is important to consider the joint strength which has to be of maximum possible value when designing the brazing process. This can be accomplished, in many instances, by preplacing the joining alloy in such a way that it will flow into the joint by gravity and capillary attraction, showing uniform and complete penetration by appearance of the alloy at the opposite end of the joint. Materials like stainless steel, copper and brass, carbon steel, aluminium and its various alloys, carbides (nonmetal, a ceramic material that is impossible to weld) are commonly induction brazed.
It is common knowledge that copper has superior thermal conductivity than aluminium. And it is also known that aluminium is about one third the density of copper (2.7g/cm³ for Al and 8.9g/ cm³ for Cu). One might conclude then that you use copper/brass when you want heat transfer efficiency (good cooling) and use aluminium when you want weight savings. However, as will be explained in more detail in the section below, aluminium radia- tors can be significantly lighter than similar copper/brass units and still provide better cooling. Therefore, better heat transfer efficiency would result if the tubes were wider, thereby increasing the fin-to-tube contact area.
A typical copper radiator uses 3/8” (9.5250 mm) to 5/8” (15.8750 mm) wide tubes. However, increasing the width of the tubes would also require an increase in tube wall thickness to prevent ballooning and for copper, the penalty in weight gain could be severe. Increasing the tube wall width to 1” (25 mm) would require double the wall thickness of 5/8” (15.8750 mm) tube resulting in a radiator weighing up to 60 lbs (approx. 30 kgs).
The answer to the above dilemma is to use aluminium. For example, a radiator could be manufactured with 1” (25 mm) to 1 ¼ “ (31 mm) wide tubes with a suitable wall thickness to prevent ballooning and still be up to 60% lighter than the same radiator built from copper. Furthermore, the increased tube-to-fin contact area in this example increases cooling capacity by roughly 25%. Perhaps the most demanding applications for induction brazing come from aerospace components. These are made from aluminium and titanium alloys, superalloys based on chromium and cobalt, which are all tricky to weld with conventional techniques.
High precision parts, with zero defect regimes are routinely made. Lack of space does not permit us to explore ten-twelve other areas wherein induction brazing is daily used as a manufacturing technique. In addition, there are a vast number of users who have either found vacuum brazing cumbersome and expensive, or conventional brazing unreliable with corrosion topping the list of problems. Vacuum brazing can produce superior joints but furnaces are very expensive, operational costs are high, controlled atmosphere brazing (CAB) is a viable alternative but again like vacuum brazing it needs expensive chambers. Induction brazing can be performed in the open, at lower costs, with enviable precision with lack of chemical contamination, at very high speeds and with total reproducibility. It straddles the pride of place among competing techniques.