Choosing the Right Parameters for Friction Welding

(1) Welding Parameters of Continuous Drive Friction Welding

The welding parameters of continuous drive friction welding mainly include spindle rotation speed, friction pressure, friction time, upsetting pressure, upsetting time, and deformation amount. These parameters directly impact the welding quality and also affect the welding production rate, metal material consumption, and welding machine power.

1) Rotation Speed and Friction Pressure

Rotation speed and friction pressure are the most crucial welding parameters. During the welding process, rotation speed and friction pressure directly affect friction torque, friction heating power, joint temperature field, plastic layer thickness, and friction deformation speed.

When the diameter of the workpiece is fixed, the rotation speed represents the friction speed. The average friction speed on the friction interface of a solid round cross-section workpiece is the friction line speed at a place 2/3 of the radius away from the center. The rotation speed when the welding temperature is reached is generally termed the critical friction speed.

To heat the deformation layer of the interface to the metal material’s welding temperature, the rotation speed must be higher than the critical friction speed. Typically, the critical friction speed of low carbon steel is about 0.3m/s, and the average friction speed range is 0.6~3m/s.

During the stable friction stage, the impact of rotation speed on the friction deformation layer thickness of the welding surface, the position of the deep plastic zone, and flash is shown in Figure 4-15.

Figure 4-15 Impact of Rotation Speed on Deformation Layer Thickness, Position Displacement in the Deep Plastic Zone, and Flash

(Φ19mm Low Carbon Steel Rod, Friction Pressure of 86MPa)

At a rotation speed of 1000r/min, due to the high friction speed of the outer circle, the temperature of the outer metal increases, the temperature of the friction surface is lower than during high-speed friction, the friction torque and friction deformation speed increase, and shift towards the outer circle, making the deformation layer of the outer circle thicker than the center.

At this time, the deformed layer metal easily flows out of the friction surface, forming an asymmetrical, oversized flash (Figure 4-15a). The temperature distribution gradient of such a joint is large, the deformed layer metal can be squeezed out in large quantities, the weld metal is quickly renewed, which effectively prevents oxidation.

When the rotation speed increases, the temperature of the friction surface rises, the friction torque and friction deformation speed decrease, and the deep plastic zone moves towards the center.

At this time, the high-temperature viscous metal in the deformation layer, while flowing outward under the influence of friction pressure and friction torque, faces substantial resistance, forming a symmetrical thin-winged flash (Figure 4-15c). Such a joint, due to the small torque and less extruded metal, has a broader temperature distribution, and the deformed layer metal is more likely to oxidize.

Friction pressure greatly impacts the quality of the welded joint. To generate sufficient friction heating power and ensure full contact of the friction surface, the friction pressure cannot be too small.

During the stable friction stage, as the friction pressure increases, the friction torque increases, the friction heating power increases, the friction deformation speed increases, the deformation layer thickens, the deep plastic zone widens and moves towards the outer circle, forming a large and asymmetrical flash under the pressure.

When the friction pressure is high, the temperature distribution gradient of the joint is large, and the deformed layer metal is less likely to oxidize. During the friction heating process, the friction pressure is generally a fixed value, but to meet the special requirements of the welding process, the friction pressure can also continuously rise or adopt two-stage or three-stage pressurization.

The selection range of rotation speed and friction pressure is broad, and there are two most common combinations: one is strong regulation, i.e., lower rotation speed, higher friction pressure, and shorter friction time; the other is weak regulation, i.e., higher rotation speed, lower friction pressure, and longer friction time.

2) Friction Time

The friction time determines the stages and degree of friction heating, directly affecting the joint’s heating temperature, temperature distribution, and welding quality.

If the time is short, the friction surface is not sufficiently heated, a complete plastic deformation layer cannot form, and the joint temperature and temperature field cannot meet the welding requirements; if the time is long, the joint temperature distribution is broad, the metal in the high-temperature zone is prone to overheating, the friction deformation amount is large, the flash is also large, and the consumption of material and energy is high.

The friction time for carbon steel workpieces is generally within the range of 1~40s.

3) Friction Deformation Amount.

