Controlling Weld Energy

Advances made in resistance welding process control

Resistance welding is a well-documented, reliable process that has long been used fin joining metallic parts. However, the trend toward miniaturization brings a host of new problems associated with controlling the welding process.

Recent advances in high-frequency inverter technology allow for very precise control over weld energy, a factor that is critical to successful welding. This article discusses advances in this technology and how they are being used in industry.

How Resistance Welding Works

Resistance welding is just one of many techniques used for joining metals. In this process, a controlled amount of electrical current is passed through two or more pieces of metal that have been pressed together, creating heat at their interface. The amount of heat generated is expressed by the following formula:

Q=I2Rt

where: Q = heat generated, joules
I = weld current, amperes
R = resistance of workpieces, ohms
t = duration of current, seconds

The weld current (I) and duration of current (t) are controlled by the resistance welding power supply. The resistance of the workpieces (R) is a function of the weld force and the materials used.

Slight changes in the amount of cur rent or time can greatly affect the weld heat. This is especially true when weld ing miniature parts.

For example, catheter lead wires with diameters of 0.003 inch can require weld times from 1 to 2 milliseconds and cur rents as low as 100 amps. The ability to control weld current and time in very small increments becomes increasingly important as the sizes of the workpieces decrease.

Power Supplies The quality control of the resistance welding process is based on the ability of the equipment to duplicate results under controlled conditions. Different power supply technologies control current and time in different ways.

Power supplies can be open-loop or closed-loop. Open-loop supplies have no feedback method to control the output. Closed-loop supplies use adaptive feed back to control the output precisely.

Conventional resistance welding power supply technologies include, but are not limited to, capacitive discharge (CD) and direct energy (AC [current]):

1. Capacitive discharge. In CD power supplies (also referred to as stored energy power supplies), the weld energy is stored in a capacitor bank and dis charged through a welding transformer.

The charge on the capacitor bank determines the amount of energy output, which is very consistent from weld to weld. The weld time, or pulse width, is set by changing taps on the welding transformer. Typically, three pulse width choic es are available: short, medium, and long.

Because of the limited pulse width choices, CD supplies have limited control over the duration of current (t). Additionally, because there is no feedback control, changes in the secondary loop resistance can change the amount of weld current (I) delivered to the parts.

The typical output pulse from a CD supply has a high peak current with a very fist rise time. This is advantageous when welding highly conductive parts, but can cause problems when welding very small parts for which part fit-up is difficult.

Because the weld energy is stored in a capacitor bank before welding, line volt age fluctuations do not affect the weld energy in CD supplies.

2. Direct energy. In AC power sup plies, energy is taken directly from the power line as the weld is being made. Coarse current control is achieved by changing the tap settings on the welding transformer, which changes the amplitude of the secondary AC signal.

Fine adjustment of weld current (I) is achieved by controlling the percentage of the AC power that is applied to the primary of the welding transformer. The duration of current (t) is controlled in line cycles (1 cycle = 16.67 milliseconds at 60 hertz), the minimum usually being 1 half cycle.

Line voltage fluctuations can affect the weld current delivered by AC supplies. For this reason, the input line must be well-regulated.

Because time control is in line cycles, only coarse adjustments to time can be made. Even the minimum setting of 1 half cycle may not be suitable for very small or refractory materials.

High-Frequency Inverters

High-frequency inverters (see Figure 1) were introduced to the microjoining market in the late 1980s in response to the need for greater current and time control in resistance welding.

The first inverters used a 1-kilohertz feedback frequency to control the output. Soon, 2-kilohertz inverters followed.

The newer inverters use control frequencies of 20 to 25 kilohertz. The higher frequencies provide faster update rates, resulting in quicker response to variations in the welding process.

High-frequency inverter technology uses pulse width modulation circuitry to control the weld energy. Three-phase in- put current is full-wave-rectified to direct current (DC) and switched at up to 25 kilohertz to produce AC at the primary of the welding transformer. The secondary current is then rectified to pro duce DC with an imposed, low-level AC ripple.

The feedback circuitry in a high-frequency inverter allows the inverter to adapt to changes in the secondary loop resistance. A 25-kilohertz inverter adjusts the output current every 20 microseconds, which allows the weld time, or duration of current, to be controlled in increments as small as 0.1 millisecond. This degree of control can be advantageous when developing weld schedules in the 0.5-to 5-millisecond range.

Although specific techniques for con trolling the weld energy will vary with manufacturer and model, inverter power supplies can offer constant current, constant voltage, or constant power modes of operation. The constant current mode uses primary or secondary current sensors to provide feedback.

Voltage feedback is provided by pick up cables that are mounted to the weld ing electrodes or electrode holders. Power is derived by calculations from the cur rent and voltage feedback signals.

Weld Monitoring. In addition to providing a means of control, feedback circuitry also can be used for monitoring the weld process. Weld monitoring can provide numeric and graphical data used for process setup, process enhancement, or statistical process control (SPC).

For power supplies that offer SPC data output, it is advisable that the calibration of the monitor be traceable to the National Institute of Standards and Technology (NIST). Medical device manufacturers in particular need this traceability to support reporting requirements to the Food and Drug Administration (FDA).

Pulse Shaping. Small contact areas and surface irregularities can increase contact resistance in the workpieces.

High contact resistance at the beginning of the weld pulse can cause expulsion and weld flash. Closed-loop systems have the ability to shape the weld pulse to help overcome these problems.

