Traditional Friction Welding: Quality Control and Safety Techniques

Welding Quality and Control

Welding Defects and Their Causes

Once the materials of the metals or dissimilar metals are determined, the quality of the friction weld joint largely depends on the appropriate selection of welding parameters and the control of these parameters during the welding process.

Table 4-14 lists common joint defects and their causes during friction welding of carbon structural steel, low alloy steel, and dissimilar steel (such as high speed tool steel – medium carbon steel, heat resistant steel – low alloy steel, stainless steel – carbon steel). Measures should be taken to eliminate these defects based on their causes.

Table 4-14 Main Defects of Friction Welded Joints and Their Causes

Defect NameCauses of Defects
Eccentric JointLow machine rigidity, fixture eccentricity, inclined or overly extended workpiece end face
Unclosed FlashHigh rotation speed, excessive or insufficient friction pressure; too long or too short friction time, resulting in a narrow deformation layer and high-temperature zone in the joint before forging; slow stopping
Incomplete PenetrationPoor pre-welding surface cleaning; low rotation speed, excessive or insufficient friction pressure; short friction time, low forging pressure
Twisted Joint StructureLow speed, high pressure, slow stopping
Overheated JointHigh speed, low pressure, long friction time
Hardened JointWhen welding hardened steel, short friction time, fast cooling speed
Welding CracksWhen welding hardened steel, short friction time, fast cooling speed
Oxide SpotsPoor cleaning of workpiece before welding, machine vibration; low pressure, short friction time; before forging, the deformation layer and high-temperature zone in the joint are narrow
Brittle Alloy LayerOverly high heating temperature, long friction time, relatively low pressure

Quality Control

After the materials to be welded, joint type, and welding parameters have been determined, the quality of friction welding primarily depends on the preparation of the workpiece blanks, their clamping and alignment, the adjustment of the welding machine, and the control of welding parameters. There are several methods for controlling the parameters of friction welding:

(1) Time Control

Control the friction heating time to keep it constant, typically using a time relay for control. This control requires consistent specifications for the workpiece blanks and stable welding parameters. The results are generally poor when used for mass production with strict specifications (low rotation speed, high friction pressure, short time).

(2) Peak Power Control

This is essentially a control of welding energy and temperature, suitable for carbon structural steel and low alloy steel with strict friction welding specifications.

When friction welding carbon structural steel and low alloy steel with strict specifications, the welding process is immediately stopped and forging is begun as soon as the friction heating power exceeds the peak and drops close to the stable value. This can achieve a good welding quality.

(3) Temperature Control

The heating temperature of the joint is measured using a photoelectric head of a photoelectric relay. Once the temperature rises to the desired value, the relay is activated to control the machine stop and forging. The key to control is selecting the optimal welding temperature and improving measurement accuracy and reproducibility.

(4) Deformation Control

The friction shortening of the workpiece is controlled, and as soon as it equals the selected value, the machine is immediately stopped and forging begins. This deformation reflects the heating temperature and plastic deformation of the joint to some extent.

When the mechanical properties of the joint metal change or the welding parameters fluctuate during the friction heating process, the welding parameters automatically adjust. For example, when the high-temperature strength of the workpiece material increases, the rotation speed correspondingly increases and the friction pressure decreases.

If the workpiece diameter increases or the welding end face of the workpiece is oily and has an oxide film, the deformation resistance of the joint metal increases, and the friction deformation speed decreases. If the friction deformation (i.e., the shortening of the workpiece) is fixed, the friction time automatically extends; otherwise, it is reversed.

This method is better than time control and is suitable for the weak specification welding of steel. It can also monitor the friction time.

(5) Comprehensive Parameter Control

This involves the simultaneous control of power, deformation, and the time at each stage of the welding process, currently controlled by microcomputer. If these parameters exceed the specified values, immediate feedback and alarm are provided, and automatic control and adjustment are carried out.

The main parameters such as rotation speed, pressure, and deformation can also be displayed and their time variation curves recorded. The comprehensive parameter control method can comprehensively and reliably ensure the joint quality of friction welding.

Safety Techniques

The end of a friction welding machine is like the head of a lathe, used for clamping the workpiece and making it rotate at high speed. The tail of the welding machine is like a press, used to clamp the workpiece and apply pressure. Therefore, the safety measures and operating rules of lathes and presses apply to friction welding machines.

For continuous production friction welding machines, the interlock and protection between various actions must be maintained.

For example, only after the rotating chuck clamps the workpiece can the main spindle be driven to rotate, otherwise the workpiece can easily fly out; the chuck can only be moved and returned to its original position after the main spindle stops rotating and the rotating chuck is loosened, then the workpiece can be exited;

the interlock protection of the clutch and brake must be considered during the rotation and parking of the main spindle; collisions between the two chucks and overload and overheating of the main spindle motor must be prevented.

The emergency stop buttons for the machine main spindle stop and pressure stop should be installed in conspicuous and convenient places for operation. The operators of the friction welding machine should wear work clothes and safety goggles.

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