Although aluminum and its alloys have been used to weld into many important products, the actual welding production is not without difficulties. The main problems are: pores in the weld, welding hot cracks, and "equal strength" of joints. Due to the strong chemical activity of aluminum and its alloys, it is easy to form an oxide film on the surface, and most of them have refractory properties (for example, the melting point of Al2O3 is 2050 °C, and the melting point of MgO is 2500 °C). In addition, aluminum and its alloys have strong thermal conductivity. It is easy to cause non-fusion phenomenon during welding. Since the density of the oxide film is very close to that of aluminum, it is also easy to become inclusions in the weld metal. At the same time, the oxide film (especially the oxide film with the presence of MgO, which is not very dense) can absorb more moisture and often become one of the important reasons for weld pores.
In addition, aluminum and its alloys have a large coefficient of linear expansion and strong thermal conductivity, and are prone to warping deformation during welding. These are also quite difficult problems in welding production. In the following, an in-depth analysis of the relatively serious cracks generated during the test is carried out.
1. Cracks and their characteristics in aluminum alloy welded joints
In the process of aluminum alloy welding, due to the different types, properties and welding structures of materials, various cracks can appear in the welded joints, and the shape and distribution characteristics of the cracks are very complex. According to their generated parts, they can be divided into the following two types of cracks form:
(1) Cracks in weld metal: longitudinal cracks, transverse cracks, crater cracks, hair or arc cracks, root cracks and microcracks (especially in multi-layer welding).
(2) Cracks in the heat-affected zone: weld toe cracks, laminar cracks and microscopic thermal cracks near the fusion line. According to the temperature range of crack generation, it is divided into hot crack and cold crack. Hot crack is generated at high temperature during welding, which is mainly caused by the segregation of alloy elements on the grain boundary or the existence of low melting point substances.
Depending on the material of the metal to be welded, the shape, temperature range and main reasons for the occurrence of hot cracks are also different. Hot cracks can be divided into three categories: crystallization cracks, liquefaction cracks and polygonal cracks. Crystallization cracks are mainly produced in hot cracks. During the crystallization process of the weld, near the solidus line, due to the shrinkage of the solidified metal, the residual liquid metal cannot be filled in time.
Intergranular cracking occurs under the action of solidification shrinkage stress or external force, which mainly occurs in carbon steel, low-alloy steel welds and some aluminum alloys with more impurities; liquefaction cracks are heated in the heat-affected zone to Produced under the action of shrinkage stress during high temperature grain boundary solidification.
During the test, it was found that when the surface of the filler material was not sufficiently cleaned, there were still many inclusions and a small amount of pores in the weld after welding. In the three sets of tests, since the welding filler material is a cast structure, and the inclusions are high melting point substances, it will still exist in the weld after welding;
In addition, the casting structure is relatively sparse, and there are many holes, which are easy to absorb the components containing crystal water and oil quality, which will become the factors that generate pores during the welding process. When the weld is under tensile stress, these inclusions and pores often become the key sites for inducing microcracks.
Further observation by microscopy revealed that there was a clear tendency for these inclusions and pore-induced microcracks to intersect with each other. However, it is still difficult to judge whether the harmful effect of the inclusions is mainly manifested as a stress concentration source to induce cracks, or it is mainly manifested as a brittle phase to induce cracks.
In addition, it is generally believed that pores in aluminum-magnesium alloy welds do not have a significant impact on the tensile strength of the weld metal. phenomenon of cracks.
Whether the phenomenon of porosity-induced microcracks is only a secondary phenomenon or one of the main factors causing a substantial decrease in the tensile strength of welds remains to be further studied.
2. The process of hot crack generation
At present, Prokhorov's theory is considered to be more complete at home and abroad about the theory of welding hot cracks. Generally speaking, the theory believes that the occurrence of crystalline cracks mainly depends on the following three aspects: the size of the brittle temperature range; the ductility of the alloy in this temperature range and the deformation rate of the metal in the brittle temperature range.
Usually people call the size of the brittle temperature range and the ductility value in this temperature range as the metallurgical factor that produces hot welding cracks, and the deformation rate of the metal in the brittle temperature range is called the mechanical factor.
