In addition to process factors, other welding process factors, such as groove size and gap size, inclination angle of electrode and workpiece, spatial position of joint, etc., can also affect weld formation and weld size.
1. Influence of welding current on weld formation
Under certain other conditions, with the increase of arc welding current, the penetration depth and residual height of the weld increase, and the penetration width increases slightly. The reasons for this are as follows:
1) As the welding current of arc welding increases, the arc force acting on the weldment increases, the heat input of the arc to the weldment increases, and the position of the heat source moves down, which is conducive to the conduction of heat to the depth of the molten pool and increases the penetration depth. The penetration depth is approximately proportional to the welding current, that is, the weld penetration depth H is approximately equal to Km×I. In the formula, Km is the penetration coefficient (the number of millimeters that the welding current increases by 100A to increase the weld penetration), which is related to the arc welding method, wire diameter, current type, etc. See Table 1-1.
2) The melting speed of the core or wire of arc welding is proportional to the welding current. As the welding current of arc welding increases, the melting speed of the welding wire increases, and the melting amount of the welding wire increases approximately proportionally, while the increase of the melting width is less, so the welding seam height increases.
3) When the welding current increases, the diameter of the arc column increases, but the depth of the arc into the workpiece increases, and the moving range of the arc spot is limited, so the increase of the melting width is small.
During gas-shielded metal arc welding, the welding current increases and the weld penetration increases. If the welding current is too large and the current density is too high, finger-shaped penetration is likely to occur, especially when welding aluminum.
2. Influence of arc voltage on weld formation
Under certain other conditions, when the arc voltage is increased, the arc power increases accordingly, and the heat input by the weldment increases. However, the increase of arc voltage is achieved by increasing the arc length. The increase of arc length increases the radius of the arc heat source, the heat dissipation of the arc increases, and the energy density of the input weldment decreases, so the penetration depth decreases slightly and the penetration depth increases. At the same time, since the welding current remains unchanged, the melting amount of the welding wire is basically unchanged, which reduces the weld height.
For various arc welding methods, Russia and Japan need to obtain suitable weld formation, that is, maintain a suitable weld formation coefficient φ, increase the arc voltage appropriately while increasing the welding current, and require an appropriate matching relationship between the arc voltage and the welding current. . This is most common in molten electrode arc welding.
3. The effect of welding speed on weld formation
Under certain other conditions, increasing the welding speed will lead to the reduction of welding heat input, thus reducing the weld width and penetration depth. Since the amount of wire metal deposition on a unit length of weld is inversely proportional to the welding speed, it also leads to a reduction in the weld height.
Welding speed is an important index to evaluate welding productivity. In order to improve welding productivity, welding speed should be increased. However, in order to ensure the weld size required by the structural design, the welding current and arc voltage should be correspondingly increased while increasing the welding speed. These three quantities are related to each other. At the same time, it should also be considered that when the welding current, arc voltage and welding speed are increased (that is, high-power welding arc, high welding speed welding), welding defects may occur in the process of forming the molten pool and during the solidification process of the molten pool, such as galling. Edges, cracks, etc., so there is a limit to increasing the welding speed.
4. Influence of welding current type and polarity and electrode size on weld formation
1). Type and polarity of welding current
The types of welding current are divided into DC and AC. Among them, DC arc welding is divided into constant DC and pulsed DC according to the presence or absence of current pulse; according to the polarity, it is divided into DC positive connection (welding part is connected to positive) and DC reverse connection (welding part is connected to negative). AC arc welding is divided into sine wave AC and square wave AC according to the different current waveforms. The type and polarity of welding current affect the amount of heat input by the arc into the weldment, so it can affect the weld formation, and also affect the droplet transfer process and the removal of the oxide film on the surface of the base metal.
When argon tungsten arc welding is used to weld steel, titanium and other metal materials, the penetration depth of the weld formed when the DC is connected is the largest, and the penetration is the smallest when the DC is reversed, and the AC is between the two. Since the penetration of the weld seam is the largest during the DC positive welding, and the tungsten electrode burning loss is the smallest, the DC positive welding should be used when welding steel, titanium and other metal materials. When TIG welding adopts pulsed DC welding, since the pulse parameters can be adjusted, the welding seam forming size can be controlled as required. When welding aluminum, magnesium and their alloys by argon tungsten arc welding, it is necessary to use the cathode cleaning effect of the arc to clean the oxide film on the surface of the base metal. It is better to use AC. Since the waveform parameters of square wave AC are adjustable, the welding effect is better. .
In fusion electrode arc welding, the weld penetration depth and width of DC reverse connection are greater than those of DC positive connection, and the penetration depth and width of AC welding are between the two. Therefore, in submerged arc welding, DC reverse connection is used to obtain a larger penetration depth; while in submerged arc surfacing welding, DC forward connection is used to reduce the penetration depth. In gas-shielded gas-shielded arc welding, it is widely used because the DC reverse connection not only has a large penetration depth, but also the welding arc and droplet transfer process are more stable than DC positive connection and AC, and have a cathode cleaning effect, so it is widely used. Communication is generally not used.
2). Influence of tungsten electrode tip shape, wire diameter and extension length
The angle and shape of the front end of the tungsten electrode have a great influence on the concentration of the arc and the arc pressure, and should be selected according to the size of the welding current and the thickness of the weldment. Generally, the more concentrated the arc and the greater the arc pressure, the greater the penetration depth and the corresponding reduction in the penetration width.
