The method of applying pressure after weldment combination through the electrode, the use of current through the resistor thermal contact surfaces and adjacent areas generated by the welding joint is called resistance welding. Resistance welding has important welding technology for high efficiency, low cost, saving materials, ease of automation and other characteristics, it is widely used in aviation, aerospace, energy, electronics, automobiles, light industry and other industrial sectors, it is.
First, the influence of welding heat output and factors
The heat generated by spot welding is determined by: Q = IIRt (J) ---- (1)
Where: Q-- heat generated (J), I-- welding current (A), between R-- electrode resistance (ohms), t-- welding time (s)
1. The resistance R and R factors influence
Resistance between electrodes comprising a workpiece itself resistance Rw, the contact resistance between the two workpieces Rc, the contact resistance between the electrode and the workpiece Rew i.e. R = 2Rw + Rc + 2Rew -. (2)
When a certain workpiece and the electrode, the resistance of the workpiece depends on its resistivity. Thus, the resistivity of the material is important to the performance of the weld. Higher resistivity than metallic conductivity difference (e.g., stainless steel) which metal having low resistance, good conductivity (such as aluminum). Therefore, spot welding stainless steel heat production and heat dissipation difficult and easy, spot welding aluminum heat production and heat dissipation difficult and easy. When welding, the former can be used a smaller current (several thousand amperes), which you must use a large current (several ten thousand amperes). Resistivity depends not only on the type of metal, but also with the heat treatment of metals, processing methods and the temperature.
Contact resistance exists time is short, typically present in the initial welding, formed of two reasons:
1) the workpiece and the electrode surface oxide or soiled material layer having a high resistivity, would have been larger current obstacles. Oxide and soiled material layer is too thick can not even make the current conduction.
2) Under the surface is very clean conditions, due to the microscopic surface roughness of the workpiece only partial rough surface to form the contact points. Shoulong current lines formed at the contact point. Due to narrow the current path increases the resistance of the contact.
Resistance between the electrode and the workpiece Rew compared with Rc and Rw, since copper alloy resistivity and hardness is generally lower than that of the workpiece, so little effect on the formation of a nugget of smaller, less we consider its impact.
2. The impact of the welding current
From equation (1) shows the impact of the current heat production is bigger than both the resistance and time. Thus, during the welding process, which is a parameter must be strictly controlled. Caused by current variation is mainly due to voltage fluctuations and exchange welder secondary loop impedance changes. Impedance changes because of the geometry of the loop or by the introduction of changes in the secondary loop different amounts of magnetic metal. For DC welding machine, the secondary loop impedance changes, no significant effect on the current.
3. Impact of weld time
To ensure the nugget size and joint strength, welding time and welding current within a certain range can complement each other. In order to obtain a certain strength of solder joints, it can be used high-current and short-time (strong condition, also known as hard specification), may also be a small current and long (weak condition, also known as soft specification). Use a hard or soft specification standard, depending on the properties of the metal, and the thickness of the welder power. For current and time different metal properties and thickness required, there is an upper limit, are based on this use.
4. Impact of the electrode force
Electrode force has a significant impact on the total resistance between the two electrodes R, with increasing pressure electrode, R significantly reduced, while increasing the amplitude of the welding current is not large enough to affect the heat production due to reduced R-induced neutropenia. Therefore, as the weld joint strength total pressure increases. The solution is to increase the welding pressure at the same time, increase the welding current.
5. The impact of the electrode shape and material properties
Since the contact area of the electrode determines the current density, resistivity and thermal conductivity of the electrode material related to heat generation and dissipation, and therefore, the shape and material of the electrodes have a significant impact on the formation of a nugget. With the deformation and wear of the electrode tip, the contact area increases, joint strength will be reduced.
6. The impact of surface condition
Oxides, dirt, oil and other impurities workpiece surface increases the contact resistance. Thick oxide layer can not even make the current. Partial conduction, since the current density is too large, it will produce spatter and surface burning. The presence of the oxide layer will also affect the individual joints of uneven heating, causing fluctuations in the quality of welding. Thoroughly clean the surface of the workpiece is therefore necessary to ensure access to quality joint.
Second, the heat balance and heat dissipation Spot welding, heat generated only a small portion for the formation of solder joints, due to the greater part of the material near the conduction or radiation is lost, the heat balance equation:
Q1 - heat forming heat, Q2-- loss of nugget: Q = Q1 + Q2 ---- (3) wherein
Thermo-physical properties useful heat Q1 depends on the amount of metal and molten metal, regardless of the welding conditions used. Q1 = 10% -30% Q, good thermal conductivity metal (aluminum, copper alloys, etc.) take the lower limit; high resistivity, poor thermal conductivity of metal (stainless steel, high temperature alloys, etc.) to take maximum. Q2 include heat loss through heat conduction electrode (30% -50% Q) and the heat conduction through the workpiece (20% Q or so). Radiant heat in the atmosphere to around 5%.
Third, the weld cycle
Spot and projection welding welding cycle consists of four basic phases (as shown in spot welding process):
1) pre-press stage - down to the electrodes current-on stage, to ensure that the electrode clamping the workpiece between the workpiece so that adequate pressure.
2) welding time - the welding current is formed through the workpiece nugget heat production.
3) to maintain the time - cut off welding current, electrode force to continue to maintain sufficient strength nugget solidification.
4) Time rest - Lift the electrodes start to the electrode began to decline again, at the beginning of a weld cycle.
To improve the performance of welded joints, and sometimes need the following items in addition to one or more of the basic cycle:
1) increase pre-pressure to eliminate the gap between the workpiece thickness, making it tight fit.
2) improve the plasticity of the metal with preheat pulse, so that the workpiece is easy to fit closely to prevent splashing; doing so can have multiple projection welding bumps before power-uniform contact with the welding plate to ensure uniform heating of each point.
3) increase the compaction force to forging a nugget, prevent cracks or craters.
4) with slow cooling or tempering of quenched steel pulse eliminate organization, to improve the mechanical properties of the joint, or without increase in forging force conditions to prevent cracks and craters.
Fourth, the type and scope of welding current
1. AC amplitude modulated by a current ramp, slow down in order to achieve the purpose of pre-heating and slow cooling, which is very beneficial for aluminum welding. AC pulses can also be used for multi-spot welding, i.e., for the left between the two or more pulses of cooling time, to control the heating rate. This method is mainly used in welding thick steel plates.
2. DC is mainly used for large current situations, since most of the three-phase DC welding power supply to avoid a three-phase unbalanced load single-phase power supply.
Fifth, when the weld metal resistance welding
The following are the main indicators of assessment of resistance welding:
Electrical conductivity and thermal resistivity of the material
1. The thermal conductivity is small and the large power required welding metal, weldability is poor.
2. The material strength at high temperature (0.5-0.7Tm) yield big strength of the metal, prone to splashing, shrinkage, cracks and other defects when welding, requires the use of large electrode force. Large forging force if necessary, after the application of power needs, poor weldability.
Plastic plastic temperature range narrow temperature range of metallic material
3. (such as aluminum), fluctuations welding parameters are very sensitive, require precise control of welding process parameters, and requires a good follower electrode. Poor welding.
4. The material sensitivity to thermal cycling under the influence of welding heat cycle, there is a tendency hardened metal, easy to produce hardened tissue, cold cracking; fusible alloy with a low melting point impurities tend to form easily produce hot cracks; cooled as reinforced metal easy to produce soft zone. Prevent these defects should take appropriate technical measures. Thus, the sensitivity of thermal cycling large metal welding is also poor.