The welding process is the synthesis of a series of unbalanced process processes. This feature is essentially related to the metallurgical and mechanical factors of the metal fracture of the welded joint. For example, the products of the welding process and the metallurgical process are physical and chemical. and structural inhomogeneity, slag and inclusions, gas elements and vacancies in supersaturated concentrations, etc.
All of these are metallurgical factors closely related to the initiation and development of cracks. From the perspective of mechanical factors, the specific temperature gradient and cooling rate of the welding thermal cycle, under certain restraint conditions, will make the welded joint in a complex stress-strain state, thus providing necessary conditions for the initiation and development of cracks.
In the welding process, the combined effect of metallurgical factors and mechanical factors will be attributed to two aspects, that is, whether to strengthen the metal connection or weaken the metal connection. If a strength connection is being established in the metal of the welded joint during cooling, it can be compliantly strained under certain rigid restraint conditions, and when the weld and the metal near the weld can withstand the action of the applied restraint stress and the intrinsic residual stress, cracks are not easy to occur. , the metal crack susceptibility of welded joints is low,Conversely, when the stress cannot be tolerated, the strength connection in the metal is easily interrupted, and cracks will occur. In this case, the crack susceptibility of the welded joint metal is high. The welding joint metal starts from the temperature of crystallization and solidification, and cools to room temperature at a certain rate, and its crack sensitivity is determined by the comparison of deformation capacity and applied strain, and the comparison of deformation resistance and applied stress.
However, during the cooling process, at different temperature stages, due to the different growth of intergranular strength and grain strength, the distribution of deformation between grains and within grains, the diffusion behavior induced by strain is different, and the stress concentration is different. The conditions and the factors that cause metal embrittlement are different, the specific weak links of the welded joint and the factors and degrees of its weakening are also different.
The metallurgical factors and mechanical factors that cause cracks in the welded joint metal are closely related. The stress gradient in the mechanical factors is related to the temperature gradient determined by the thermal cycle characteristics, and the latter is closely related to the thermal conductivity of the metal, such as the thermoplastic change of the metal. Metallurgical factors such as characteristics, thermal expansion and microstructure transformation play an important role in the stress-strain state of the welded joint metal to a large extent.
In addition, as the temperature decreases and the cooling rate changes, the metallurgical and mechanical factors are also changing, and the strength of the welded joint metal is different in different temperature ranges. For example, if the crystallization temperature range is large, the solid The phase line temperature is low, and it is more likely to cause stress concentration at the low-melting liquid metal remaining between the grains, resulting in cracks in the solid phase metal;
Similarly, as the temperature decreases, if the shrinkage amount is large, especially under the condition of rapid cooling, when the shrinkage strain rate is high and the stress-strain state is more severe, cracks and so on are prone to occur.
In the later stage of the solidification and crystallization of the weld metal during welding of aluminum alloys, the low-melting eutectic is squeezed out at the center where the crystals meet, forming a so-called "liquid film". When the free shrinkage produces a large tensile stress, the liquid film forms a relatively weak link at this time, and under the action of the tensile stress, it may crack in the weak area to form a crack.
3. The mechanism of hot crack generation
In order to study the most likely time for hot cracks to occur when aluminum alloys are welded, the crystallization of the weld pool during aluminum alloy welding is divided into three stages.
The first stage is the liquid-solid stage. When the weld pool begins to crystallize from high temperature cooling, only a small number of crystal nuclei exist. With the decrease of temperature and the prolongation of cooling time, the crystal nucleus gradually grows up, and new crystal nucleus appears, but in this process, the liquid phase always occupies a large amount, and there is no contact between adjacent crystal grains. The free flow of the unsolidified liquid aluminum alloy does not form a hindrance.
In this case, even if there is tensile stress, the opened gap can be filled in time by the flowing aluminum alloy liquid metal, so the possibility of cracks in the liquid-solid stage is very small.
The second stage is the solid-liquid stage. When the crystallization of the welding molten pool continues, the solid phase in the molten pool continues to increase, and the previously crystallized nuclei continue to grow. When the temperature drops to a certain value, the solidified aluminum alloy metal The crystals are in contact with each other and are continuously rolled together. At this time, the flow of the liquid aluminum alloy is hindered, that is to say, the crystallization of the molten pool has entered the solid-liquid stage.
