Aluminum steel dissimilar metal connection can realize the lightweight of automobile parts, but the intermetallic compound cracks caused by aluminum steel welding seriously affect the performance of the joint.
This post introduces the main welding methods of laser welding of aluminum steel, and explains the research status in recent years.
Galvanized steel is coated with zinc layers of different thicknesses on the steel substrate. Zinc plating is divided into electroplating and hot plating.
The zinc layer not only has the function of physical shielding but also has the function of electrochemical protection on the steel substrate.
Because galvanized steel has good corrosion resistance, it is widely used in transportation, electric power, construction, heating facilities, instruments and furniture and other industries. Especially in the automobile industry, the application of ordinary galvanized steel, high-strength galvanized steel and ultra-high-strength galvanized steel in automobile improves the corrosion resistance and service life of the body and other parts.
However, due to the existence of zinc layer, the melting welding of galvanized steel plate is very difficult.
This is because the melting point of zinc is 420 ℃, the boiling point is 908 ℃, the melting point of base steel is 1300 ℃, and the boiling point is 2861 ℃. During the melting welding process, zinc will evaporate and oxidize seriously, forming defects such as pores, incomplete fusion and cracks.
Aluminum and aluminum alloys have light weight (aluminum density is 2.7g/cm3), high specific strength, good conductivity, heat conduction and corrosion resistance, and can maintain good mechanical properties at low temperatures.
Nowadays, energy, safety and environmental protection have become the theme of the times. With the gradual intensification of environmental pollution and energy crisis, the realization of vehicle lightweight has become the focus of attention all over the world.
Therefore, a lot of research has been carried out on aluminum/magnesium, aluminum/titanium, titanium/aluminum and aluminum/steel composite structures at home and abroad. Aluminum/steel composite structures make full use of the advantages of their respective materials to reduce weight and pollution, and are widely used in automobile, aviation, shipbuilding and other industries.
The application of aluminum steel connection in the automobile door panel is shown in the below figure.
However, due to the great difference in thermophysical properties between aluminum and steel, it is very difficult to connect them.
The melting point of iron is 1538 ℃, the melting point of aluminum is 660 ℃, the density of iron is 7870 kg / m3, and the density of aluminum is 2700 kg / m3.
During the melting welding of aluminum/steel, when the steel is completely melted, the aluminum will float on the steel and it is difficult to form a weld.
Moreover, the linear expansion coefficient of aluminum is almost twice that of iron, which will produce great thermal stress and easy to crack during welding.
According to the Fe-Al binary phase diagram, iron and aluminum can form not only solid solution, intermetallic compound but also eutectic.
The solid solubility limit of iron in aluminum is very small.
At 225～600℃, the solid solubility of Fe in Al is 0.01%～0.022%;
When the eutectic temperature is 652 ℃, the solubility of Fe in Al is 0.53%.
A series of studies show that Fe and Al can form intermetallic compounds such as Fe3Al, FeAl, FeAl2, Fe2Al5 and FeAl3.
These intermetallic compounds will form brittle structures in the weld. According to the thermodynamic analysis of Fe-Al, not all intermetallic compounds grow up in the process of welding heat.
According to the thermodynamic calculation based on the Gibbs free energy formed by the compounds, only Fe2Al5 and FeAl3 are the last stable brittle structures.
The existence of these brittle structures will reduce the mechanical properties of welded joints and easily cause cracks in welds.
Because Fe is almost insoluble in aluminum, it is difficult to obtain a good joint for aluminum/steel fusion welding.
In order to prevent brittle intermetallic compounds between steel and aluminum, it is necessary to coat the steel surface with metals compatible with aluminum and iron, such as Ni, Zn, Ag, Cu, etc.
In this way, the formation or growth of Fe-Al intermetallic compounds can be avoided or reduced.
Different coating metals have different effects on the steel/aluminum interface reaction, and the weldability of steel and aluminum is different. In addition to improving the weldability of steel/aluminum, the selection of filler materials and flux will also improve the weldability of steel/aluminum dissimilar metals and expand the application range of steel/aluminum in various industries.
Laser deep penetration welding
The main feature of laser deep penetration welding is deep penetration hole. High power density laser irradiates the metal surface, the surface metal reaches the boiling point, melts and vaporizes rapidly.
The air pressure generated by metal evaporation makes the surface concave and forms small holes.
The generated small hole increases the energy absorption of the laser.
