Hello Readers, welcome to your own website to understand each and every topic related to the manufacturing process where we transform complex content into simpler ones. In this article, we are focused to cover the general principles of resistance welding, bulk resistance, contact resistance, resistance spot and seam welding, high frequency welding, resistance projection welding, flash, stud and percussion welding, advantages, disadvantages, and applications.
Resistance welding is a process in which heat generation takes place on the parts to be joined by passing an electrical current for a precisely controlled time and under a controlled pressure.
The fact that the resistance of the workpieces and electrodes are used in combination or contrast to generate the heat at their interface, from this the resistance welding is derived.
Resistance welding is one of the most simple heat generation process in which the passage of current through a resistance generates heat.
The same principle mentioned above is used for the operation of heating coils.
For the addition to the bulk resistances, the contact resistances is also an important factor.
Due to the surface condition (surface roughness, cleanliness, oxidation, and
platings) the contact resistances are get influenced.
The resistance welding heat generation formula is given below.
Heat = I2 x R x t x K
I = weld current through the workpieces
R = electrical resistance (in ohms) of
t = weld time (in hertz, milliseconds or microseconds
K = thermal constant.
Power supply, the weld current (I) and duration of current (t) is controlled in the resistance welding.
The resistance of the workpieces (R) is a function of the weld force and the materials used.
From the part geometry, fixturing and weld force the thermal constant K can be affected.
The surface roughness and the cleanliness of the mating surfaces, on this two factor the strength of the bond is dependent.
So because of this the oil films, paint, and thick oxide layers should be removed before welding.
The includation of uniform, thin layers of oxide and of other contaminants is not as critical.
Resistance welding process requires specialized machinery, which was developed in the early 1900s.
Much of it is now operated by programmable computer control.
Generally, the machinery is not portable, and the process is suitable primarily for use in manufacturing plants and machine shops.
The operator skill required is minimal, particularly with modern machinery.
The workpieces, electrodes, and their interfaces both cause and affect the amount of heat generated, from the values of bulk and contact resistance.
It is a function of temperature.
All metals exhibit a Positive Temperature Coefficient (PTC), it means that their bulk resistance increases with temperature.
Bulk resistance becomes a factor in longer weld.
It is a function of the extent to which two surfaces mate intimately or come in contact.
At the time of first few milliseconds, weld contact resistance plays an important role.
If sometime we examined the surfaces of the metals on a molecular scale, it will be quite rough.
When the metals are forced together with some relatively small amount of force, some of the peaks make contact.
There are three types of resistance welding bonds mentioned below.
SOLID STATE BOND
The Solid State Bond also known as thermo-compression bond, in which dissimilar materials are with dissimilar grain structure.
In a very short heating time, high weld energy, and high force the molybdenum to tungsten, are joined.
There is a definite bond and grain interface but little melting and minimum grain growth.
Due to this the materials are actually bond when still they are in the solid state.
There is excellent shear and tensile strength, but poor peel strength in the bonded materials.
Whether there is similar or dissimilar materials with similar grain structures they are heated generally to the melting point.
With the help of nugget alloy of the two materials the larger grain growth is formed due to the subsequent cooling and combination of the materials.
Specially high weld energies at either short weld time or long weld times, is dependent on the physical characteristics, which are used to produce fusion bonds.
The bonded materials usually exhibit excellent tensile, peel and shear strengths.
REFLOW BRAZE BOND
A resistance heating of a low temperature brazing material, such as gold or solder, in a reflow braze bond, is used to join either dissimilar materials or widely varied thick/thin material combinations.
The brazing material must wet to each part and should have a lower melting point than the two workpieces.
The resultant bond has definite interfaces with minimum grain growth.
Specifically this process requires a more heating time at low weld energy.
For eg .2 to 100 ms.
There is an excellent tensile strength, but poor peel and shear strength, in the resultant bond.
RESISTANCE SPOT WELDING
In resistance spot welding the tips of two opposing solid, cylindrical electrodes touch a lap joint of two sheet metals, and resistance heating produces a spot weld.
The pressure is applied until the current is turned off and the weld is solidified for obtaining a strong bond in the weld nugget.
In resistance welding the accurate control and timing of the alternating electric current and the pressure are essential.
The number of cycles range around 30 at a frequency of 60 Hz in the automotive industries.
RESISTANCE SEAM WELDING
The modification of spot welding where the electrodes are replaced by rotating wheels or rollers is called as resistance seam welding.
The electrically conducting rollers produce a spot weld using a continuous AC power supply when,
(a) Seam-welding process in which rotating rollers act as electrodes.
(b) Overlapping spots in a seam weld.
(c) Roll spot welds.
(d) Mash seam welding.
HIGH FREQUENCY RESISTANCE WELDING
High frequency current up to 450 kHz is employed, other than this high frequency resistance welding is similar to seam welding.
The production of butt welded tubing or pipe where the current is conducted through two sliding contacts to the edges of roll-formed tubes, this is one of the typical application of this process.
Structural sections can be fabricated using HFRW welding.
RESISTANCE PROJECTION WELDING
By embossing more than one projection dimples on one of the surfaces to be welded, the electrical resistance at the joint is developed.
The projections may be round or oval for design or strength purposes.
By modifying the electrode, spot welding equipment can be used for resistance projection welding.
The flash welding also known as flash butt welding, the heat generated is very rapid from the arc as the ends of the two members begin to make contact and develop an electrical resistance at the joint.
The quality of the weld is good because of the impurities and contaminants which are squeezed out during these operations.
Stud welding (SW) is also called stud arc welding and is similar to flash welding.
The stud (which may be a small part or, more commonly, a threaded rod, hanger, or handle) serves as one of the electrodes while being joined to another component, which is usually a flat plate.
In the automobile bodies, electrical panels, and shipbuilding this process has major application.
This process is also used for the construction of the buildings.
The resistance welding process already described usually employ an electrical transformer to meet the power requirements.
Side by side the electrical energy for welding may be stored in a capacitor.
• Very short process time.
• No consumables, such as brazing materials, solder, or welding rods.
• Operator safety because of low voltage.
• Clean and environmentally friendly.
• A reliable electro-mechanical joint is formed.
Tools cost will be high.
Due to the current requirement the work section thickness is limited.
It is less proficient for high-conductive equipment.
It consumes high electric-power.
For the weld joints it contain small tensile & fatigue power
In automotive industries most of the time Resistance welding is widely used.
For the production of nut and bolt projection welding is widely used.
Seam welding is also used most of the time for producing the leak prove joint required in small tanks, boilers.
For welding pipes and tubes, flash welding is mostly used.
We have covered all the important concepts related to resistance welding process. Hope you all are crystal clear with understanding all the concepts mentioned here. If you have any questions please use the comments section to get in touch with us. Till then have fun and always keep reading!