In this case, due to the lack of liquid aluminum alloy metal, the deformation of the crystal itself can be strongly developed, the liquid phase remaining between the crystals is not easy to flow, and the tiny gaps generated under the action of tensile stress cannot be filled, as long as there is a slight The presence of tensile stress has the potential to generate cracks. Therefore, this stage is called the "brittle temperature zone".
The third stage is the complete solidification stage. The weld formed after the molten pool metal is completely solidified will show good strength and plasticity when subjected to tensile stress. The possibility of cracks in this stage is relatively small. .
Therefore, when the temperature is higher or lower than the brittle temperature zone between a-b, the weld metal has a greater ability to resist crystallization cracks and a smaller crack tendency. In general, for metals with less impurities (including base metal and welding materials), due to the narrow brittle temperature range, the tensile stress acts in this range for a short time, so that the total strain of the weld is relatively small.
Therefore, the tendency of cracks generated during welding is less. If there are more impurities in the weld, the brittle temperature range is wider, the tensile stress in this range is longer, and the tendency to crack is larger.
4. Prevention measures for aluminum alloy welding cracks
According to the mechanism of hot cracks during welding of aluminum alloys, improvements can be made from two aspects of metallurgical factors and process factors to reduce the probability of hot cracks in aluminum alloys welding.
In terms of metallurgical factors, in order to prevent intergranular thermal cracks during welding, it is mainly by adjusting the welding seam metal system or adding a modifier to the filler metal. The focus of adjusting the welding suture system, from the perspective of crack resistance, is to control an appropriate amount of fusible eutectic and narrow the crystallization temperature range.
Since aluminum alloys are typical eutectic alloys, the maximum crack tendency corresponds to the "maximum" solidification temperature range of the alloy, and the presence of a small amount of eutectic always increases the solidification crack tendency. The element content exceeds the alloy composition where the crack tendency is greatest, so that a "healing" effect can occur.
As modifiers, trace elements such as Ti, Zr, V, and B were added to the filler metal in an attempt to improve the plasticity and toughness by refining the grains, and to prevent welding hot cracks. , and achieved results. Figure 3 shows the crack resistance test results of Al-4.5%Mg welding wire with modifier added under the condition of rigid lap fillet weld.
The Zr added in the test was 0.15%, and the Ti+B was 0.1%. It can be seen that adding Ti and B at the same time can significantly improve the crack resistance. The common feature of elements such as Ti, Zr, V, B and Ta is that they can form a series of peritectic reactions with aluminum to form refractory metal compounds (Al3Ti, Al3Zr, Al7V, AlB2, Al3Ta, etc.). Such small refractory particles can become non-spontaneous solidification nuclei when the liquid metal solidifies, thereby producing the effect of grain refinement.
In terms of process factors, mainly welding specifications, preheating, joint form and welding sequence, these methods are all based on welding stress to solve welding cracks. The welding process parameters affect the unbalance of the solidification process and the microstructure state of the solidification process, and also affect the strain growth rate during the solidification process, thus affecting the generation of cracks.
The welding method with concentrated heat energy is conducive to the rapid welding process, which can prevent the formation of coarse columnar crystals with strong directionality, thereby improving crack resistance. Using a small welding current and slowing down the welding speed can reduce the overheating of the molten pool and improve the crack resistance.
The increase of the welding speed promotes the increase of the strain rate of the welded joint, which increases the tendency of hot cracking. It can be seen that increasing the welding speed and welding current promotes the increase of the crack tendency. During the assembly and welding of the aluminum structure, the welding seam is not subjected to great rigidity, and measures such as segmented welding, preheating or appropriate reduction of the welding speed can be adopted in the process.
Through preheating, the relative expansion of the test piece can be made smaller, the welding stress can be reduced accordingly, and the stress in the brittle temperature range can be reduced; try to use butt welding with open grooves and small gaps, and avoid the use of cruciform joints and Improper positioning and welding sequence; when welding ends or is interrupted, the arc crater should be filled in time, and then the heat source should be removed, otherwise it will easily cause arc crater cracks. For the welded joints of 5000 series alloy multi-layer welding, microcracks are often generated due to local melting of the intergranular, so the heat input of the next layer of weld bead must be controlled.
According to the test in this paper, for the welding of aluminum alloy, the surface cleaning of the base metal and the filler material is also very important. The inclusion of material in the weld will become the source of cracks and the main reason for the decline of weld performance.
