KR100622747B1 - Method for suppressing brittle fracture along the friction stir welded zone for automotive body application - Google Patents

Abstract

The present invention relates to a method for preventing brittleness of a welded portion during friction stir welding of a vehicle body, and by applying magnesium to the surface of the shoulder part and the fin part of a welding device rotating body mainly made of tool steel by physical vapor deposition (PVD) Elongation and fatigue resistance of the welded portion of the dissimilar metal sheet, which is an aluminum-magnesium alloy, by suppressing the constitutional Liquation in which the coated magnesium is deformed together with the base metal to form an intermetallic compound during welding (FSW). The present invention relates to a method for preventing brittleness of a welded portion during friction stir welding of an automobile body capable of improving quality by improving the back and the like.

Automobile, aluminum alloy, magnesium alloy, friction stir welding, brittle

Description

Method for suppressing brittle fracture along the friction stir welded zone for automotive body application}

1 is a view showing a magnesium layer coated on the rotating body of the welding apparatus according to the present invention,

Figure 2 is a state diagram of the aluminum-magnesium base material according to the present invention.

<Explanation of symbols for the main parts of the drawings>

10: shoulder portion 11: pin portion

The present invention relates to a method for preventing brittleness of a welded portion during friction stir welding of a vehicle body, and by applying magnesium to the surface of the shoulder part and the fin part of a welding device rotating body mainly made of tool steel by physical vapor deposition (PVD) The present invention relates to a method of preventing brittleness of a welded portion during friction stir welding of a vehicle body that can suppress compositional supercooling in which a coated magnesium is deformed together with a base material to form an intermetallic compound during welding.

In general, the automobile industry is a typical assembly manufacturing industry of a complex nature of assembling about 20,000 parts that go through different production processes using various types of materials, such as iron-based alloys and non-ferrous alloys to complete the automobile.

In the automotive industry, in order to satisfy safety and environmental legislation, automakers have developed and applied various production technologies to increase body rigidity and improve fuel efficiency. In addition, various materials such as general steel, high tensile strength steel, steel, and non-ferrous steel, which require omnidirectional characteristics resistant to impact, have been applied to the automotive industry.

In addition, in 1927, the fuel efficiency of 1 liter / 100 km has been improved by reducing the weight of 100 Kg in 1927. One of the easiest ways to reduce the weight of automobiles is to reduce the weight of the body parts.

In this way, it is because the burden from the part that does not greatly affect the driver's safety is light and effective for the designer or the producer.

In order to reduce the weight of the outer shell of the body, the most popular material is AA6XXX aluminum-magnesium-silicon alloy sheet.

Since the alloy plate has bake hardening, the strength is improved by 20% or more after the coating process, and thus the application amount is gradually increased because it has an advantage in securing dent resistance and outer plate rigidity.

However, when the outer plate of the aluminum alloy plate is applied, a lot of bonding failure occurs between the plate, the conventional steel-aluminum plate is used for the self-piercing rivet joining method (excellent bonding strength and fatigue resistance) is used. However, there were disadvantages in terms of bonding cost and maintenance.

Recently, friction stir welding (FSW) has been in the spotlight as a method of joining dissimilar metals.

This method was developed in 1991 by the Welding Institute (TWI) in the United Kingdom, and uses the friction heat source of the rotating body and the specimen to perform the joining with lower indentation heat than the conventional melt joint. It is a bonding method that emphasizes the same advantages.

The friction stir welding is a solid state bonding process. After fixing the bonding base material, the mechanical heat-affecting portion is generated around the fin part inserted into the rotating body by generating frictional heat by the relative motion of the non-consumable rotating body having a hard material and the bonding base material. Will be created.

In addition, a portion where the fin portion is heated along the joining line is extruded from the front of the fin portion to the rear and a solid state joint is formed by frictional heat and mechanical combination.

Such friction stir welding is a joining with high quality and economical efficiency.

First, it is possible to prevent cracks (solidification cracks) that may occur in the melt joint, and there is almost no deformation, so the mechanical properties are excellent.

Second, as a joining method that does not require filler metal, bonding is performed by friction with the specimen.

Third, as an environmentally friendly method, no smoke is generated during bonding, and no harmful rays of infrared rays and ultraviolet rays are generated.

According to the above main features, it is easy to apply to materials that are difficult to melt welding, such as aluminum alloy, and is expected to be an effective method for joining these materials in the future.

In the case of the friction stir welding, defects such as solidification cracks, pores, and oxidation, which are problematic in conventional melt welding, can be minimized, and a variety of welded designs can be secured due to a mechanism different from the conventional friction welding method. It has the advantage of ensuring superior quality and economic efficiency than welding method.

