Resistance spot welding

Classification of welding processes according to DIN 1910-100 with serial numbers according to

Resistance spot welding (common abbreviation RP, EN ISO 4063n: Process 21) is a resistance pressure welding process for welding mostly sheet-metal parts of various dimensions and materials. According to DIN 1910-100, resistance spot welding belongs to the group of pressure welding processes using electricity as an energy source.

The basics of the process, the preparation and implementation of the welding production with resistance spot welding of steels are described in DVS data sheet 2904-4 and in the monograph "Resistance pressure welding".

Fields of application and procedural advantages

Resistance spot welding is used to join sheet metal in bodywork and vehicle construction and generally in sheet metal manufacturing. It is also used to weld the smallest parts made of a wide variety of metals and metal combinations in the electrical industry and electronics manufacturing, e.g. B. in the production of capacitors , contact sets for relays and circuit breakers or connections of coils and motor windings. The connections in this application area often lead to variants of resistance projection welding . With certain restrictions, materials that are otherwise poorly suitable for welding can also be joined together.

The advantages of the process compared to fusion welding are good energy efficiency, low component distortion due to the welding heat, high productivity, good automation and welding without additional material. However, no continuous, tight seam is created. The quasi-static strength (Kopfzug- and shear strength) of certain steel sheets is higher in resistance spot welding than that of mechanical connections as those of the clinching , however, the dynamic strength is better.

Procedural principle

The principle is that the parts to be welded are supplied with electricity via electrodes under the action of force. The Joule heat generated by the local flow of electricity through the parts to be connected is used for heating.

${\displaystyle W_{\text{s}}=I_{\text{s}}^{2}R_{\text{s}}t_{\text{s}}}$

With

${\displaystyle W_{\text{s}}}$ ... welding energy,

${\displaystyle I_{\text{s}}}$ ... welding current,

${\displaystyle R_{\text{s}}}$ ... resistance at the welding point,

${\displaystyle t_{\text{s}}}$ ... welding time.

Due to the resistance heating, the connection partners are heated until the required welding temperature is reached. The parts are point-welded at their point of contact between the electrodes under the action of an electrode force by solidification of the melt, by diffusion or in the solid phase. Within a relatively short time, the thermal energy is concentrated on a small area of ​​the work piece, whereby the permanent connection is created under the effect of pressure. Setting variables ( welding parameters ) for the process are generally the welding time, the welding current and the electrode force, which vary widely depending on the welding task. To set the welding parameters for welding sheet steel connections, the DVS or users (e.g. Mercedes-Benz) specify guide values.

Equivalent circuit diagram of the secondary circuit with 2 × 1 mm sheet steel and electrode diameter 5 mm (according to M. Krause)

Resistances in spot welding

A number of contact and material resistances contribute to the heating in the welding area :

• Material resistances ( ): These resistances are strongly temperature dependent. ${\displaystyle R_{\text{M}}}$

${\displaystyle R_{M}=\sum _{i=1}^{n}{\frac {s_{i}}{\kappa _{i}A_{i}}}\left(1+\alpha _{i}\Delta T_{i}\right)}$

With

${\displaystyle s}$ ... sheet thickness,

${\displaystyle \kappa }$... electrical conductivity ,

${\displaystyle A}$... area ,

${\displaystyle \alpha }$... temperature coefficient ,

${\displaystyle \Delta T}$ ... temperature difference to reference temperature.

The material resistances of the electrodes R ₁ and R ₂ (picture "Resistances in spot welding") should be kept as low as possible, their cross-sections should be as large as possible, their lengths short and their specific electrical resistances low.

