Cracks in Aluminum Alloy Welded Joints: Characteristics and Mechanism Analysis

Although aluminum and its alloys have been used to weld many important products, it is not without difficulties in actual welding production.

The main problems are: porosity in the weld, welding hot cracks, “equal strength” of joints, etc.

Due to the strong chemical activity of aluminum and its alloys, the surface is very easy to form oxide film, and most of them are refractory (for example, the melting point of Al2O3 is 2050 ℃, and the melting point of MgO is 2500 ℃).

In addition, aluminum and its alloys have strong thermal conductivity, which is easy to cause non fusion during welding.

As the density of oxide film is very close to that of aluminum, it is also easy to become inclusions in weld metal.

At the same time, the oxide film (especially the less dense oxide film with MgO) can absorb more water and is often one of the important reasons for weld porosity.

In addition, aluminum and its alloys have large linear expansion coefficient, strong thermal conductivity and are prone to warp deformation during welding.

These are also very difficult problems in welding production.

Next, the relatively serious cracks generated during the test are analyzed in depth.

Cracks in Aluminum Alloy Welded Joints: Characteristics and Mechanism Analysis 1

1. Cracks in aluminum alloy welded joints and their characteristics

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.

The shape and distribution characteristics of the cracks are very complex.

According to the positions where they occur, they can be divided into the following two types of cracks:

(1) Cracks in weld metal: longitudinal cracks, transverse cracks, crater cracks, hairlike or arc cracks, root cracks and microcracks (especially in multi-layer welding).

(2) Cracks in the heat affected zone: weld toe cracks, lamellar cracks and micro thermal cracks near the fusion line.

According to the temperature range of crack generation, it can be divided into hot cracks and cold cracks.

Hot cracks are generated at high temperature during welding, and are mainly caused by alloy element segregation on the grain boundary or the existence of low melting point materials.

According to the different materials of the welded metal, the shape, temperature range and main causes of the hot cracks are also different.

The hot cracks can be divided into three types: crystallization cracks, liquefaction cracks and polygonization cracks.

Crystallization cracks are mainly generated in the hot cracks.

It is in the process of weld crystallization, near the solidus, due to the shrinkage of the solidified metal, the residual liquid metal is insufficient and 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 caused by the shrinkage stress during solidification of grain boundaries heated to high temperature in the heat affected zone.

During the test, it was found that when the filler material surface was not cleaned sufficiently, there were still many inclusions and a small amount of pores in the weld after welding.

In the three group number test, since the welding filler material is a cast structure, and the inclusion is a high melting point material, 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, and they will become the factors that cause porosity during welding.

When the weld is under tensile stress, these inclusions and pores often become the key parts to induce microcracks.

Through further observation by microscope, it is found that there is an obvious trend of interaction between these inclusions and micro cracks induced by pores.

However, it is difficult to determine whether the harmful effect of inclusions here is mainly manifested as stress concentration source to induce cracks, or mainly manifested as brittle phase to induce cracks.

In addition, it is generally believed that the porosity in the aluminum magnesium alloy weld will not have a significant impact on the tensile strength of the weld metal.

However, in this study, it was found that there are microcracks induced by inclusions and porosity in the weld tensile samples at the same time.

Whether the phenomenon of micro cracks induced by pores is only a secondary associated phenomenon, or one of the main factors that cause a significant decline in the tensile strength of welds, remains to be further studied.

Cracks in Aluminum Alloy Welded Joints: Characteristics and Mechanism Analysis 2

2. Process of hot crack generation

At present, the theory of welding hot crack is considered to be more perfect by Prokhorov at home and abroad.

To sum up, the theory believes that the generation of crystal cracks mainly depends on the following three aspects:

The size of brittle temperature range;

The ductility of the alloy in this temperature range and the deformation rate of the metal in the brittle temperature range.

Generally, the size of the brittle temperature range and the ductility value in this range are called metallurgical factors that produce welding hot cracks, and the deformation rate of the metal in the brittle temperature range is called mechanical factors.

The welding process is the synthesis of a series of unbalanced technological processes.

