Spring 2016 (Issue No. 30) Contents:

1. Welding of Ni Plated Components

2.Upset Welding

3. Breaking In

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Welding of Ni Plated Components

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Industrial components are often Ni plated for a variety of reasons including appearance, diffusion barrier layer, corrosion protection, and surface hardness.  In most applications, the plating process is the final step.  However, there are situations where the parts have to be welded after plating which introduces a whole bunch of challenges.  To resolve such issues, it is important to step back and first understand the types of plating and their effect on welding.   There are two types of plating processes: electroplated Ni and electroless Ni.  Electroplated Nickel, as the name implies, is a process that plates a metallic component in an electroplating bath with a Nickel anode.   The plating process is controlled to produce Ni plate of required thickness; usually, the plated layer is almost pure Ni. On the other hand, an electroless Ni plating deposits a layer that incorporates a small amount of phosphorous along with Ni; phosphorous can be in the range of 3-15%.  Electroless Ni process is able to produce plated layer of uniform thickness even if the component has many small features such sharp steps or blind holes.

 

For electrical applications where a Cu conductor with Ni plating and gold flash is to be soldered, the Ni plating provides a barrier layer and prevents dissolution of copper into the solder.   In situations where higher application temperatures are required, a weld is preferred instead of a solder.  To make a weld, the two terminals are typically resistance welded where the nickel layers provide a convenient resistive interlayer and help to focus the welding heat at the interface.  For a strong bond, the user may have to push enough energy to melt the nickel layer which then fuses with the copper base form a fusion nugget (see photo below).

 

Ni plated structural components are commonly made of Ni-plated carbon steels and are chosen as cheaper alternative to stainless steels.  When such components are fusion welded as with laser or TIG welding, the user is faced with two problems.  First, the Ni layer melts and mixes with the base steel in the fusion zone; the resulting weld nugget no longer has the protective Ni layer and is susceptible to corrosion.  The second problem is encountered when welding electroless Ni plating where the phosphorous in the plating reacts with Ni and Fe to form low melting phases that have poor strength. Such phases are segregated to the center of the weld and along grain boundaries, and under suitable stress conditions, can form cracks in the weld.  The amount of cracking can be reduced by using a low phosphorous plating; and if possible one should switch to electroplating which is practically pure Ni.  However, using electroplated Ni is no sure bet to avoid cracks as can be seen in the weld section shown below.  In this particular example, the plating process was not properly controlled and plated Ni layer was full of surface cracks where sulfur compounds from the plating bath had the opportunity to hide.  A follow-on weld test by removing the Ni plating layer produced welds without any cracks.  Information fed back to the plating vendor led to improved plating quality and defect-free welds.

 

In both types of Nickel plating processes, the plating vendor may decide to add some pixie dust and try to impress you with making a bright Ni plating.  The pixie dust is typically proprietary chemicals that turn a dull matt finish on the Ni plating into a bright shiny surface.  Unfortunately, the brighteners added are almost always detrimental to the welding process.  This is one situation where the welding engineer would be smart and choose to be dull.

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Upset Welding

Upset welding is one of those processes whose name can adequately describe the process as well the emotional state of the user under vexing circumstances. Upset welding a variant of resistance welding which is used to make a butt joint. Parts to be welded are held in grippers and pushed against each other while the required welding current flows through the parts and the weld interface. Resistive heating softens a narrow region of metal volume on either side of the interface. At the appropriate time, the force is increased to upset the joint which pushes the softened material out of the joint; this action helps to push out any oxides and impurities at the weld and allows clean parent metal on both sides to come in intimate contact. After the required upset action has taken place, the welding current is turned off and the weld is allowed to cool under action of the upset force.

Upset welding is commonly used in wire and bar mills to joint wire coils and bars to facilitate continuous processing. The upsetting action does pushout some of the metal which is then removed by grinding and leaves a joint that is often indistinguishable from the original parts. The welds can be made strong and ductile enough to pass through subsequent rolling and forming operations. One of the most challenging applications of upset welding, and possibly all of resistance welding, is upset welding of metallic tubes to form hermetic seals for aerospace o-rings. In this application a tube of suitable alloy is cut to length, and rolled to form a ring; tube diameters are of the order of 1/8 inch (3 mm). The two ends of the ring are held in grippers and upset welded to form a ring; excess flash is removed by grinding. A common method for testing upset welds is the bend test where the sample is bent at the weld and the weld joint is expected to withstand a full bend without failure.

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Breaking In

Resistance welding is one of the few material joining processes where the welding tip comes in direct contact with the work piece. Consequently, tip shape, surface smoothness, and cleanliness are important factors that can affect weld quality. Even with a carefully machined tip, a brand new tip can take a few welds to break-in where the tip conforms to the parts being welded. Breaking in a tip is difficult to do offline and is best done on the machine with actual parts or simulated parts that are practically identical. During the break-in process, the tip conforms to the part surface, smoothens tip surface asperities, and in some cases also transfers a small amount of material from the part being welding to the tip surface. After break-in, the tip can produce consistent weld quality over its lifetime, after which it has to be redressed or replaced. Break-in can be a couple of welds when using copper electrodes or can be up to 100 welds with tungsten electrodes. Life of the tip after break-in depends on the application (materials, welding frequency, and tip cooling) and has to be established by testing.

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