Up-slope can be used at the beginning of the pulse to help seat the electrodes and reduce contact resistance. Down-slope can be added at the end to help reduce marking and embrittlement of the parts. Dual pulse energy (see Figure 2) is often used to break through plating or temper a welded part.

High-Frequency Inverter Feedback Modes

As previously stated, the weld energy can be delivered in the form of constant current, constant voltage, or constant power. Selecting an appropriate feedback mode for the application should be considered before developing the weld schedule.

Constant Current. The constant current mode can be used for 70 to 75 percent of all inverter applications.

Referring to the heat generation formula, Q = I the weld current (I) and duration of current (t) are directly controlled by the power supply. The resistance of the workpieces (R) is a combination of contact and bulk resistance values, affected by the weld force and the materials used.

High weld force and large contact areas result in low contact resistance. Low weld force and small contact areas result in high contact resistance.

If the contact resistance does not change dramatically during the weld or from weld to weld, using constant cur rent will result in a predictable, consistent generation of heat (Q). This will occur when welding flat parts with consistent part-to-part and electrode-to-part contact areas.

With constant current, changes in weld cable length, diameter, or loop resistance do not change the weld current. Multiple weld stations operate at the same weld current, regardless of weld head or cable length variations.

Constant Voltage. Voltage control is suitable for applications in which the parts being welded do not have flat surfaces.

Typically, the resistance of the work- pieces (R) changes greatly if there is considerable change in the part-to-part or electrode-to-part contact area as the weld occurs. A common example of this is the welding of two overlapping wires.

Substituting E for 1 in the heat generation formula, changes in the heat generated (Q) are inversely proportional to changes in the resistance of the work- pieces (R), as long as the voltage across the workpieces (E) remains constant (see Figure 3):
Q = (E2/R)t

where: Q = heat generated, joules
B = voltage across workpieces, volts
R = resistance of workpieces, ohms
t duration of current, seconds

This can be demonstrated with the example of welding two wires together (see Figure 4). In this case, the contact resistance, and hence the resistance of the workpieces (R), starts out high, so the heat generated (Q) starts out low. The wires quickly deform, decreasing the contact resistance and increasing the amount of heat generated.

As the parts melt, the workpiece resistance again increases because of the increase in bulk resistance, reducing the amount of heat generated. The end result of using constant voltage for wire welds is a minimum deformation of the wire, with greater control over the amount of heat generated.

Another application suitable for volt age feedback is a weld of stainless steel coil to stainless steel tube (see Figure 5). In this case, the number of current paths that are present depends on the number of coil windings at the electrode-to-coil interface.

The current through each coil winding is the same because the voltage across them is the same. Thus, the amount of heat generated is consistent.

Constant Power. Constant power can be used effectively for applications with changes in electrical resistance from weld to weld. Changes in resistance can occur with a buildup of heat in the electrodes, as is sometimes seen in automated welding systems.

The buildup of heat increases the resistance of the workpieces (R). The constant power control feature decreases the weld current (I) to consistently maintain the product (1 resulting in a consistent amount of heat generated (Q).

Slight variations in plating or a buildup of plating on the electrode face also affects the resistance of the workpieces. Constant power should be used in both of these cases to maintain consistent generation of heat.

Weld Monitoring

Prepulse Energy Limit. An effective use of built-in weld monitors is to program a prepulse energy limit to monitor and control errant workpiece variables. For example, misaligned or missing parts can be detected before weld current is applied.

By inhibiting weld current, "blown welds" are avoided. With constant current applied, the voltage across the work-pieces will be consistent if the parts are aligned consistently from weld to weld.

To use a prepulse energy limit, a very small prepulse of current is added to the weld schedule. This first pulse should be very short (about 1 or 2 milliseconds) at very low current (50 to 100 amps).

The first pulse is used to test the resistance of the workpieces using maximum and minimum voltage limits. If the parts are misaligned, the workpiece resistance will be high, causing a high- voltage, out-of-limits condition.

The out-of-limits alarm is then used to inhibit the weld pulse. The lower voltage limit is used in the same fashion to detect missing parts.

This technique is used in applications in which multiple welds are performed on each workpiece. In lamp manufacturing, fix example this technique is used in automated assembly lines on which each lamp requires several welds.

One blown weld would require the entire assembly to be scrapped and also would cause severe damage to the electrodes, which would have to be replaced before any more welds were made.

Alternately, the part that caused the voltage alarm can be removed and taken to another station to be welded manually, resulting in scrap reduction with corresponding cost savings.

SPC Software. Nondestructive, continuous monitoring of the resistance welding process is the only way to detect negative process trends.

Some built-in weld monitors offer RS-232 or RS-485 transmission of weld cur rent and voltage data that can be collected on a personal computer. SPC software packages can perform statistical calculations, generate X bar and R control charts, and provide summary information of the weld data.

Resistance Wrap-up

Advances in high-frequency inverter technology, including higher control frequencies, built-in monitors, and data output capabilities, are being used to address the growing demand for high- precision weld energy control. The ability to select, control, and monitor weld energy output has a direct impact on quality, reliability, and cost savings.

Monitors are being used to supply both improved process control and statistical data. Different feedback modes can be used and customized to the process.

The flexibility for selecting current, voltage, or power control modes provides users with wider choices in addressing their welding applications.

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