In gas metal arc welding, when the welding current is constant, the thinner the welding wire is, the more concentrated the arc heating is, the penetration depth increases, and the fusion width decreases. However, when selecting the wire diameter in the actual welding project, the current size and the shape of the molten pool should also be considered to avoid poor weld formation.
When the extension length of the welding wire of MIGAW arc welding increases, the resistance heat generated by the welding current through the elongated part of the welding wire increases, so that the melting speed of the welding wire increases, so the residual height of the welding seam increases, while the penetration depth decreases. Due to the relatively large resistivity of the steel wire, the influence of the wire extension length on the weld formation is obvious in the welding of steel and thin wires. The resistivity of aluminum welding wire is relatively small, and its influence is not large. Although increasing the extension length of the welding wire can improve the melting coefficient of the welding wire, there is a permissible range of variation in the extension length of the welding wire considering the stability of the welding wire melting and the welding seam formation.
5. The influence of other process factors on the welding seam forming factors
In addition to the above-mentioned process factors, other welding process factors, such as groove size and gap size, inclination angle of electrode and workpiece, and spatial position of joint, can also affect the weld formation and weld size.
1). Groove and gap
When welding butt joints by arc welding, it is usually determined whether to reserve a gap, the size of the gap and the form of the groove according to the thickness of the welded plate. Under certain other conditions, the larger the size of the groove or the gap, the smaller the residual height of the welded seam, which is equivalent to the decrease of the position of the welding seam, and the fusion ratio is reduced at this time. Therefore, the gap or bevel can be used to control the size of the overhang and adjust the fusion ratio. Compared with beveling with a gap and without a gap, the heat dissipation conditions of the two are somewhat different. Generally speaking, the crystallization conditions of the bevel are more favorable.
2). Electrode (welding wire) inclination
During arc welding, according to the relationship between the electrode tilt direction and the welding direction, it can be divided into two types: electrode forward tilt and electrode backward tilt. When the welding wire is tilted, the arc axis is also tilted accordingly. When the welding wire is tilted forward, the effect of the arc force on the backward discharge of the molten pool metal is weakened, the liquid metal layer at the bottom of the molten pool becomes thicker, the penetration depth decreases, the depth of the arc into the weldment decreases, the moving range of the arc spot expands, and the melting width decreases. increase, the residual height decreases. The smaller the angle α of the wire forward inclination is, the more obvious the effect is. When the wire is tilted back, the opposite is true. In electrode arc welding, the electrode back-tilt method is mostly used, and the inclination angle α is more suitable between 65° and 80°.
3). Welding angle
Weldment inclination is often encountered in actual production, which can be divided into uphill welding and downhill welding. At this time, the molten pool metal tends to flow down the slope under the action of gravity. When uphill welding, gravity helps the molten pool metal to discharge to the tail of the molten pool, so the penetration depth is large, the melting width is narrow, and the excess height is large. When the upslope angle α is 6°～12°, the excess height is too large, and undercuts are easy to occur on both sides. During downhill welding, this effect prevents the molten pool metal from being discharged to the tail of the molten pool, and the arc cannot deeply heat the metal at the bottom of the molten pool. If the inclination angle of the weldment is too large, it will lead to insufficient penetration and overflow of liquid metal in the molten pool.
4). Weldment material and thickness
The weld penetration is related to the welding current, as well as the thermal conductivity and volumetric heat capacity of the material. The better the thermal conductivity of the material and the larger the volumetric heat capacity, the more heat required to melt the metal per unit volume and raise the same temperature. Therefore, under certain conditions such as welding current, the penetration depth and penetration width are decrease. The greater the density of the material or the viscosity of the liquid, the more difficult it is for the arc to dislodge the metal in the liquid molten pool, and the shallower the penetration. The thickness of the weldment affects the heat conduction inside the weldment. When other conditions are the same, the thickness of the weldment increases, the heat dissipation increases, and the fusion width and penetration depth decrease.
5). Flux, electrode coating and shielding gas
The composition of the flux or electrode coating is different, resulting in different arc voltage drop and arc column potential gradient, which will inevitably affect the weld formation. When the flux density is small, the particle size is large or the stacking height is small, the pressure around the arc is low, the arc column expands, and the arc spot moving range is large, so the penetration depth is small, the melting width is large, and the residual height is small. When high-power arc welding is used to weld thick parts, the use of pumice-like flux can reduce the arc pressure, reduce the penetration depth, and increase the fusion width. In addition, the welding slag should have a suitable viscosity and melting temperature. If the viscosity is too high or the melting temperature is too high, the slag will be poorly ventilated, and it is easy to form many pressure pits on the surface of the weld, and the surface of the weld will deteriorate.
The composition of the shielding gas (such as Ar, He, N2, CO2) for arc welding is different, and its physical properties such as thermal conductivity are different, which makes the arc pole pressure drop and arc column potential gradient, arc column conductive cross section, plasma flow force. , specific heat flow distribution, etc., which all affect the formation of the weld.
In short, there are many factors affecting weld formation. In order to obtain a good weld formation, it is necessary to select according to the material and thickness of the weldment, the spatial position of the weld, the joint form, and the requirements of the working conditions on the joint performance and weld size. Suitable welding methods and welding conditions are used for welding, and the most important thing is the attitude of the welder towards welding! Otherwise, the weld formation and its performance may not meet the requirements, and even various welding defects may appear.
