Fall 2013 (contents):
1. Core Value of Welding
2. Conduction Mode Laser Welding
3. Etching-The Good Corrosion
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Core Values of Welding
Every welding process has a few key parameters that have major impact on the outcome. Typical parameters are current, voltage, force, time, and power. These parameters can be directly programmed into the controller or set on the machine by turning a knob. For example, you can change the welding force for a resistance weld by increasing air (actuator) pressure or increase peak power of a laser weld by entering a new number on the programming interface. The user can then run some experiments and find optimized settings that will produce a robust weld. However, there is often a need to change the machine setup to meet new specifications or part design. For example, a user may want to make a bigger laser spot with a new focusing lens, or prefer to use a new ultrasonic horn with a larger footprint. Such changes require corresponding changes to key process parameters. The larger laser spot will require more laser power and a bigger horn will require higher welding force. In such situations, the best way to get to a new baseline is to use simple math and go back and calculate the core values such power/unit area or force/unit area. Assuming no change in materials being welded, the core values should not change much either.
Knowing core values of your welding process will help you easily scale the process up or down when changes are required. You may quickly find out that the required modifications may be beyond the capability of your machine. For example, a 40% increase in laser spot size will require a 100% increase in peak power to produce the same power density (kW/mm2). Similarly, a 40% increase in resistance welding electrode tip diameter will require a 100% increase in welding force to keep the same electrode pressure (kg/mm2) and correspondingly a 100% increase in welding current to keep the same current density (kA/mm2). Similar assessment can be made for welding and grounding cables to make sure that they are scaled accordingly to prevent overheating. Core values can be used to directly compare your weld settings to those in handbooks, published literature, or what your colleagues are using on other side of the world.
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Conduction Mode Laser Welding
When laser light is incident on a metal surface it has two options: it can be absorbed or reflected. While most of the initial light incident is reflected, some is absorbed; absorption rates are based on absorptivity of that metal at room temperature. For example, steel absorbs more of the incident light compared to aluminum. Laser light absorbed on the surface is converted into heat. As the surface temperature increases, surface absorptivity of the metal increases as well, and thus sets up a chain reaction which leads to a rapid increase in temperature and eventually leads to melting of metal on the surface. Heat generated on the surface is quickly conducted into the bulk of the material being welded. Based on factors such as thermal conductivity, rate of heat input, rate of heat loss, and melting point of the metal, the molten metal volume extends into the bulk to form a weld pool. With proper control of laser parameters, a condition can be setup where energy density incident on the surface is high enough to sustain the melt and yet low enough make sure that surface does not overheat leading to vaporization of the metal and opening up of keyhole ( see Fall 2013 Newsletter for Keyhole Mode Welding). In this situation, the laser light is only absorbed on the surface of the melt and heat is transferred inside by conduction, and hence the name conduction mode welding.
Conduction mode welds tend to be shallow and wide, with a typical width-to-depth ratio of the order of 3:1 or more. Conduction welds are commonly observed in pulsed welding which results in individual weld spots. Keep in mind that the weld diameter is usually bigger than the laser spot diameter since the heat is rapidly conducted sideways as well as into the bulk. As a result, a laser spot size of 0.4 mm can produce weld spot of 1.0 mm diameter. Higher peak power increases weld depth, whereas longer pulse duration increases weld width. Hermetic seam welds can be produced with a series of overlapping spot welds. Conduction mode weld profile has many unique applications. For example, a shallow weld would be ideal for butt welding thin sheets. A wider weld spot can be useful where it is difficult to perfectly align the laser beam to the weld seam. A shallow weld can also be used to control mixing of two metals in a lap weld configuration where only limited mixing of the bottom layer is preferred, as can be seen in the section shown below. A good weld engineer should never be embarrassed of making a shallow weld to meet specifications; it does show some deep thought.
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Etching - The Good Corrosion
Corrosion costs an enormous amount and is one of the main culprits for reduced life of any product including planes, trains, and automobiles. However, corrosion is very useful when it comes to weld analysis. A sectioned and polished weld sample can be used to identify porosity and cracks, but is often difficult to see the weld nugget, grain boundaries, and HAZ (newsletter). To bring out these features, the polished sample is unceremoniously dunked into a beaker of highly corrosive liquid mixtures (acids, alkalis, peroxides,..) which react preferentially with different phases and grain boundaries to reveal intricate details of the weld. A whole slew of these witches brews (politely referred to as reagents) are available commercially for etching. Oftentimes a weld engineer is primarily interested in getting basic information about the size of the weld nugget. If that is the case, simpler solutions can be used such as nitric acid for steels and sodium hydroxide for aluminum. A quick and crude polish, followed by an etch to see the nugget size, and the engineer is back at work fine tuning the weld parameters to get the required nugget size.
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