The heat generated is the melting of the metal around the small hole, the liquid flow outside the small hole, the surface tension of the inner wall of the small hole and the continuous steam pressure in the inner cavity of the small hole.
The light beam continuously enters the small hole, and the material outside the small hole continuously melts and flows.
With the movement of the light beam, the small hole is always in dynamic stability.
The molten metal of the small hole and the surrounding hole wall move with the light beam, the molten metal continuously fills the small hole, and finally the molten metal cools to form a weld. When laser deep penetration welding is used to connect aluminum steel, it mostly adopts the joint form of steel upper and aluminum lower.
The laser acts on the steel surface, and the steel plate and aluminum plate melt to form a welding hole.
Gsierra et al. studied the laser deep penetration welding of the steel in the form of upper aluminum lap and lower lap.
The research results show that controlling the weld penetration below 500 μm can reduce the formation of Fe-Al intermetallic compounds and reduce the brittleness of the weld.
The weld penetration is controlled below 500 μm, and the joint strength can reach 250MPa.
It is found that a small amount of intermetallic compounds and the white molten band formed by aluminum-rich compounds appear in the weld.
When the penetration depth is less than 500 μm, the joint failure position is at the junction of weld and aluminum alloy.
With the increase of penetration depth, the joint failure position changes and the joint strength decreases significantly.
Kouadri David et al. studied the microstructure and properties of laser deep penetration welding and laser thermal conductivity welding of galvanized steel and aluminum alloy.
By controlling the weld penetration within 600μm, the strength of the laser deep penetration welded joint reaches 140MPa.
It is pointed out that the penetration along the steel thickness direction has an important influence on the joint strength.
Similarly, studies by katsyama et al. show that the penetration depth of steel in aluminum is the key factor affecting the performance of joints.
Toryamany et al. studied Nd: YAG pulsed laser welding of low carbon steel / 5754 aluminum alloy.
Laser deep penetration welding overlapping structure was used in the experiment.
The effects of laser power, pulse width and lap factor on the formation of metal compounds were studied.
The results show that the amount of intermetallic compounds increases with the increase of laser peak power (constant pulse energy), pulse width (constant peak power) and lap factor (constant pulse energy and peak power).
Jinyang et al. studied the relationship between penetration and weld formation in laser deep penetration welding of pure aluminum/stainless steel.
The results show that under the condition of large penetration (354μm), aluminum rich Fe-Al intermetallic compound with microcracks is formed at the interface of the aluminum/fusion zone. The joint strength is (27.2 ± 1.7) MPa.
There are three forms of fracture: shear brittle fracture, cleavage brittle fracture and mixed fracture.
When the penetration is small (108), the interface of the Al / Fe fusion zone is crack free intermetallic compound, the joint strength is (46.2 ± 1.9) MPa, and only one fracture form is cleavage brittle fracture along the weld.
The advantages of laser deep penetration welding are the high utilization rate of laser energy and high welding efficiency.
The keyhole has an important influence on the weld penetration and weld width, and the keyhole is the key factor in the process of laser deep penetration welding.
However, the plasma and deep penetration holes produced in the welding process make the welding process unstable and difficult to control.
Moreover, in the process of laser deep penetration welding, the gas is easy to enter the small hole, and the solidification is easy to produce pores.
Due to the steam pressure generated by the metal steam, the metal shrinkage is easy to produce surface depression during solidification, and the weld is not beautiful.
Laser heat conduction welding
When the laser irradiates the material surface, part of the laser is reflected and part is absorbed by the material, which converts the light energy into heat energy and melts it.
The heat on the material surface continues to transfer to the depth of the material in the form of heat conduction, and finally the two weldments are welded together.
Laser thermal conductivity welding is an important welding mode in laser welding, which is widely used in the welding of thin parts.
In laser thermal conduction welding, heat conduction plays a dominant role in the process of heat propagation, and radiation and convection account for only a small share in the process of heat propagation, which can be ignored.
In addition, the weld pool of laser thermal conductivity welding is very small, so the release of phase change latent heat in the weld pool and the influence of thermophysical parameters with temperature and state on the welding thermal process can be ignored.
MECO and others use laser heat conduction welding mode to connect 2mm thick steel plate and 6mm thick aluminum plate, and the overlapping form of steel on the top and aluminum on the bottom.
When laser irradiates the surface of the steel plate, the transmitted heat reaches the melting point of aluminum alloy to melt it.
