With the engineering industry undergoing a radical transformation involving a steep step-up in production technology as well as production quantities, the need has arisen for welding systems designed for automation with consistent weld quality and dealing in materials much more complex than simple mild steel.
1. Thyristorised Universal Welding Rectifiers:
These electronically regulated power sources which can take full care of voltage fluctuations are designed as versatile, highly adaptable units which can serve as the heart of a multipurpose welding system.
Providing both constant current (CC) and constant potential (CP) type of output characteristics, these rectifiers offer stable output, step less control of welding parameters, easy change over from CC to CP mode and vice-versa, low spatter loss and protection against thermal overloads or fan failure.
The rectifiers are available in a choice of five job rated models ranging from 400 to 1,500 amperes. While models upto 600 amps are mobile on rubber tired wheels, the higher capacity units are foot mounted and can be transported using the lifting eye-bolts provided.
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Depending on the model and characteristics selected, these rectifiers can be used for manual metal arc, carbon air-arc gouging, gas metal arc or submerged are welding applications.
2. Transistorised Pulsed GTAW Welders:
Pulsed GTAW (gas tungsten arc welding) helps to overcome the problems of improper fit-up, excessive distortion, uneven penetration and difficulties encountered in joining thick to thin sections. It is used for joining of pipes and tubes in heat exchangers as well as fabrication of chemical vessels covering aerospace, instrumentation, thermal power and nuclear applications. It is ideal for joining ferrous and non-ferrous metals except light metals like aluminium, magnesium and their alloys.
In essence, pulsed GTAW is a modification of the GTAW process in which the current alternates between a ‘low’ or background level and a high or peak level. The duration and amplitude of both peak and background currents can be varied independently to suit the job.
The melting takes place during the peak current period and welded pool solidifies between pulses as the heat is dissipated in the job during the background current phase. This current pulsing leads to intermittent melting along the joint seam giving a series of discrete melt spots which overlap each other.
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The most significant advantage of this process is the apparent tolerance to external variables such as joint geometry, clamping fit-up, dissimilar thicknesses or other factors causing variation in thermal heat-sink.
In conventional GTAW welding delicate manipulation by the welder is needed to maintain the balance between heat input from the arc and heat-sink due to the job in mechanised welding, this is difficult to achieve resulting in significant variations in joint quality. This problem is avoided in pulsed GTAW since the weld pool is allowed to solidify during background time so that the effect of heat built-up is largely overcome.
Some of the salient features of pulsed GTAW welding outfits are:
i. Completely solid state circuitry.
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ii. Independent control and presetting of parameters like gas pre-flow time, current up-slope and down- slope, peak and back-ground current amplitudes and gas post-flow time.
iii. Automatic welding sequence initiation through 4- stroke torch switch.
iv. Easy arc ignition through HF (high frequency) coupled with booster voltage circuits. This ensures welds free from tungsten contamination and also prolongs life of the tungsten electrode.
v. While HF is automatically cut-off after arc initiation a special circuit switches off the complete system if welding does not commence within 10 seconds after pressing the torch switch, thus avoiding wastage of expensive argon gas.
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vi. Special over-current limiting circuitry shuts off power source when welding current exceeds rated values. The welding cycle will not start and the machine will automatically be switched off if these is fan failure causing inadequate ventilation. Safeguards are also provided against inadequate cooling water of failure of circulatory system.
3. Micro Plasma Arc Welding Outfit:
Plasma arc welding is closely related to GATW welding. Plasma is present in all arcs but in the plasma are process a constriction containing an orifice is placed around the arc so that the amount of ionisation or ‘plasma’ is greatly increased.
The result is that the arc becomes more concentrated and the high velocity and temperature ionised gas stream takes the form of a very stable, unidirectional and high intensity jet which is used for welding.
In the micro plasma arc process, a DC power source capable of delivering extremely low and precise welding currents (0.20 to 20A) and a specially designed torch are used to make it suitable for welding thin sections (0.10 to 1.25 mm). The precise control of heat input is achieved through “pulsed mode” operation.
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The micro plasma arc welding outfit is suited for welding thin sections of carbon and alloy steels, stainless steels, heat-resisting alloys, copper alloys and nickel alloys.
4. Plasma Arc Cutting-Cum-GTAW and Manual Arc Welding Outfit:
This multi-purpose machine can cut stainless steels and carbon steels upto 25 mm thickness and aluminium and its alloys upto 30 mm using the plasma arc process.
The outfit can also be used for DC/GTAW welding of all ferrous and non-ferrous metals except aluminium and its alloys. It is also ideal for DC manual welding using all types of electrodes.
The outfit consists of a 3-phase full wave, dual OCV transduction type rectifier power source with built-in controls for fine stepless adjustment of output current and water circulatory system. The output current is available in the ranges of 30 to 250 A for plasma cutting, 70 to 400 A for GTAW welding and to 350 A for manual welding. By adding a travel carriage or profile cutting unit the equipment can be further automated.
Plasma arc cutting is basically an extension of GTAW welding and in both processes the workpiece forms a part of the electrical circuit. Retraction of the electrode inside the GTAW nozzle constricts the arc into virtually a parallel side jet. Consequently, the heat of the arc is concentrated on a small area of workpiece which melts. The gas which is preheated by the arc plasma expands and is accelerated through the constricting orifice.
The metal which is melted is then removed by the kinetic energy of the gas stream to lave a clean cut and narrow “kerf”. Cutting speeds with plasma arc are several times greater than those obtained with conventional oxy-fuel gas cutting. Plasma arc cutting produces high quality straight and bevelled cuts at greatly reduced costs.
Wire mesh welder: Automatic wire mesh welder is capable of making wire mesh of width upto 2.5 metres from wire ranging in dia from 2 to 5 mm. The fully mechanical system adopted for the movement of wires and across wire feeding maintains absolute accuracy of pitches. The machine is capable of making wire machine of pitch ranging from 25 to 150 mm at a speed of 75 strokes per minute.
The electrical system consists of nine transformers; each rated 30 kVA in order to give balanced loading to the machine. An electronic control system ensures consistent weld quality.
5. Automatic Container Side Seam Welder:
The various stages of manufacturing like destacker feeder, flexer, roll forming, overlapping the edges for welding, transfer for welding, welding and ejection of welded shells etc. are all automatically sequenced.
The machine has a fully solid state electronic control system along with two counters to record batch and cumulative production data. A special feature of the machine is reduced current at the beginning and end of welding in order to avoid cracking at the edges while seaming.
6. Energy Saver:
An electronic energy device has been developed which can prove a boon to all fabrication shops using manual welding rectifiers.
It is a common experience in such fabrication shops that in an eight-hour working shift the actual arcing time in a manual welding unit is only two hours. The rest of the time the machine is working on No Load. Even on No Load, the power consumption remains high.
The energy saver is a voltage sensing device which on being connected to the rectifier, cuts off the supply from the mains some 15 to 20 seconds (adjustable) after arcing has stopped. Moreover, to resume welding, the operator need not come to the machine for reconnecting the power. All that he has to do is strike the arc and the unit is ready for welding.
Not only is this device an energy saver, it also improves the power factor of the workshop by disconnecting the welding rectifier during No Load operation.