• Contact resistances ( ): These resistances arise where two surfaces meet. They are very much subject to random influences, such as impurities and oxide layers on the component surface, alloys on the electrode caps, component fit. The level of resistance depends on systematic influencing variables. These include the electrode forces and the coating of the workpiece surface. ${\displaystyle R_{\text{K}}}$

Material dependency of the dynamic resistance in resistance spot welding according to Matsuyama

The total contact resistance is made up of the individual resistors R _{3, 4} and R ₅ at the beginning of the current flow, depending on the sheet thickness . In the case of steel, depending on the sheet thickness, it is 5 to 10 times greater than the material resistance of the workpieces (R _6.7 ).

The total resistance has a characteristic curve during the current flow time and is called dynamic resistance. With increasing electrode force and temperature, the contact resistances decrease due to higher surface pressure, leveled roughness peaks and enlarged contact surfaces. The contact resistances in the electrode-sheet metal planes (R _3,4 ) fall faster than in the sheet metal-sheet plane (R ₅ ). After passing through a resistance minimum, the resistance increases, which is due to the increase in the material resistance (R _6.7 ) with increasing temperature in the area of ​​the weld. After a maximum the total resistance falls again. This is a result of the electrodes sinking into the workpiece surface. The sinking depth increases with higher electrode force, longer current time, smaller electrode working area and lower heat resistance of the parts to be joined. The course described is typical for unalloyed steel with a bright surface. It differs greatly from this with other materials. With online process monitoring , the dynamic resistance can be used as a criterion for the connection quality, since the size of the weld nugget is reflected in this process.

Energy balance and temperature at the welding point

Energy balance at the welding point

In the workpiece, electrical energy is converted into thermal energy . Only the smaller part of this supplied energy is used directly for the welding process, the active heat . Only this effective heat thus contributes to the temperature increase and the formation of the welded joint . The other part is lost for the welding process due to heat conduction via the cooled electrodes ( ) and the workpiece ( ), as well as due to thermal radiation ( ). ${\displaystyle W_{\text{el}}\ }$ ${\displaystyle Q_{\text{zu}}\ }$ ${\displaystyle Q_{\text{w}}\ }$ ${\displaystyle Q_{\mathrm {V_{E}} }}$ ${\displaystyle Q_{\mathrm {V_{W}} }}$ ${\displaystyle Q_{\mathrm {V_{S}} }}$

${\displaystyle Q_{\text{w}}=Q_{\text{zu}}-\left(Q_{\mathrm {V_{E}} }+Q_{\mathrm {V_{W}} }+Q_{\mathrm {V_{S}} }\right)}$

The temperature increase during the spot welding process can be calculated, at least roughly

${\displaystyle \Delta T={\frac {Q_{\text{w}}}{c\rho V}}}$

With

${\displaystyle \Delta T}$... temperature difference ,

${\displaystyle c}$... specific heat ,

${\displaystyle \rho }$... density ,

${\displaystyle V}$... volume of the nugget

Current density and temperature distribution at the end of the current flow in resistance spot welding (according to DVS 2902-4)

Current density and temperature distribution at the beginning of the current flow in resistance spot welding (according to DVS 2902-4)

From this formula it can also be seen that the current density is relevant for the weld spot shape. The current density and the resulting temperature strongly depend on the electrode shape and cooling. Before the current begins to flow, relatively few and small contact areas are formed under the influence of the electrode force. The passage of current generates heat in all of these areas. The spatial distribution of the welding current results from the resistance ratios and the skin effect . As the current flow progresses, the temperature rises locally and the current paths shift, and with it the position and intensity of the heat sources. Since the area through which current flows in the sheet-metal plane is always larger than that of the electrode-sheet metal plane and the resistance in the center of the point increases as a result of the rise in temperature, the current flow increasingly shifts to the outer zones. Depending on the shape of the electrode, this results in different temperature concentrations and connection forms.

Connection formation

Metals combine because the atoms of both parts to be joined interact with one another chemically or metallurgically. There are three types of connection in resistance spot welding:

• Joining in the molten state, whereby the molten pools of the parts to be joined mix and solidify together,
• Diffusion bonding with bonding in a mix of molten and doughy material,
• Solid phase bonding, with atoms in a very limited area of ​​the materials being brought together to interact and form a solid, coherent bond.

