Have you considered the following question? During our welding operations, while the arc light flashes and the welding sparks produce glare, we, the operators, are constantly burned. What could be the cause of this phenomenon?
1. The principle of resistance spot welding
Welding is a process that uses heat, pressure or both, with or without filler material, to achieve atomic bonding between two separate metal surfaces, forming a permanent connection.
The essence of welding:
The reason solids like metals can maintain a fixed shape is because the distance (lattice) between their internal atoms is very small and they form strong binding forces between the atoms.
Unless enough external force is applied to break these bonds between atoms, a solid metal will not deform or separate into two parts.
To connect two separate metallic components, from a physical point of view, it is necessary to bring the atoms on the connecting surface of these two components closer together to the distance between the metallic lattice.
Resistance spot welding:
Resistance welding is a method of using electrical current to heat and melt or plasticize objects being welded by trapping them between electrodes and passing the current through the contact surface and adjacent areas of the object being welded.
Basic principle of resistance welding:
The heat generated during welding and the factors affecting heat generation, the amount of heat generated during spot welding is determined by Joule's law according to the following formula:
Total heat: Q = I2RT
- Q – heat generated (joules),
- I – welding current (ampere),
- R – electrode resistance (ohm),
- T – welding time (seconds).
Where R = 2R parts +R Contacts + 2 Relays (as shown in figure 1).
The heat generated by the contact resistance R Contacts +2R electrodes accounts for about 10% of the total heat, while the heat generated by the internal resistance 2R parts of the welded joint accounts for about 90% of the total heat. The highest temperature is always in the center of the welding area, where the fusion zone forms.
R Contacts are harmful to welding and are the main cause of spatter and burns at the welding point. During welding, the metal on the contact surface first reaches the welding temperature.
As the temperature continues to rise, the contact resistance disappears and the thermal resistance of the plate itself continues to act, forming a weld spot evenly distributed on both sides of the contact surface.
R electrodes are harmful to welding because they overheat the plate and reduce the life of the electrode or even burn the electrode and the surface of the plate.
In spot welding, it is impossible for the entire surface of the part to make contact, therefore there is contact resistance. The number of contact points and the size of the contact area depend on the hardness of the metal material, the smoothness of surface processing and the pressure applied to both ends of the workpiece.
Obviously, the softer the part material, the smoother the surface, and the higher the pressure, the lower the contact resistance.
For low carbon steel, when the temperature exceeds 6000°C, the contact resistance disappears. The higher the pressure, the lower the temperature required for the contact resistance to disappear.
Once the material has been determined, the main factors affecting contact resistance are electrode pressure, surface condition and heating temperature.
As shown in the figure above, when there is an oxide film or dirt on the surface of the board, the contact resistance increases. As the plate temperature increases, the number and area of contact points increases as the crushing resistance of the contact point decreases, resulting in a decrease in contact resistance.
When the electrode pressure increases, the convex points on the surface of the plate are crushed, the oxide film is destroyed, and the number and area of contact points increase, resulting in a decrease in contact resistance.
2. Causes and classifications of welding spatter
In the welding process, a plastic ring and a fusion zone are formed under the action of heat and mechanical force, and increase with the progress of electrical heating until the required weld size is obtained.
Generally speaking, the metal between the two electrodes and the contact surface of the workpiece undergoes the most intense heating and reaches the highest temperature, which can exceed 300 ℃ above the melting point of the metal. The temperature distribution of the metal around the center of the weld is shown in the figure on the left.
During spot welding, the heating rate of the weld is extremely fast, and the core temperature of the weld can be heated to more than 1800℃ in 0.06-0.1 seconds or even less time. The heating rate can reach 2,000-30,000 degrees/second.
Due to the strong cooling of the water in the electrode, a large amount of heat will be transferred by the electrode, so the temperature of the contact surface between the workpiece and the electrode will not be very high, generally only about 550 ℃.
Therefore, the hottest place during spot welding is the center of the small cylinder, where the liquid metal is surrounded by a ring of plastic metal that has not yet melted and is still in a plastic state when the central metal is melted. We call this metal-plastic ring a “plastic ring” (Figure 3).
During the welding process, a plastic ring is first formed and then a fusion zone is formed in the center of the plastic ring where heat is concentrated. The plastic ring surrounds the fusion zone and expands radially.
When the expansion rate of the fusion zone is greater than that of the plastic ring under high pressure, the fusion zone ruptures the plastic ring and spreads, forming welding spatter, which adheres to the weld surface and is called welding burr. (Figure 4).
Welding spatter can be divided into two categories: early spatter and late spatter.
1. Initial Splashes:
During the heating process of spot welding, if the heating is too fast and the surrounding plasticity is not yet formed or is not compact enough, the contact point that is heated quickly due to the rapid increase in temperature will cause internal gasification of the metal . Under the action of electrode pressure, the liquid metal in the ring will be squeezed and sprayed in the form of splashes towards the space between the plates.