The results of the thickness of the intermetallic compound were 4 ~ 20μm.
The maximum microhardness of Fe2Al5 is 1145HV.
Laser arc hybrid welding
Laser arc hybrid welding technology (the experimental schematic diagram is shown in Fig. 2) is a new and efficient welding method developed in the 1970s.
Laser high energy density can get deeper penetration, but the bridging of the gap is poor and the assembly accuracy is high.
The heating range of arc is wide, the wider weld can be obtained, but the bridging of arc to the gap is good.
The laser arc hybrid welding technology can obtain the weld with a wide top and large penetration by using their respective characteristics.
The plasma produced by the laser can stabilize the arc, so the hybrid welding increases the welding adaptability and welding efficiency.
Honggang donga et al. invented a method of connecting dissimilar metals with large spot laser and arc composite heat sources.
The patent aims at laser arc hybrid heat source welding with small spot, which can not be applied to the connection between steel and aluminum, steel and copper and other dissimilar metals.
In this method, the laser has an obvious stabilizing effect on the arc. The arc is used to melt the filler metal and low melting point base metal. The large spot laser can realize the accurate control of heat input.
The tensile property test of 5A02 aluminum alloy and galvanized steel joint obtained by this method shows that the failure position of the sample occurs in the welding heat affected zone on one side of the aluminum alloy base metal, rather than the brazing connection zone.
The joint strength can reach 153.1mpa. Qin and others adopt laser MIG composite welding of galvanized steel/aluminum alloy and AlSi5 welding wire.
The experimental results show that the brazing interface produces 2 ~ 4 μm, and the phase groups are FeAl2, Fe3Al5 and Fe4Al13.
The maximum tensile strength of the joint is 247.3MPa.
Wangshujun et al. also used laser MIG hybrid welding technology, and experimentally used three different solders: AlSi5, AlSi12 and AlMg5 to study the effects of intermetallic compounds and weld microstructure and forming after the addition of Si and Mg.
The results show that increasing Si content can refine the grains in the melting zone and increase the microhardness of the melting zone.
The microhardness of Al-Si solder in the melting zone is greater than that of Al-Mg solder.
For AlSi12, AlSi5 and AlMg5 solder, the average thickness of the intermetallic compound layer is 0.90, 1.49 and 2.64μm respectively.
It is concluded that Si can inhibit the diffusion of Fe and reduce the formation of intermetallic compounds.
XRD analysis of the intermediate layer shows that the phase groups of intermetallic compound layers corresponding to AlSi5 and AlSi12 are Fe2Al5, Fe4Al13 and Al0.5Fe3Si0.5.
The phase groups corresponding to AlMg5 are FeAl2, Fe2Al5 and Fe4Al13.
The corresponding joint strength of AlMg5, AlSi5 and AlSi12 solder is 178.9, 172.43 and 144MPa respectively.
It is concluded that the increase of Si content is unfavorable to the joint strength, while the addition of Mg is beneficial to the joint strength.
It is not explained how Si and Mg affect joint strength.
Thomy et al. studied the interaction between laser and plasma arc in the process of laser arc hybrid welding and developed coaxial laser arc hybrid welding head.
Brazing is to use solder with a lower melting point than the base metal.
By heating to a temperature higher than the melting point of the solder and lower than the melting point of the base metal, the solder melts, but the base metal does not melt.
The liquid solder is used to wet the base metal. Under the capillary action of the brazing gap, the liquid solder automatically fills the base metal gap and diffuses with the base metal to form a connection.
Fusion brazing has the characteristics of brazing and fusion welding, which is suitable for the connection between two dissimilar materials with large differences in physical properties.
Fusion brazing of steel and aluminum means that steel does not melt, aluminum and solder melt, brazing connection is on the steel/solder side, and fusion welding is on the solder/aluminum side.
The essence of fusion brazing of steel and aluminum is that molten aluminum and solder are combined with solid steel through interface reaction.
In this method, solder can be added or not added for welding.
Peyre et al. studied the connection between galvanized steel and aluminum alloy by laser fusion brazing without solder.
The results show that a 2 ~ 20μm thick interface layer is formed along the steel aluminum interface.
It is found that the layer is mainly Fe2Al5 phase, and its hardness is up to 1200HV, which causes cracks in the joint and significantly reduces the mechanical properties of the joint.