The physical binding mechanism depends on the materials of the parts to be joined.

Resistance spot welding cannot connect all materials equally well. The causes lie in the physical properties and in the interaction with the electrode materials. Ideal material properties for resistance spot welding are:

• equal or close melting points
• low electrical conductivity
• low thermal conductivity
• high deformability (hot deformability)

These conditions are seldom encountered in real welding tasks. Materials with high electrical and thermal conductivity are difficult to weld because the welding heat is dissipated very quickly and the required welding temperature cannot be achieved. Hard and brittle materials have only a small temperature range with plastic properties and tend to break during the welding and cooling process. The deformability in the range of the welding temperature is important. Metals of low ductility in this temperature range tend to develop so-called hot cracks during cooling.

There are three groups of materials:

Group 1:

Gold , aluminum , silver , copper , brass and bronze alloys . These materials are face-centered cubic in the lattice structure with high ductility due to the large number of dislocation structures . This group of materials has relatively high electrical and thermal conductivity and therefore limited suitability for welding . Metals of group 1 with those of groups 1, 2 and 3 combine in the solid state. After the tensile test, a weld point can be seen in the fracture pattern of these connections, which is often misinterpreted as the result of a fusion weld.

Group 2:

Nickel , titanium , platinum , CrNi18-8 and CrNi16-10 with a hexagonal grid. This group can weld to one another using any type of connection. Metals of group 2 combine in a solid state with partners in metal groups 1 and 2.

Group 3:

Chromium , iron , molybdenum , niobium , tantalum , tungsten and martensitic stainless steels (e.g. X12CrS13, X14CrMoS1) with body-centered cubic lattice. The melting temperature is high, the material is hard and brittle and the electrical conductivity is in the medium range. The connection with metals of all groups takes place in the solid state.

Process variants

Electrode arrangement

The electrodes can be arranged in different ways. With two-sided spot welding, the electrodes work on both sides of the workpieces. Indirect spot welding has electrodes arranged on one side.

Variants of resistance spot welding with different electrode arrangements according to DVS 2902-1

Simultaneously created points arranged in parallel

If only a single point can be produced during a welding process, this is known as single-point welding. This is done either on two sides or on one side with the aid of a dummy electrode.

If two or more electrodes are used one after the other or at the same time to produce several welding points in one welding cycle, we speak of multi-point welding. With one-sided double spot welding in one welding cycle, two individual spot welds are produced. The same current flows through two electrodes arranged on one side within a circuit. With multi-spot welding, two or more welding points are created in one welding cycle.

Simultaneously created points in the electrode axis

With single-shear spot welding (double sheet welding), a spot weld is created between two sheets. Several sheets can also be connected in the stack at the same time. This is referred to as multi-step spot welding (multi-sheet welding). In the majority of cases, three-sheet welding is used in practice.

Procedure

Electrode path, electrode force and welding current curve of a 2-pulse welding

Resistance spot welding - process sequence

During welding, a controlled interaction of the electrode movement, the electrode force and the welding current leads to the expected welded connection. The electrode movement is triggered by switching valves via simple pneumatic or hydraulic cylinders, or servo drives with servo-pneumatic cylinders or servo-electric motors are used. With the latter, the electrode movement is carried out by the welding control, the robot control or an additional drive control.

During the lead time (VHZ), the upper electrode is moved over the closing (SLZ) onto the sheet metal surface, the force builds up over the force increase time (KAZ) until the welding current is switched on. Depending on the requirements of the materials to be joined, the current can be controlled to the desired welding current via a current rise time (SAZ - so-called up- slope ). The welding current is kept constant over the welding current time (SZ) when using a constant current control. If welding is carried out with two current pulses, the current is switched off during a pause time (PZ). Pause times are currentless times within a weld, the electrodes remain closed. The pause time is also known as the cooling down time or heat equalization time. The current is switched on again for the duration of welding current time 2 (SZ2). Switching off the current can also be controlled along a falling ramp using the current fall time (SFZ). A controlled current drop causes the weld nugget to cool down slowly. The electrodes remain closed during the hold-up time (NHZ). The weld nugget cools down under pressure. The power is switched off (vehicle power drop time). Series spot welding is followed by a hold-open time (OEZ) as a powerless and currentless time. It is the time between two welds.