2. Late splash:
After the plastic ring is formed during the heating process, the heating continues and the melting zone and the plastic ring continue to expand outward. When the radial expansion rate of the fusion zone is greater than that of the plastic ring, the fusion zone will break through the weakest part of the plastic ring and be pulverized.
The edge where the electrode cap contacts the sheet metal during the welding process is the narrowest part of the plastic ring. After spraying, sharp welding burrs often remain on the weld surface.
3. Splashes caused by liquid bridge rupture
The liquid bridge refers to the thinnest part that connects the welding wire or rod to the droplet formed at the end.
Characteristics of splashes caused by liquid bridge rupture:
When the liquid bridge breaks, spatter is controlled by the bell shape of the end of the welding rod. Furthermore, the gravity of the droplet and the force of the ionized gas cause the spatter to spread from the breaking point of the liquid bridge. The entire spatter band falls from top to bottom in a fan shape formed by the angle of the bell-shaped end of the welding rod.
4. Splashes caused by temperature differences
Here, the temperature difference refers to the difference between the arc, the droplet and the weld pool.
Firstly, the temperature of the welding arc is between 5370 and 7730°C.
Drop temperature:
The moment the drop detaches from the welding electrode, it transforms into a sphere surrounded by a layer of slag. At this point, the gasifying agent (CO gas produced from the oxides and carbides in the rod coating) creates a steady, continuous gas flow, removing some of the heat from the droplet, resulting in a droplet temperature of about 4,000°C.
3. Factors and measures to control welding spatter
1. Operational factors:
(1) Poor quality of electrode end face: During welding, the electrode cap end face should be kept flat and the size controlled within 6 ~ 8 mm (Fig.5).
(2) Electrode misalignment: The amount of misalignment of the electrode end face should be less than 1 mm (Fig.6).
(3) Edge welds: The distance between the welding point impression and the edge should be 1mm, allowing the environmental protection of the plastic to be released.
(4) Oil stains on the surface of the sheet metal: Before welding, make sure the surface of the sheet metal is clean.
2. Welding parameter factors:
Based on the principle of welding, it can be seen that the parameters that affect welding include welding current, welding resistance and welding time. If the welding parameters are too large, the molten metal in the weld pool will expand sharply, causing spatter. This may result in defects such as electrode sticking, electrode explosion, welding breakdown, etc.
(1) Excessive welding current and welding time:
Set a reasonable welding current and time, and check the current output status according to the corresponding frequency.
(2) Excessive welding resistance:
Confirm the surface and fitting status of the sheet before welding, and select a reasonable welding pressure to check the current output status according to the corresponding frequency.
(3) The welding specification is very difficult:
Reasonably match the welding current and welding time or add a preheating program before the welding procedure so that the sheet metal can form an initial connection and eliminate contact resistance, thereby reducing welding spatter.
As the welding current increases, the size of the fusion zone or penetration rate also increases. Under normal circumstances, there is a reasonable upper and lower limit for the current in the welding area.
When the current is less than the lower limit, the heat input is too small to form a standard melting zone; when the current is higher than the upper limit, the heating speed is too fast, which may cause welding spatter.
To ensure welding strength and reduce welding spatter, welding parameters should be selected at the critical point between spatter and non-spatter (Fig.7).
Complex welding cycle diagram:
When adding the preheating program and using the acceleration current, the current gradually increases to reduce the heating speed (Fig.8).
Through preheating, the plasticity of the sheet is improved, facilitating the fitting of the panels, reducing the contact resistance of the panels to a certain extent and reducing spatter during welding.
Validation of welding parameters:
On-site validation workstation: XX left/right front longitudinal beam inner panel assembly
Workstation details: X30-2512H: a total of 51 points
Welding tongs status: normal
Before validation: Welding parameters
| Workstation name. | Welding pliers model. | Pre-pressing time. | Time to press. | Preheating time | Preheating current | Thermal cooling | Welding time | Welding current | Acceleration time | Acceleration current. | Waiting time. |
| XX | X30-2512H | 25 | 30 | 0 | 0 | 0 | 25 | 9.5 | 0 | 0 | 20 |
Number of splashes: 30-35
Number of burrs: 18-25
Checked: welding parameters.
| Workstation name. | Welding pliers model. | Pre-pressing time. | Time to press. | Preheating time | Preheating current | Thermal cooling | Welding time | Welding current | Acceleration time | Acceleration current. | Waiting time. |
| XX | X30-2512H | 15 | 30 | 5 | 5 | two | 22 | 9.0 | 3 | 1.0 | 15 |
Number of splashes: 6-12
Number of burrs: 2-6
Checked effect diagram:
Tracking effect: Significant improvement in welding spatter and burrs by adjusting welding parameters through process optimization and operation control.
4. Conclusion:
Currently, welding spatter control mainly depends on process optimization and operation control. Due to the welding characteristics and the complex environment on site, it is not yet possible to completely eliminate welding spatter.
Therefore, every welder needs to improve his sense of responsibility, observe more, debug more and improve more, optimize our welding environment and improve the quality of our body welding, providing higher quality cars for each user.
