Welding Handbook: Metals and Their Weldability VOLUME 4 7th edition

Welding Handbook Seventh Edition Volume by Kearns

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For the capacitor or mag- of the weld zone. The pressure developed at the system. Single-Impulse Welding The functions of this force or pressure are to 1 The use of one continuous application of bring the various interfaces into intimate contact, current to make an individual weld is called 2 reduce initial contact resistance at the inter- single-impulse welding see Fig.

Up or faces, 3 suppress the expulsion of molten weld down current slope may be included in the time metal from the joint, and 4 consolidate the period. Forces may be applied during the welding Multiple-Impulse Welding cycle as follows: Multiple-impulse welding consists of two or I A constant weld force. The basic application of this high initial level to reduce initial contact resis- sequence is for spot welding relatively thick steel tance and bring the parts into intimate contact, sheet to control the rate of heating at the interface.

Forging is used to reduce porosity and the electrodes and the work is assured by the hot cracking in the weld nugget. This combina- application of electrode force. This force is pro- tion is shown in Fig. EQUIPMENT Spot, seam, and projection welding equip- Alternating Current ment consist of three basic elements: an electrical circuit, the control equipment, and a mechanical Most resistance welding machines produce system.

Depending upon the thickness and type of The secondary circuit includes the electrodes material to be welded, currents may range from that conduct the welding current to the work and to amperes. The typical electrical the work itself.

In some cases, a means of storing circuit design for this type of machine is shown electrical energy is also used in the circuit. Both in Fig. The welding machine con- verts 60Hz line power to low voltage, high am- Welding machines may produce direct cur- perage power in the secondary circuit of the rent of continuous polarity, pulses of current of welding machine.

The latter type is produced by stored 3. Resistance welding equipment is covered inCh. Throat area l shaded. Rectifier Type Machines. These machines are over a relatively long time between welds, de- direct energy type in that ac power from the plant livering it to the electrodes during a short weld distribution system passes through a welding time.

The equipment for electrostatic stored en- Silicon diode rectifiers are widely used in the ergy welding consists primarily of a bank of secondary circuit because of their inherent relia- capacitors, a circuit for charging these capacitors bility and efficiency.

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The system can be single to a predetermined voltage, and a system for phase. However, one of the advantages of direct discharging the capacitors through a suitable current systems is the ability to use a three-phase welding transformer. High voltage capacitors transformer to feed the rectifier system in the are generally used, the most common varying secondary circuit.

This makes it possible to use from to volts. Frequency Converter Machines. The primary current flow is controlled I Conduct the welding current to the work by ignitron tubes or silicon-controlled rectifiers and, with spot and seam welding, determine the SCRs. Half cycles of three-phase power, either current density in the weld zone. In projection positive or negative, are conducted to the trans- welding, the current density is determined by former for a timed period that depends upon the the size, shape, and number of projections.

The transformer output is a 2 Transmit a force to the workpieces. By switching the polarity 3 Dissipate part of the heat from the weld oft he primary half cycles, the polarity of the sec- zone. A weld may be made 4 Maintain relative alignment and posi- with one or more de pulses. Stored Energy Machines. Stored energy ma- If the application of pressure were not in- chines are of electrostatic design.

They draw volved, electrode material selection could be power from a single-phase system, store it, and then suddenly discharge it to make the weld. Resistance welding electrodes are discussed more These machines draw power from the supply fully inCh. Since the electrodes to the welding transformer are subjected to forces that are often of consider- 2 Control the magnitude ofthe current able magnitude, they must be capable of with- 3 Actuate and release the electrode force standing the imposed stresses at elevated temper- mechanism at the proper times atures without excessive deformation.

Proper They may be divided into three groups electrode shape is important because the current based on their functions: welding contactors, must be confined to a fixed area to achieve timing and sequencing controls, other current needed current density. When only one spot or seam weld is to be The welding contactor connects and dis- made at a time, only one pair of electrodes is connects the primary power to the welding trans- required.

In this case, the force and current are former.

It may be mechanical, magnetic, or applied to each weld by shaped electrodes. Sev- electronic in operation. Mechanical contactors eral closely spaced projection welds can be made are operated with a foot pedal or a motor-driven with one pair of welding dies electrodes.

Magnetic contactors are actuated by an Electrodes of several copper alloys with electromagnet operating against a spring and satisfactory physical and mechanical properties gravity that open the contactor when the electro- are available commercially. Generally speaking, magnet is de-energized. Electronic contactors the harder the alloy, the lower its electrical and use silicon-controlled rectifiers SCRs or igni- thermal conductivities.

The choice of a suitable tron or thyratron tubes to control the flow of alloy for any application is based on compro- primary current to the welding transformer. Electrodes selected the welding sequence and the duration of each for aluminum welding, for instance, should have function of the sequence. This includes applica- high conductivity at the expense of high com- tion of electrode force and current as well as the pressive strength to minimize electrode sticking time intervals following each function.