In specific welding processes, the various process sections are combined in a wide variety of ways. Welding controls offer the possibility of programming optimized process sequences.

Welding equipment

→ Main article : Welding equipment for resistance welding

A wide variety of welding devices are used to carry out resistance spot welding. They consist of a mechanical machine body and an electrical system that consists of a power unit and a controller. The electrode force is applied by a force generation system. In most cases, a cooling system is used to cool the electrodes .

Welding parameters

→ Main article : Spot welding parameters

Welding parameters for resistance spot welding are called the setting values ​​for a resistance welding process , which can be set on the welding devices or can be changed in a controlled manner during the welding process .

For resistance spot welding these are the sizes:

• Welding current I _s ,
• Welding time t _s ,
• Electrode force F _E .

These values ​​must be set so that the required weld quality is achieved.

Spot weld quality

→ Main article : spot weld quality

Possible shape and position of the weld nugget for resistance spot welding in section (according to DVS 2902-4)

Resistance spot welding is used to connect components to one another in such a way that they can meet the requirements placed on them over an expected service life. To do this, the spot welds must meet the quality criteria or quality features that meet the requirements. General specifications for the quality characteristics cannot be made, because these are always closely linked to the respective component requirements. In any case, the strength of a welded point is important, which depends on the point size, shape and location. The selected welding parameters but also other influencing variables such as material, material thickness, arrangement of the electrodes and cooling lead to welding points of different geometrical structures. Unsymmetrical weld nuggets can be created by the cooling conditions, physical material properties, and electrode shape. Depending on the electrode shape (flat or convex), electrode size, electrode force and selected welding parameters, electrode impressions of different depths arise in the sheet metal surface.

Individual evidence

1. ↑ ^{a b} DIN 1910-100: 2008-02 Welding and related processes - Terms - Part 100: Metal welding processes with additions to DIN EN 14610: 2005.
2. ↑ ^{a b} DIN EN ISO 4063: 2011-03 Welding and related processes - List of processes and serial numbers.
3. ↑ ^{a b c d e f g} DVS : "Resistance spot welding of steels up to 3 mm thick - Basics, preparation and implementation" , DVS data sheet 2902-4, 2001
4. ↑ ^{a b c} M. Krause: Resistance pressure welding . DVS-Verlag, Düsseldorf 1993, ISBN 3-87155-531-2
5. ↑ M. Reinstettel: Laboratory studies on process stability in rivet clinching. Diss. TU Chemnitz, 2007, urn : nbn: de: bsz: ch1-200800492 .
6. ↑ Resistance spot welding of steel up to 3 mm sheet thickness . Mercedes-Benz, 2005.
7. ↑ Kin-ichi Matsuyama: Quality Management of Resistance Welds. IIW-Doc. III-1496-08, 2008.
8. ^ David Steinmeier: Laser & Resistance Welding - Metal Bond Types . (PDF) microJoining Solutions - microTips ™.
9. ↑ DVS : Resistance welding in electronics and precision engineering - overview and basics. DVS Leaflet 2801-1, 1996.
10. ^ David Steinmeier: Laser and Resistance Weldability Issues, Part I - Bulk Material Properties . (PDF) microJoining Solutions - microTips ™.
11. ↑ ^{a b} DVS : Resistance spot welding of steels up to 3 mm thick - overview , DVS data sheet 2902-1, 2001
12. ↑ DVS : Controls and power units for resistance welding , DVS bulletin 2904, 2010