After weldment combination through the electrode pressure, using the current through joint contact area and the resistance of the heat generated in the adjacent area in welding method known as resistance welding. Resistance welding with high production efficiency, low cost, save material, easy automation, etc, therefore widely used in aviation, aerospace, energy, electronics, automobile, light industry and so on various industry sectors, is one of the important welding process.

The heat output and influencing factors

When spot welding heat is determined by the type: Q = IIRt (J) -- - (1)

Type: Q - the heat generated by the (J), I - welding current (A), R - t - welding electrodes resistance (ohms), time (s)

1. Resistance R and the influencing factors of R

Resistance including the artifact itself resistance between the electrodes Rw, contact resistance between the two artifacts Rc, contact resistance between the electrode and workpiece designs, R = 2 Rw + Rc + 2 designs, (2)

When the workpiece and electrode, the resistance of the workpiece depends on its resistivity. Therefore, the resistivity is one of the important performance by welding materials. High resistivity of the metal its poor electrical conductivity, such as stainless steel low resistivity of the metal its conductivity is good (e.g., aluminum alloy). Therefore, generate heat easily when spot welding of stainless steel and heat dissipation, yi and heat dissipation when spot welding of aluminum alloy heat production difficult. Spot welding, the former small current (several thousand amperes) are available, and the latter will have to use a large current (tens of thousands of amp). Resistivity is not only depends on the metal species, but also with metal heat treatment state, processing methods and temperature.

Contact resistance of time is short, generally exist in the welding of the early, formed from two aspects of reasons:

1) the workpiece and the electrode surface has high resistance coefficient of oxide layer or dirty substances, can make the current large obstacles. A thick layer of oxide and dirty material can''''t make even current conduction.

2) under the condition of the surface is clean, because the surface micro roughness of the workpiece can only be formed in the rough surface of local point of contact. Current line at the contact point of the ether. As a result of the current path narrow and increases the contact resistance.

The resistance between the electrode and workpiece designs compared with Rc and Rw, because the resistivity of copper alloy and the hardness is generally lower than the artifacts, so small, the influence on the formation fusion nuclear smaller, less we consider the effect of it.

The influence of welding current

From the formula (1) can be seen, the influence of the electric current to generate heat is larger than both the resistance and time. In the welding process, therefore, it is a must strictly control parameters. Is the main reason of the change of the current power grid voltage fluctuation and ac welder secondary loop impedance change. Because the loop impedance change of geometry or because of changes in the introduction of the secondary loop in different amount of magnetic metal. For dc welding machine, the secondary loop impedance change, had no obvious effect on current.

The influence of welding time

In order to guarantee molten nuclear size and strength of solder joints, welding time and welding current can complement each other in a certain range. In order to obtain a certain strength of solder joints, can be used in a large current and short time (strong condition, also called hard specification), also can use a small current and long (weak conditions, also known as soft specification). Choose or soft hard specification standard, depends on the performance of the metal, thickness and power of the welding machine used. For the different performance and current and the time required for the thickness of the metal, there is a lower limit, when used with this shall prevail.

The influence of electrode pressure

Electrode pressure between the two electrodes has distinct effect on the total resistance R, with the increase of electrode pressure, R decreased significantly, and increase the size of the welding current is small, can''''t influence caused by R reduce heat production decrease. Therefore, solder joint strength of total and decreases as the pressure welding. The solution is at the same time of increasing welding pressure, welding current.

The influence of electrode shape and material performance

Due to the electrode contact area decided the current density, resistivity of electrode materials and the relationship of thermal conductivity with heat generation and lost, therefore, the shape of the electrode and the material has a significant influence on the formation of a melt nuclear. As the electrode end of deformation and wear, the contact area increases, solder joint strength will reduce.

The influence of the workpiece surface condition

Oxide on the surface of the workpiece, dirt, oil and other impurities increases the contact resistance. A thick oxide layer and even made the current cannot pass. Local conduction, due to the large current density, can produce splash and surface burn. The existence of the oxide layer can also affect the non-uniformity of the solder joint heating, welding quality fluctuations caused. So clean up the workpiece surface is the necessary condition to ensure access to quality connectors.

Heat balance and heat dissipation

Spot welding, the heat generated only a small part is used to form the solder joint, the most lost by to the adjacent material conduction or radiation, the heat balance equation:

Q = Q1 plus Q2 -- - (3) : Q1 - forming molten nuclear heat, Q2 - the loss of heat

Effective heat Q1 depends on the thermal physical properties of the metal and the amount of molten metal, and has nothing to do with the welding conditions used. Q1 Q = 10% - 10%, good thermal conductivity of metal (aluminum, copper alloy, etc.) in the lower; High resistance rate, poor thermal conductivity of metal (stainless steel, high temperature alloy, etc.). Heat loss Q2 mainly includes the through electrode conduction heat (30% - 50% Q), and through the workpiece heat conduction of around 20% (Q). Radiation heat to the atmosphere around 5%.

The welding cycle

Spot welding and the convex welding welding cycle consists of four basic stages (pictured) spot welding process:

1) preloading stages - down to current electrode connected, ensure that the electrode clamping workpiece, make appropriate pressure between the artifacts.

2) welding time, welding current through artifacts, heat production formation of molten core.

3) maintain time - to cut off the welding current, electrode pressure to maintain enough strength to melt nuclear microstructure.

4) resting time - electrode began to mention they began to fall again, begin the next welding cycle.

In order to improve the performance of the welded joint, sometimes need to add one or more of the following in basic cycle:

1) increase the pressure to eliminate the gap between the workpiece thick, make it fit closely.

2) improve the plasticity of metal, with preheating pulse parts easy to fit closely, prevent splash; Projection welding when doing so can make multiple bumps in electric welding with flat even before contact, to ensure the consistent of each point to heat.

3) forging pressure increased with compaction melt nuclear, prevent cracks or shrinkage cavity.

4) with tempering or slow cooling pulse elimination of alloy steel quenching organization, improve the mechanical properties of joint, or under the condition of no forging pressure increased, to prevent the cracks and porosity.