The friction deformation amount is related to rotation speed, friction pressure, friction time, the condition of the material, and deformation resistance. To obtain a solid joint, a certain amount of friction deformation is necessary. When welding carbon steel, the commonly chosen range for the friction deformation amount is 1~10mm.

4) Stoppage Time.

Stoppage time affects the thickness of the joint deformation layer and the welding quality, and it is generally chosen based on the thickness of the deformation layer. When the deformation layer is thick, the stoppage time should be short; when the deformation layer is thin and it is desired to increase the thickness of the deformation layer during the stoppage phase, the stoppage time can be extended. The typical range for brake stoppage time is 0.1~1s.

5) Upsetting Pressure, Upsetting Deformation Amount, and Upsetting Speed.

Upsetting pressure is applied to squeeze out oxides and other harmful impurities from the friction plastic deformation layer and forge the joint metal to produce a close bond, refine the grains, and improve joint performance.The selection of upsetting pressure is related to the material, joint temperature, thickness of the deformation layer, and friction pressure.

For materials with higher high-temperature strength, a larger upsetting pressure should be chosen. When the joint temperature is high and the deformation layer is thick, the required upsetting deformation amount can be achieved with a smaller upsetting pressure; when the friction pressure is high, the corresponding upsetting pressure should be a bit smaller.

Generally, the upsetting pressure should be 2~3 times the friction pressure, and the smaller the friction pressure, the larger the multiple. For carbon structural steel and low alloy steel, the upsetting pressure is typically in the range of 103~414MPa; heat-resistant alloys and stainless steels require higher upsetting pressures.

The upsetting deformation amount is the result of the action of upsetting pressure, and it is typically chosen in the range of 1~6mm. Upsetting speed greatly impacts the welding quality; if the upsetting speed is too slow, it will not achieve the required upsetting deformation amount. The general upsetting speed is 10~40mm/s.

(2) Welding Parameters of Inertia Friction Welding.

The selection of welding parameters for inertia friction welding differs from that of continuous drive friction welding. The main welding parameters are flywheel rotational inertia, initial flywheel rotation speed, and axial pressure. The impact of the main welding parameters of inertia friction welding on joint morphology is shown in Figure 4-16.

Figure 4-16 Impact of Inertia Friction Welding Parameters on Joint Morphology
  • a) Impact of Flywheel Rotational Inertia
  • b) Impact of Initial Flywheel Speed
  • c) Impact of Axial Pressure

1) Flywheel Rotational Inertia

Both the flywheel rotational inertia and initial speed affect the welding energy.

When the initial speed and axial pressure are constant, increasing the rotational inertia of the flywheel increases the total welding energy, extends the welding time, enhances the upsetting action, and therefore increases the amount of plastic metal at the interface, the amount of upsetting and extruded metal, and the size of the welding flash.

Conversely, if the rotational inertia of the flywheel is too small, the upsetting action is insufficient to compact the weld seam and eliminate impurities from the interface. Figure 4-16a shows the impact of flywheel rotational inertia on joint morphology.

The rotational inertia of the flywheel depends on the shape, diameter, and mass of the flywheel (including the mass of the flywheel, chuck, bearing, and transmission components). When the mass or shape of the flywheel changes, the wheel energy at a specific speed will also change.

2) Initial Flywheel Speed

Every metal has a range of external circumferential speeds that can give the joint the best performance. For solid steel rods, the recommended range of initial flywheel speeds is 2.5~7.6m/s. If the speed is too low, even if the required energy level is reached, it will be difficult to form a solid bond across the entire interface due to insufficient heat in the center, and the burrs will be rough and uneven.

As the speed increases, the heating area of the interface gradually flattens from a pinched shape. When the initial speed is higher than 6m/s, the weld seam bulges, with the center being thicker than the periphery. The impact of the initial flywheel speed on joint morphology is shown in Figure 4-16b.

3) Axial Pressure

The effect of axial pressure generally contrasts with the effect of speed. When the axial pressure increases, the extrusion amount of thermoplastic metal at the interface increases, the amount of flash increases, and the welding heat-affected zone narrows.

But if the pressure is too high, the joint in the center will not bond well, and the upsetting amount will be considerable. For medium carbon steel rods, the effective range of axial pressure is 150~210MPa. The impact of axial pressure on joint morphology is shown in Figure 4-16c.