This feature is essentially related to the metallurgical and mechanical factors of metal fracture of welded joints, such as the physical, chemical and organizational heterogeneity, slag and inclusions, gas elements and vacancies in supersaturated concentration of the products of the welding technological process and metallurgical process.

All these are metallurgical factors closely related to crack initiation and development.

In terms of mechanical factors, the specific temperature gradient and cooling rate of welding thermal cycle will make the welded joint in a complex stress-strain state under certain constraint conditions, thus providing necessary conditions for the initiation and development of cracks.

In the welding process, the comprehensive effect of metallurgical factors and mechanical factors will be attributed to two aspects, namely, strengthening metal connection or weakening metal connection.

If the strength connection is being established in the weld joint metal during cooling, the strain can be obedient under certain rigid constraint conditions.

When the weld and the metal near the seam can withstand the external constraint stress and internal residual stress, the crack is not easy to occur.

The metal crack sensitivity of the weld joint is low.

On the contrary, when the stress cannot be withstood, the strength connection in the metal is easy to be interrupted, and the crack will occur.

In this case, the crack sensitivity of the weld joint metal is high.

The weld joint metal is cooled to room temperature at a certain rate from the temperature of crystallization solidification.

Its crack sensitivity depends on the comparison of deformation capacity and applied strain, as well as the comparison of deformation resistance and applied stress.

However, in the cooling process, at different temperature stages, due to the different growth of intergranular strength and grain strength, the different distribution of deformation between and within grains, the different diffusion behavior induced by strain, the different conditions of stress concentration and the factors leading to metal embrittlement, the specific weak links of welded joints and the factors and degrees of its weakening are also different.

Metallurgical and mechanical factors that lead to cracks in the weld joint metal are closely related.

The stress gradient in 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 metallurgical factors composed of the metal’s thermoplastic change characteristics, thermal expansion and structural transformation.

To a large extent, it plays an important role in the stress – strain state of the welded joint metal.

In addition, with the decrease of temperature and the change of cooling speed, metallurgical factors and mechanical factors are also changing.

The strength of welded joint metal is affected differently in different temperature ranges.

For example, the large crystallization temperature range and low solidus temperature are more likely to cause stress concentration at the residual low melting liquid metal between grains, leading to cracks in the solid metal;

Similarly, with the temperature decreasing, if the shrinkage is large, especially under the condition of rapid cooling, cracks are easy to occur when the shrinkage strain rate is high and the stress strain state is harsh, etc.

In the later stage of solidification and crystallization of weld metal during aluminum alloy welding, eutectic is squeezed in the center of crystal intersection to form a so-called “liquid film”.

At this time, due to the large shrinkage during cooling, free shrinkage is not available to generate large tensile stress.

At this time, the liquid film forms a weak link, which may crack in the weak zone under the effect of tensile stress.

Cracks in Aluminum Alloy Welded Joints: Characteristics and Mechanism Analysis 3

3. Mechanism of hot cracks

In order to study the hot cracks that are most likely to occur during aluminum alloy welding, the crystallization of the welding pool during aluminum alloy welding is divided into three stages.

The first stage is the liquid-solid stage.

When the welding molten pool starts to crystallize from the high temperature cooling, only a small number of crystal nuclei exist.

With the decrease of temperature and the extension of cooling time, the crystal nucleus gradually grows up, and new crystal nuclei appear.

However, during this process, the liquid phase always occupies a large number, and there is no contact between adjacent grains, which does not hinder the free flow of the liquid aluminum alloy that has not been solidified.

In this case, even if there is tensile stress, the opened gap can be filled by the flowing aluminum alloy liquid metal in time, so the possibility of cracks in the liquid-solid phase is very small.

The second stage is the solid-liquid stage.

When the welding molten pool crystallization continues, the solid phase in the molten pool continues to increase, and the previously crystallized crystal nucleus continues to grow.

When the temperature drops to a certain value, the solidified aluminum alloy metal crystals contact each other and continuously roll together.

At this time, the flow of the liquid aluminum alloy is blocked, that is, the molten pool crystallization enters the solid-liquid stage.

