TechArchive

This article was originally written in the period 1995-2000

Paul Brackell of Townsend Coates covers the basics of a technique which is often overlooked – resistance welding can be the answer to joining small parts together

The four basic bonds

• Braze/soldered bond:-resistance heating of the workpieces produces sufficient heat to melt a third metal, such as gold or silver, which alloys to both workpieces.

• Forge weld:-the workpieces are “forged” together without any apparent melting. This is usually accomplished by using a very short weld time. This type of bond is advantageous when two dissimilar materials with radically different crystal structures are joined.

• Diffusion weld:-the surfaces at the workpiece interface are heated to the plastic state. There is no melting, although a definite mixing of the crystals from both workpieces takes place. This type of bond is typical of those obtained from short pulse welding and is ideal for dissimilar metals with similar crystalline structure.

• Fusion weld:-the interface is heated to the melting point of both workpieces. The subsequent cooling and recombination of the materials forms a “nugget” which contains all possible alloys of the two materials, some of which may have undesirable characteristics. This type of bond is typical of long pulse welding and is ideal for welding similar materials.

Resistance welding is often the best solution for difficult joining jobs with small metal parts, for instance when the materials are conductive or dissimilar.

In resistance welding, electrode pressure is used to force the metals together. Heat, generated by the resistance of the workpieces to the flow of electricity, either melts the material at the interface or at least reduces its strength to a level where the surface becomes plastic. When the flow of current stops, the electrode force is maintained, for a fraction of a second, while the weld rapidly cools and solidifies. The weld current is distributed over a large area as it passes through the bulk metal. When the current stops, the electrodes cool the molten metal rapidly. This solidifies, forming a weld.

The common forms of resistance welding include direct and indirect single spot welds, multiple spot welds, seam welds, projection welds and parallel gap welds. Direct single, opposed, spot welds are those in which all the welding current flows through the welding interface. Indirect (commonly called series) welds are those in which a portion of the welding current by-passes the interface. Parallel gap welding is a form of series welding in which the electrodes are less than 0.025in (0.64mm) apart and a single weld is formed between the electrodes.

A seam weld is an opposed weld in which the electrodes or the workpieces are moved to form a continuous series of overlapping spots. Projection welds derive their name from projections extending from one of the two workpieces. This is used to decrease the amount of energy required to make a weld, to improve the heat balance when thin materials are welded to thick materials, or to allow several welds to be made at pre-determined locations with a single weld pulse.

Conventional joining methods – whether a crimp, pin or solder – have inherent disadvantages and limitations. For instance, solder contains lead, employs volatile components and cannot be used to form pitch joints of less than 10 mil. It is also useless on glass, which presents problems for liquid crystal displays. Resistance welding has also taken over from solder in the battery pack market.

Resistance brazing and laser reflow equipment can heat parts locally that require soldering, rather than having to heat the whole assembly. Often, many components, due to their nature or assembly requirements, cannot be soldered by infra-red or reflow soldering systems. These applications benefit the most from localised heating techniques.

Fundamentally, there are four types of fine spot resistance welding systems: direct energy (alternating current – AC), stored energy (capacitor discharge – CD), the high frequency inverter and constant voltage/power microjoining. High frequency inverters offer the highest production welding speeds, with greatest control over welding parameters. Direct energy offers high welding speed at low cost, whereas the uni-polar stored energy systems deliver a precise burst of weld energy ideal for welding highly conductive materials such as copper and its alloys.

The AC welder derives its name from having a sinusoidal output of the same frequency as the power line. It extracts energy from the power line as the weld is being made. For this reason, the power line must be well regulated and capable of providing the necessary energy. Some AC welders include a line voltage compensation feature to adjust for power line fluctuations automatically.

In its simplest form, the AC welder consists of a welding transformer which steps down the line voltage (normally between 115V and 460V) to the welding voltage (typically 2-20V). The welding current which flows through the secondary of the transformer and its connected load is very high, ranging from 10 to more than 100,000A. The welding current is allowed to flow for very short periods of time – typically 0.001s to 1s.

The stored energy welding power supply, commonly called a capacitor discharge (CD) welder, extracts energy from the power line over a period of time and stores it in welding capacitors. This stored energy is discharged through a pulse transformer, producing a flow of electric current through the welding head and workpieces.

Capacitor discharge power supplies are rated in accordance with the amount of energy they store and their welding speed. Some power supplies provide a dual-pulse feature which allows the use of two pulses to make a weld. The first pulse is used to displace surface oxides and plating and the second pulse welds the base materials. This feature also reduces spitting. Recently, some CD power supplies have become sequence controls, since they control the operation of the welding head.

Pulse transformers are designed to carry high secondary currents, typically up to 10,000A. Welds made with a capacitor discharge system are generally accomplished with a single, very short weld pulse with a duration of 1-16ms. This produces very rapid heating, which is localised at the welding interface. The length of the output pulse width can normally be modified by changing taps on the pulse transformer. Polarity switching is a convenience when welding a wide variety of polarity-sensitive, dissimilar materials.

High frequency inverter welders use switching technology with weld current to provide constant weld current feedback or a combination of weld current and voltage feedback to provide constant weld power. Switching technology minimises internal power losses in the control circuits, providing immunity to fluctuations in input line voltage and using smaller weld transformers to deliver high weld energy. The high frequency switching provides fast response, adaptive feedback on welds down to 2ms.

The actual weld pulse is a combination of direct current and high frequency. The direct current component of the weld pulse produces stronger welds on polarity-sensitive materials, or similar materials of differing thickness. The high frequency component generates weld heat at the surfaces of each part and is thus particularly useful when welding conductive materials such as copper to copper.

The combination of direct and high frequency weld currents, coupled with real-time weld current or power feedback, produces stronger welds, less part deformation and longer electrode life when welding difficult materials. These include brass-brass, nickel or steel to brass or copper, molybdenum or tungsten or molybdenum, silver or silver alloy to brass or beryllium-copper.

• Townsend Coates
• Tel: 0116 276 9191
• Fax: 0116 274 2236