(3) Principles for Selecting Welding Parameters

When selecting welding parameters, in addition to considering factors such as the material, shape, size, preparation of the welding surface, and quality requirements for the joint, the technical data and performance of the welding machine should also be understood.

Generally, the range for selecting the welding parameters of carbon steel continuous drive friction welding is: friction speed 0.6~3m/s; friction pressure 20~100MPa; friction time 1~40s; deformation amount 1~10mm; stoppage time 0.1~1s; upsetting pressure 100~200MPa; upsetting deformation amount 1~6mm; upsetting speed 10~40mm/s.

For the friction welding of medium carbon steel, high carbon steel, low alloy steel, and their combinations, the welding parameters can refer to those of low carbon steel. In order to prevent the formation of quenched structures in the welds of medium carbon steel, high carbon steel, and low alloy steel and to reduce the post-weld annealing process, a weaker welding specification should be selected.

When welding high alloy steel with a high difference in high-temperature strength, the friction pressure and upsetting pressure need to be increased, and the friction time should be appropriately extended.

When welding dissimilar steels with a large difference in high-temperature strength or certain dissimilar metals that do not produce brittle compounds, in addition to adding a mold on the side of the material with lower high-temperature strength, the friction time should be extended, and the friction pressure and upsetting pressure should be increased.

When welding dissimilar metals that are prone to form brittle compounds, a specific mold needs to be used to enclose the joint metal, and the welding temperature should be controlled, the friction speed should be reduced, and the friction pressure and upsetting pressure should be increased.

For small diameter workpieces, the main welding specification is strong; for large diameter workpieces, a weaker specification should be adopted. When the friction speed remains constant, the rotation speed should be correspondingly decreased.

The larger the diameter of the workpiece, the more uneven the distribution of friction pressure on the friction surface, the greater the resistance to friction deformation, and the longer the time required to expand the deformation layer.

In order to reduce the power of the welding machine and the axial pressure, the friction pressure often increases from small to large, with multi-stage pressurization, or inertia friction welding is adopted. When welding carbon steel and low alloy steel with unequal end faces, due to different heat conduction conditions, the temperature distribution and thickness of the deformation layer on the joint are different.

To ensure welding quality, strong specification welding or inertia friction welding should be adopted. When welding pipes, in order to reduce internal burrs, efforts should be made to reduce the friction deformation amount and upsetting deformation amount while ensuring welding quality.

In large-scale production, the welding surface should be smoothed and cleaned before welding, which is conducive to ensuring and stabilizing welding quality.

Currently, the welding parameters of friction welding cannot be determined by calculation, and are mainly determined through experimental methods. Tables 4-8 and 4-9 list the commonly used welding parameters for continuous drive friction welding and inertia friction welding for several typical materials, respectively.

Table 4-8 Welding Parameters for Continuous Drive Friction Welding of Typical Materials

MaterialsJoint Diameter
/mm
Rotation Speed
/rpm
Friction Pressure
/MPa
Friction Time
/s
Upset Forging Pressure
/MPa
Notes
#45 Steel + #45 Steel162000601.5120
#45 Steel + #45 Steel252000604120
#45 Steel + #45 Steel6010006020120
Stainless Steel + Stainless Steel2520008010200
High-Speed Tool Steel + #45 Steel25200012013240Utilizing mold
Copper + Stainless Steel2517503440240Utilizing mold
Aluminum + Stainless Steel251000503100Utilizing mold
Aluminum + Copper252082806400Utilizing mold
Aluminum + Copper, End Face Taper Angle 60°~120°8~501360~300020~1003~10150~200Mold used on both ends
CH41692023709010125
GH3536202370651695
30CrMnSiNi2A20237030655
40CrMnSnMoVA20237035378

Table 4-9 Welding Parameters of Inertia Friction Welding for Selected Materials

MaterialsRotational Speed
/(r/min)
Rotational Inertia
/kg·m²
Axial Load
/kN
#20 Steel57300.2369
#45 Steel55300.2983
20CrA Alloy Steel55300.2776
40CrNi2Si2MoVA Ultra-High Strength Steel38200.73138
Pure Titanium95500.0618.6

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