High-Frequency Welding: Principle, Features & Applications

Case Study Introduction

Structural steel, with its ease of installation, high precision, and high degree of industrialization, has become an essential material for constructing bridges, buildings, and other structures. High-frequency resistance welding can be used for the production of various structural steels such as T-shaped, I-shaped, and H-shaped.

The structural steel produced by high-frequency welding can reach a web height of 600mm, a flange width of 300mm, and a thickness of 12mm. High-frequency welding is particularly suitable for profiles that cannot be rolled by conventional hot rolling due to significant differences in thickness, asymmetric shapes, or are composed of different materials.

Basic Principles of High-Frequency Welding

High-Frequency Resistance Welding (HFRW) was invented in the early 1950s and quickly applied to industrial production. High-frequency welding is a welding method that utilizes resistance heat generated by a high-frequency current of 10~500kHz passing through the weld joint surface, reaching atomic bonding under applied pressure or no pressure.

Currently, high-frequency welding is mainly used in highly mechanized or automated pipe and profile production lines. The welding materials can be steel or non-ferrous metals, with a pipe diameter range of Φ6~Φ1420mm, and a wall thickness of 0.15~20mm. Smaller diameter pipes often use straight weld seams, while larger diameter pipes often use spiral weld seams.

Basic Types of High-Frequency Welding

Depending on the way high-frequency electric energy is introduced, high-frequency welding can be divided into high-frequency resistance welding and high-frequency induction welding. In high-frequency resistance welding, the current is introduced into the weldment for welding through direct contact between the electrode contact and the weldment.

In high-frequency induction welding, an induced current is generated inside the weldment for welding through the coupling of an external induction coil, without any physical electrical contact between the power source and the weldment.

Although these two methods introduce high-frequency current into the weldment differently, the principle of resistance heating and weld seam formation in the welding area by the electric current is the same. Both utilize the skin effect and the proximity effect of high-frequency current to achieve welding.

The skin effect is a phenomenon where the high-frequency current tends to flow on the surface of the metal conductor. The depth of current penetration is related to the type of metal material, temperature, and current frequency. As the current frequency increases, the depth of current penetration decreases, and the skin effect becomes more pronounced.

The proximity effect refers to the phenomenon where the current concentrates on the nearby side of the conductor when the high-frequency current flows in opposite directions in two conductors or in a reciprocating conductor, as shown in Figure 5-1.

Figure 5-1 Generation of Proximity Effect

a) Direct Current or Low Frequency Current
b) High Frequency Current

The proximity effect increases with frequency and intensifies with the proximity of the nearby conductor to the weldment, making the concentration and heating of the current more pronounced. If a magnetic core is added around the nearby conductor, the high-frequency current will concentrate more narrowly on the surface of the workpiece.

High-Frequency Welding Process and Its Essence

High-frequency welding is a process where the skin effect of high-frequency current concentrates the current on the surface of the workpiece to be welded.

The proximity effect is then utilized to control the flow path, position, and range of the high-frequency current, causing the current to flow only through the areas of the workpiece that need to be heated and heating them to the required welding temperature. Pressure is then applied to achieve connection.

Figure 5-2 shows the schematic diagram of the high-frequency welding principle for two small-length workpieces. Whether it’s a butt joint or a T-joint, the sections of the workpieces to be welded are parallel to each other with a certain gap. High-frequency current is introduced into the workpiece via contact, flowing in the direction of the arrow. The adjacent end faces form a reciprocating conductor.

The skin effect and the proximity effect of high-frequency current concentrate the current flow over the surface of the workpiece and quickly heat it to the welding temperature. A welded joint is formed after applying pressure.

Figure 5-2 Schematic Diagram of High-Frequency Welding Principle for Small Length Workpieces

a) Butt Joint
b) T-Joint
HF – High Frequency Power Source
F – Pressure

If the workpiece is very long, continuous high-frequency welding is used. To effectively utilize the skin effect and the proximity effect of high-frequency current, the welding end face of the workpiece must be made into a V-shaped opening structure, with a V-shaped converging angle formed between the two faces to be welded, as shown in Figure 5-3.

Figure 5-3 Schematic Diagram of Heating and Melting Process in V-shaped Opening

I – Heating Section
II – Melting Section
α – Converging Angle
1 – Electrode Contact
2 – Convergence Point
3 – Liquid Melt Pool
4 – Welding Point
5 – Converging Face

Figure 5-4 shows three products manufactured using a V-shaped opening structure. During high-frequency welding, high-frequency current is introduced into the workpiece through electrode contacts placed at the edges of the workpiece.

Figure 5-4 Products manufactured with a V-shaped open structure

a) Pipes or conduits
b) I-beams
c) Composite strips

Due to the skin effect, the current flows from one electrode contact along the converging face to the convergence point and then to the other electrode contact, forming a reciprocating circuit for the high-frequency current.

Because of the proximity effect, the closer to the vertex, the smaller the distance between the two edges, the stronger the proximity effect generated, and the higher the edge temperature, even reaching the melting point of the metal to form a liquid metal pool.

As the workpiece continuously moves forward, the welding end face is squeezed, pushing out the liquid metal and oxides. Pure metal then comes into close contact with each other in a solid state, resulting in plastic deformation and recrystallization, thereby forming a solid weld seam.

Features and Applications of High-Frequency Welding

Features of High-Frequency Welding

High-frequency welding has the following advantages compared to other welding methods:

(1) High welding speed

Due to the high concentration of current in the welding area, the heating speed is extremely fast, with a general welding speed of 150~200m/min.

(2) Small heat-affected zone

Because of the high welding speed and strong self-cooling effect of the weld, the heat-affected zone of high-frequency welding is narrow and less prone to oxidation, thereby achieving welds with good structure and performance.

(3) No need for pre-weld cleaning

The voltage of high-frequency current is very high, able to conduct through the oxide film on the surface of the weldment, and can squeeze the oxide film and dirt out of the joint during welding.

The disadvantages of high-frequency welding are:

1) High assembly quality requirement during welding, especially when welding profiles with continuous high-frequency welding. Both assembly and welding have been automated, any factors causing changes in the V-shaped opening will affect the welding quality.

2) The high-voltage part of the power supply circuit poses a threat to human and equipment safety, requiring special protection measures.

3) The working life of components in the circuit, such as oscillating tubes, is relatively short, and the maintenance cost is high.

Applications of High-Frequency Welding

(1) Suitable Materials

High-frequency welding can weld low carbon steel, low alloy high strength steel, stainless steel, aluminum alloy, titanium alloy (requires inert gas protection), copper alloys (brass needs welding flux), nickel alloys, zirconium alloys, and other metal materials.

(2) Structure Types

In addition to being able to manufacture seamless tubes, special-shaped tubes, heat sink tubes, spiral heat sink tubes, cable sheaths, etc., of various materials, high-frequency welding can also produce structural profiles (T-shaped, I-shaped, H-shaped, etc.) and plates (strips), such as automobile rims, automobile body panels, saw blades composed of tool steel and carbon steel, cutting tools, etc.

Figure 5-5 shows the basic applications of high-frequency welding.

Figure 5-5 Basic Applications of High-Frequency Welding

a), b), h) Pipe Welding
c) Pipe Roll Welding
d) Strip Butt Joint
e) T-Joint
f) Spiral Tube
g) Spiral Tube Heat Sink
i) End Joint Welding
j) Melting Point Welding
k) Strip Butt Joint
HF – High-Frequency Power Source
IC – Induction Coil

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