Electrodes for welding stainless The welding current output of a machine is steel, on the other hand, should sacrifice high controlled by transformer taps or an electronic conductivity to obtain good compressive strength heat control, or both. An electronic heat control to withstand the required electrode force. It controls current by delaying the firing depends upon the proportional limit and the of the ignitron tubes during each half cycle 60 hardness of the electrode alloy.

The proportional Hz. Varying the firing delay time can be used limit is largely established by heat treatment. This provides upslope and downslope erning factor because this is where softening control of welding current. This changes the turns ratio of the to be welded. Spot welding electrodes for steel transformer with an increase or decrease in open should have flat faces of the proper diameter circuit secondary voltage.

Decreasing the turns for the sheet thickness and should be of suffi- ratio will increase the open circuit secondary cient size to carry electrical, thermal, and me- voltage, the primary current, and the welding chanical loads. The electrodes approach and form between the electrodes and the work. The retract from the work at controlled times and arc will bum or pit the electrode faces and rates. Electrode force is applied by hydraulic, cause them to stick to the work. In some cases, pneumatic, magnetic, or mechanical means.

The the parts being welded may vaporize from the rate of electrode approach must be rapid but very high arc energy.

Welding Handbook: Metals and Their Weldability (Vol. 4)

The locally ing of the weld nugget may not be adequate to heated weld metal expands and contracts rapidly consolidate the weld metal and prevent internal during the welding cycle and the electrodes must porosity or cracking. Multilevel force machines follow this movement to maintain welding pres- may be employed to provide a high forging sure and electrical contact. The ability of the pressure during weld solidification.

The magni- machine to follow motion is influenced by the tude of this pressure varies with the composi- weight of moving parts, or their inertia, and by tion and thickness of the metal and the geometry friction between the moving parts and the ma- of the parts. The forging pressure is often two to chine frame. Since the weld If the pressure between the electrodes and cools from the periphery inward, the forging work drops rapidly during weld time, exces- pressure must be applied at or close to the cur- sive surface heating may occur and an arc may rent termination point.

WO1997019781A1 - Gmaw welding of titanium and titanium alloys - Google Patents

The contact resistance ditions and methods of cleaning follows. They oxygen causes it to become coated with a thin, must be free of high resistance materials such invisible film of oxide whenever it is exposed to as paint, scale, thick oxides, and heavy oil and air. The thin oxide film that forms on a freshly grease. If it is necessary to prime paint the fay- cleaned aluminum surface does not cause suffi- ing surfaces prior to welding, as is sometimes cient resistance to be troublesome for resistance done, the welding operation must be performed we ding.

The permissible holding period, or immediately after applying the primer or special elapsed time between cleaning and welding, conducting primers must be used. For best re- may vary up to 48 hours or more, depending sults, the prime coat should be as thin as pos- upon the cleaning process used, cleanliness of sible so that the electrode force will displace it the shop, the particular alloy, and the appli- and give metal-to-metal contact.

Paint should never be applied to outside An aluminum surface may be mechanically surfaces before welding, because it will reduce cleaned for resistance welding with a fine grade electrode life and produce poor surface appear- ance. Heavy scale should be removed by me- chanical or chemical methods. Light oil on 5. Methods for cleaning some metals and alloys are steel is not harmful unless it has picked up dust described in the appropriate chapters in Sec.

Drawing compounds containing mineral ing Handbook, 6th ed. This information may be re- fillers should be removed before welding. This oil film has no motor-driven scratch brush. Clad aluminum harmful effects on the weld, provided the oily may also be cleaned by mechanical means but surfaces are not contaminated with shop dirt or care must be taken not to damage the cladding. Numerous commercial chemical cleaners are Steels are supplied with various surface available for aluminum. Chemical cleaning is finishes.

Some of the more common are l hot- usually preferred in large volume production rolled, unpickled; 2 hot-rolled, pickled, and for reasons of economy as well as uniformity and oiled; and 3 cold-rolled with or without an control. The unpickled, hot-rolled steel must be pickled or mechanically cleaned.

The hot- Magnesium rolled pickled steel is weldable in the as- Cleaning is particularly important since received condition except for possible wiping magne3ium alloys readily alloy with copper at to remove loose dirt. The cold-rolled steel pre- elevated temperatures. The contact resistance sents the best welding surface and, if properly between the electrode and the work must be protected by oil, requires no cleaning prior to kept as low a possible. The alloys are supplied welding other than wiping to remove any loose with either an oil or a chrome-pickle coating dirt.

For resistance welding, the corrosive and usually require no involved clean- protective coating must be removed to facilitate ing before welding. When exposed to elevated the oxide removal operation if sound and con- temperatures, stainless steels will acquire an sistent welds are to be obtained. The scale is an Copper oxide of chromium which is effectively removed by pickling.

Oil and grease should be removed Cleaning of copper alloys is important.

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