In this case, due to the small amount of liquid aluminum alloy metal, the deformation of the crystal itself can be strongly developed, and the residual liquid phase between the crystals is not easy to flow.

The tiny gaps generated under the tensile stress can not be filled. As long as there is a little tensile stress, there is a possibility of cracks.

Therefore, this stage is called “brittle temperature zone”.

The third stage is the complete solidification stage.

When the weld formed after the molten pool metal is completely solidified is subjected to tensile stress, it will show good strength and plasticity, and the possibility of cracking at 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 resistance to crystal cracks and a smaller crack tendency.

In general, for metals with less impurities (including base metal and welding materials), the brittle temperature range is narrow, and the tensile stress acts in this range for a short time, so the total stress of the weld is relatively small, so the crack tendency generated during welding is small.

If there are many impurities in the weld, the range of brittle temperature is relatively wide, the action time of tensile stress in this range is relatively long, and the tendency to produce cracks is large.

4. Prevention measures for aluminum alloy welding cracks

According to the mechanism of hot cracks in aluminum alloy welding, it can be improved from two aspects of metallurgical factors and technological factors to reduce the probability of hot cracks in aluminum alloy welding.

In terms of metallurgical factors, in order to prevent intergranular hot cracks during welding, it is mainly through adjusting the weld alloy system or adding modifiers to the filler metal.

To adjust the focus of the weld alloy system, from the perspective of crack resistance, is to control an appropriate amount of fusible eutectic and narrow the crystallization temperature range.

As aluminum alloy is a typical eutectic alloy, the maximum crack tendency is just corresponding to the “maximum” solidification temperature range of the alloy.

The presence of a small amount of fusible eutectic always increases the solidification crack tendency.

Therefore, it is generally used to make the content of main alloy elements exceed the alloy component with the maximum crack tendency, so as to produce the “healing” effect.

However, as a modifier, trace elements such as Ti, Zr, V and B are added to the filler metal in an attempt to improve the plasticity and toughness by refining the grains, and preventing welding hot cracks.

The attempt started long ago and has achieved results.

Fig. 3 shows the crack resistance test results of Al-4.5% Mg welding wire with modifier under the condition of rigid lap fillet weld.

Zr added in the test is 0.15%, Ti+B is 0.1%. It can be seen that the addition of Ti and B at the same time can significantly improve the crack resistance.

The common feature of 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.).

This kind of small refractory particle can become the non spontaneous solidification crystal nucleus during the solidification of liquid metal, which can produce the effect of grain refining.

In terms of process factors, they are mainly welding specification, preheating, joint form and welding sequence.

These methods are all based on welding stress to solve welding cracks.

Welding process parameters affect the non-equilibrium of solidification process and solidification structure state, as well as the strain growth rate during solidification, thus affecting the generation of cracks.

The welding method with concentrated heat energy is conducive to the rapid welding process, and can prevent the formation of coarse columnar crystals with strong directivity, thus improving the crack resistance.

Using 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 welding speed promotes the increase of strain rate of welded joints and the tendency of hot cracking.

It can be seen that increasing the welding speed and welding current will increase the crack tendency.

During the assembly and welding of aluminum structure, the weld will not bear much stiffness.

In the process, measures such as sectional welding, preheating or appropriate reduction of welding speed can be taken.

By preheating, the relative expansion of the specimen can be reduced, and the welding stress can be reduced accordingly, which reduces the stress in the brittle temperature range;

Butt welding with groove and small gap shall be adopted as far as possible, and cross joint and improper positioning and welding sequence shall be avoided;

When welding is completed or interrupted, the crater shall be filled in time, and then the heat source shall be removed, otherwise crater cracks may be easily caused.

For the welded joints of 5000 series alloy multi-layer welding, microcracks often occur due to local intergranular melting, so it is necessary to control the welding heat input of the next layer of weld bead.

According to the test in this paper, the surface cleaning of base metal and filler material is also very important for aluminum alloy welding.

The inclusion of materials will become the source of cracks in the weld and the main reason for the decline of weld performance.

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