Summer 2014 (contents):
1. Single Failure Mode
2. Induction Brazing
3. Ms. Moly B. Denum
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Single Failure Mode
One of the key steps in developing a welding process is to test the welds to failure. Such testing often involves some form of mechanical testing including pull, peel, and shear testing. Additional tests can include vibration, thermal cycling, corrosion testing, or a combination of the above in some accelerated form to estimate life of the weld in an actual application. Failure test data can then be evaluated and compared to weld specifications in the form of a Cpk number or cycle life to make sure that the welds meet requirements.
However, just calculating Cpk or some similar metric is not enough. It is also important to make sure that the welds have a single failure mode at the optimized settings. Early on during process development when weld parameters are being optimized, a larger range of settings are evaluated; during that process the welds may have different failure modes at the extremes. For example, while developing a cross-wire resistance weld, the failure may be at the interface at low energy settings, in the parent metal at medium energy levels, and at the weld at high energy levels. But once the medium energy settings are chosen as optimum, all welds made at those settings should fail in the parent metal. If you have more than one failure modes at the optimized settings (even if the strength values meet Cpk), it could be an indication of variation that is not properly controlled or an indication of a process that is not well-centered in the process window. Variation can be introduced by having multiple part vendors, different coating batches, multiple sets of fixtures, uncontrolled contamination issues, or multiple welding stations. For example, weld parameters optimized for parts from one vendor may not turn out to be optimum for parts from another vendor. Welds that are supposed to be identical but fail in different modes will have unpredictable performance when in use and could add to the risk. A welding process at the edge of a process window may result in some welds being ductile and have long life while others may be brittle and fail prematurely under dynamic loading. A welding engineer would be wise to keep a close eye on the failure mode, not just the glorified Cpk number.
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Induction Brazing
Induction brazing is a process that generates heat required for brazing by inducing currents to flow in the parts and generate heat due to part resistance to that current flow. Currents are induced by placing the parts to be brazed in the electromagnetic field of a coil conducting alternating current. The coil is commonly made of copper tubing with cooling water flowing inside. With suitable coil design, the parts can be heated rapidly and locally to produce a uniform braze temperature at the joint while limiting temperature exposure at other areas, which prevents collateral damage. Since transfer of electrical energy is through an electromagnetic field, this process allows heating of the parts from the outside while the braze assembly is shielded inside, as in a glass tube or enclosure. Confining the braze joint permits use of suitable shielding gases to provide inert, vacuum or reducing atmospheres. Heat generated is very sensitive to position of the parts with respect to the coil and hence the parts have to be fixtured in place before brazing.
Induction brazing power supplies generate high frequency current in the rage of 10 kHz to 8 Mhz. Lower frequencies produce deeper heating through the thickness of the parts and helps equalize heating of two parts of unequal mass. Higher frequencies are more suitable for brazing conductive alloys and for parts that have a thin section. Induction brazing can be used for all metals and alloys but not for brazing ceramics due to their lack of electrical conductivity. As with any I2R heating, and similar to resistance welding, there is a potential risk of runaway heating towards the end of the brazing cycle which can lead to catastrophic meltdown of the parts with even a few extra seconds of heating. Temperature can be controlled with a taper down of input power in the controller, an external feedback loop with a pyrometer, or a manual footswitch to cut-off power once the brazing process is complete. With the advantages of non-contact, localized, shielded heating, and the ability to automate the process, induction brazing has found wide application in practically all manufacturing environments.
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Ms. Moly B. Denum
Ms. Denum, or "Moly" for those of us on a first name basis, is a high-melting point (2623C or 4753F) metal which gets its name from Molybdos, which is greek for lead, because the ore Molybdenite (MoS2) was originally thought to be a lead compound. Due to its high melting point and inert nature, it is often used as an electrode material in many applications including glass melting, electrical filaments, and resistance welding electrodes. Moly is also used as an alloying element in steels, imparting strength and corrosion resistance. A good example is 316 SS which is similar to 304 SS but with 2% Moly that imparts good corrosion resistance in marine environments exposed to chlorides. However, Moly addition also makes the 316 alloy fully austenitic resulting in increased likelihood of solidification cracking during rapid cooling, a common problem during pulsed laser welding.
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