Although zinc evaporation produces pores, the zinc coating of 10μm is conducive to the wetting and spreading of aluminum on steel.
The tensile test shows that the evaporation of zinc can be restrained by coating flux on the surface of galvanized steel. For non galvanized steel, it shows low mechanical resistance.
Some scholars use solder in the process of laser brazing to change the chemical composition of the joint, so as to control the formation of Fe-Al intermetallic compounds and improve the performance of the joint.
Sierra et al. used 4047 (Al-12Si) solder in aluminum/steel laser fusion brazing connection to obtain a continuous joint without obvious macro defects, resulting in a thin Fe Al-Si intermetallic compound layer at the steel/weld interface.
Al-12Si solder was used to study the effect of Si on the growth of Fe-Al intermetallic compounds.
It is proposed that Si in solder can reduce the melting temperature of aluminum, affect the viscosity and surface tension of molten aluminum, and then affect the weld wetting angle and weld width.
Some scholars have also added mg, Cu and other elements to the solder to study the effect of solder alloying on the microstructure and properties of the weld.
Dharmendra et al. conducted fusion brazing test on DP600 galvanized steel and aa6016 aluminum alloy lap joint with continuous pulse Nd: YAG laser.
Zn Al welding wire containing 85% Zn and 15% Al was used in the test.
Different laser power, welding speed and wire feeding speed were used in the test.
It is found that the thickness of the reaction layer is 3 ~ 23μm.
Under the heat input of 60 ~ 110J/ mm, the tensile strength of the joint reaches 220MPa, and the fracture position is far away from the weld and close to the aluminum alloy side.
When the welding speed is 0.5 and 0.8m/min, the corresponding intermetallic compound thickness is 8 and 12 μmrespectively.
The tensile test results show that when the thickness of intermetallic compound is 8 ~ 12μm, the mechanical resistance is the largest, and when it is less than 8μm, the mechanical resistance increases with the increase of intermetallic layer;
When it is greater than 12μm, the mechanical resistance decreases.
It is explained that when the compound layer is thin, the crack initiates along the brittle intermetallic compound layer, and the fracture strength is very low.
When the compound layer is thick, the mechanical resistance is very low because the layer is very brittle relative to other areas.
Laukant et al. carried out the laser fusion brazing test of aluminum/steel with ZnAl2 solder.
The results showed that about 5μm FeAl intermetallic compound layers were produced, and the joint shear force was up to 9 KN.
Rajashekhara shabadi et al. also used Zn Al solder to conduct a laser fusion brazing test on AA6016 and low carbon galvanized steel.
The solder used in the test is ZnAl30. The intermetallic compound formed by the test results is mainly Fe2Al5Znx, which may contain ZnFeAl3, with a thickness of about 10μm.
Recently, some scholars have carried out laser fusion brazing experiments on aluminized high-strength steel and aluminum alloy.
For example, Windmann et al. conducted a laser fusion brazing test with AlSi3Mn solder.
It is found that Al8Fe2Si phase is formed at the AlSi3Mn / Mn22B5 interface.
The thickness of the intermetallic compound formed at Mn22B5 / AlSi3Mn interface is 2 ~ 7μm . The shear strength of the joint is 21 ~ 74MPa.
If the steel surface is preheated before welding, the joint strength will reach 210 ~ 230MPa.
From the recent research, both Al-Si solder and Zn-Al solder inevitably produce intermetallic compounds.
However, the growth sequence of Fe Al-Si intermetallic compounds and the identification of the Zn-Al solder reaction phase need to be solved.
Laser fusion welding is a promising technology for connecting steel/aluminum.
Because of its high welding efficiency, laser arc hybrid welding is mainly used for the welding of thicker plates.
Laser fusion brazing has great application prospects in automobile lightweight.
The thin aluminum/steel dissimilar metals were connected by the laser fusion brazing process, and Al-Si and Zn-Al solders were used.
However, there are still many problems to be solved in laser welding of galvanized steel/aluminum alloy for automobiles.
For example, due to the low absorptivity of the material to be welded to the laser energy, the plasma generated by laser welding has an impact on the stability of the welding process;
Fe-Al intermetallic compound brittle joint is produced in the process of aluminum steel connection;
Metallurgical compatibility of molten solder to aluminum alloy base metal and wettability to base metal steel;
Control and prevention of welding defects such as pores, cracks, incomplete fusion and slag inclusion.