Quality assurance of steel drum repair argon arc welding

Text / Xu Guoxing

In the production of steel drums, it is inevitable that some unqualified steel drums will be produced. Most of the unqualified steel drums are leaks. In order to reduce losses and reduce costs, companies often repair these leaking steel drums and sell them after they have been repaired. Many steel drum companies have used gas welding, electric welding, brazing or bonding to repair steel drums. The best way to repair them now is to use argon arc welding for repair welding. The argon arc welding not only has good quality of repair welding, but also has a small heat affected zone around the repaired weld, and the surface of the steel drum is not deformed, so it is widely used.

However, due to the different welding techniques, many welders do not have a good grasp of the operation skills of argon arc welding, which makes the welding quality difficult to guarantee.

Argon arc welding is an arc welding using argon or an argon-rich gas as a protective medium to burn an electric arc between the wire workpieces as a heat source. When argon gas or a mixed gas of argon and helium is used as a shielding gas, it is called melt-level inert gas shielded welding, referred to as MIG (Metal Inert Gas Welding) welding; using argon gas + oxygen, argon gas + carbon dioxide, or argon gas + When carbon dioxide + oxygen is used as a protective gas, it is called active gas shielded welding, referred to as MAG (Metal Active Gas Welding) welding.

I. Effect of argon arc welding droplet transfer on weld formation

The argon arc welding droplet transfer pattern can be divided into short circuit transition, jet transition, sub-jet transition, pulse transition, etc. The use of different processes depends mainly on the material, weldment size, and welding posture.

Figure 1 shows the double pulse transition on the left and the normal pulse transition on the right.

Short circuit transition

The argon arc welding droplet short circuit process is the same as the carbon dioxide arc welding droplet short circuit transition. It is also a usable droplet transfer method produced by using a thinner wire at low voltage and small current. The difference is MIG welding melting. The short-circuit transition is carried out at a lower voltage, and the transition process is stable and has less spatter, which is suitable for high-speed welding of thin plates or welding of tantalum welds. The utility model is characterized in that when welding with small current and low voltage, the droplet directly contacts the molten pool before leaving the tip of the welding wire, the arc instantaneously extinguishes the short circuit, and the electromagnetic contraction force generated by the droplet in the short-circuit current is effected by the surface tension of the liquid metal. The next transition into the molten pool. The arc in the form of a short circuit is stable, the splash is small, and the forming is good, but the penetration is shallow.

2. Jet transition

The argon arc welding droplet ejection transition is mainly used for horizontal butt joints and horizontal joints of medium and large thickness plates. The reason why the argon arc welding arc can produce a droplet ejection transition is that the arc shape is relatively extended. Argon arc welding generally uses a wire as the anode, and the wire is connected negatively or less frequently. There are two reasons for this. One is to make full use of the cleaning effect of the arc on the base metal, and the other is to make the droplets refine and form a smooth transition. At low currents, due to the small electromagnetic binding force, the droplets are mainly caused by the action of gravity, and the particles are larger than the diameter of the wire. The weld formed by the welding transition process is prone to defects such as poor fusion, insufficient penetration, and excessive height, so it is generally not used in actual welding. When the current is increased, the tip end of the electrode is sharpened, and the droplet is finely granulated. The form of the droplet transition is called "jet transition".

1) Droplet transition: The arc at the time of droplet transition is a bell-shaped shape. Aluminum and alloy argon arc welding and pulse welding of steel wire are often in the form of droplet transfer. It is easy to form defects such as infusibility.

2) Jet transition: The tip of the wire is cut into a pencil shape in the arc, and the droplets flow out from the front end and transition with very fine particles. The transition frequency can be up to 500 times per second. At this time, the strong plasma flow force and the impact force of the high-speed droplets generate a great digging effect in the middle of the molten pool, and the liquid metal in the middle of the molten pool is discharged to the two sides and the rear side, so that the arc directly heats the metal at the bottom of the molten pool. Then, in the middle of the molten pool, a molten pool depression like a finger is formed, which is usually called a finger-like penetration. Such a weld seam is liable to form pores, unmelted and the like at its root portion, and when a small amount of carbon dioxide, oxygen gas or helium gas is added to the argon, the finger penetration can be improved. In addition, when welding aluminum and aluminum alloys, weld wrinkles are prone to occur, which requires control of shielding gas and welding current to avoid.

Figure 2 Jet transition

3. Sub-jet transition

This is a form of transition between the short circuit transition and the droplet transition. The arc feature is a short arc length. This form of transition is mainly used for the welding of aluminum and aluminum alloys in flat and horizontal welds. The advantage is that the weld profile is very uniform with the penetration depth, which avoids the finger penetration.

4. Pulse transition

The adjustment of the pulse parameters at the flat welding position allows the droplet transfer to proceed in the desired manner. When performing space position weld welding, due to the large pulse current, the droplet transition has a stronger directivity, which facilitates the smooth transition of the droplet along the arc axis into the molten pool. Since the pulse average current is small, the volume of the formed pool is also smaller, and the pulse heating and the droplet transfer are intermittent, so the molten pool metal does not flow even in the vertical welding position, and the molten pool is maintained. State stability. For materials with higher heat sensitivity, the heat input to the base material or the welding line energy is reduced by the average current to reduce the overheating of the weld metal and the heat affected zone, so that the joint has good quality. The tendency to crack is reduced. In addition, the pulse action mode can prevent unidirectional crystallization of the molten pool and improve weld performance.

Second, the impact of current, voltage, welding speed

Welding current, arc voltage and welding speed are the main process parameters that determine the weld size:

Welding current

When other conditions are constant, as the welding current increases, the weld penetration, the melt width and the residual height increase. Among them, the melting depth is most obvious with the increase of current, and the melting width is only slightly increased. This is because:

1) As the current increases, the heat input and arc force on the workpiece increase, and the heat source position moves down, so the penetration increases. The penetration depth is proportional to the welding current: H = KmI, Km penetration coefficient, which is related to the wire diameter, current type and other factors.

Diameter (mm)

Current (A)

Voltage (V)

Speed â€‹â€‹(m/h)

Melting depth coefficient Km (mm/A)

1.2 to 2.4

210ï½ž550

24~42

40ï½ž120

1.5 to 1.8

2) As the current increases, the arc cross section increases, and the depth at which the arc sneaked into the workpiece also increases, which limits the range of arc spot movement. Therefore, the actual melting width is almost constant, so the residual height is increased.

2. Arc voltage

When other conditions are constant, the weld width increases significantly with increasing arc voltage, while the penetration depth and residual height decrease slightly. This is because as the arc length increases, the distribution radius r of the heat flow on the workpiece increases, and Qm decreases, so the melt width increases and the penetration depth decreases slightly. When the amount of melting of the wire is constant, the residual height is reduced due to an increase in the melt width.

3. Welding speed

When the welding speed is increased, the welding line energy (P/lm) is reduced, the melting width and the penetration depth are significantly reduced, and the residual height is also slightly reduced. In the high-speed arc high-speed welding, the strong arc force violently discharges the molten metal to the tail, and rapidly solidifies there, and the molten pool metal is not evenly distributed over the entire width of the weld, and it is easy to form a undercut. This phenomenon limits the increase in welding speed.

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