By the way, the friction stir welding should be caused by plastic deformation in a state in which the base metal is not melted due to low indentation heat. There is a problem of lowering mechanical properties through liver compound formation, process tissue formation, and the like.

Accordingly, the present invention has been invented to solve the above problems, when the friction stir welding aluminum-magnesium alloy plate, the shoulder part and the pin part of the rotating body of the welding device mainly made of tool steel by using a physical vapor deposition method After the coating is carried out by welding, the coated magnesium is introduced into the welded portion in the solid state through the rotation of the rotating body to suppress the compositional supercooling, thereby ultimately forming the intermetallic compound causing brittleness of the welded portion. An object of the present invention is to provide a method for preventing brittleness of a welded portion during friction stir welding of a vehicle body that can prevent the occurrence of friction.

Hereinafter, the features of the present invention for achieving the above object are as follows.

The method of preventing brittleness of the welded portion during friction stir welding of a vehicle body according to the present invention comprises: coating magnesium on the surface of a rotating body of a welding device including a shoulder portion and a pin portion during friction stir welding (FSW) of an aluminum-magnesium alloy plate. It is characterized by consisting of a step of performing a welding step.

In particular, the coating process is characterized in that it is made through physical vapor deposition (PVD).

In addition, the magnesium has a thickness of 20㎛ or more characterized in that the coating treatment.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

1 is a view showing a magnesium layer coated on the rotating body of the welding apparatus according to the present invention, Figure 2 is a state diagram of the aluminum-magnesium base material according to the present invention.

According to the present invention, friction stir welding of AA6XXX-based aluminum alloy sheet (aluminum-magnesium-silicon alloy) and AZ31 magnesium alloy sheet (magnesium 2-3 wt%, aluminum 1 wt%, zinc alloy) generally used for automobile In order to suppress the formation of the Al ₁₂ Mg ₁₇ intermetallic compound of the weld, magnesium is coated on the surface of the shoulder 10 and the fin 11 of the welding apparatus rotating body by physical vapor deposition.

As shown in FIG. 1, when friction stir welding by coating magnesium using physical vapor deposition, the coated magnesium is introduced during plastic deformation of the welding base material to adjust the composition of the fluidized bed, thereby forming a constitutional liquation. ) Will be suppressed.

Figure 2 is a state diagram of the aluminum-magnesium base material according to the present invention, as shown, the portion shown in dotted line is the rotational speed of the rotor is the operating condition of the friction stir welding optimized to the composition section of the AZ31 magnesium alloy sheet And at a welding speed of 1.5 mm / s, no melting occurs at 773 K (500 ° C.), which is a riseable temperature.

However, plastic flow is generated by the rotating body, and the aluminum alloy sheet and the magnesium alloy sheet are subjected to the whirlwind plastic deformation, and the solid diffusion of the aluminum and magnesium atoms occurs at 773 K.

In this case, the composition of the diffusion layer is 60 to 70% by weight of aluminum, and magnesium, indicated by the "melting section" in FIG.

In this composition section, even if the temperature is not the temperature at which the base metal of the aluminum-magnesium alloy sheet is to be melted, this portion is melted.

This is called Constitutional Liquation.

This compositional supercooling causes the microstructures in the solidification process after melting of the welded part to change into a magnesium solid solution and an Al ₁₂ Mg ₁₇ intermetallic compound.

When the compositional supercooling does not occur, the aluminum solid solution-magnesium solid solution shows a simple structure and ductility is secured, but when the microstructure is changed through the compositional supercooling, the weld part is impacted by the brittle Al ₁₂ Mg ₁₇ intermetallic compound. And because it becomes vulnerable to deformation, the present invention has an object to suppress it.

In order to realize this, in order to prevent the compositional subcooling, the welding temperature should not be higher than 773 K, and solid diffusion should not occur.

This is difficult to control the welding temperature in reality, not only can not control the locally rising temperature, but also because it is a general phenomenon that the solid diffusion due to plastic deformation is difficult to control this.

To solve this problem, in order to suppress the formation of intermetallic compounds during compositional supercooling, the coated magnesium is introduced into the surface of the rotating body of the welding apparatus during friction stir welding. As shown in the state diagram of FIG. 2, L (liquid) + The composition is changed to magnesium or magnesium section.

In this case, even if the temperature rises, the compositional subcooling does not occur, and the solid phase of the base material is maintained.

Hereinafter, the present invention will be described in more detail with reference to preferred embodiments, but the present invention is not limited by the following examples.

Example

0.46 to 0.7 weight percent aluminum, 0.8 to 0.15 weight percent magnesium, 0.01 to 0.11 weight percent silicon, copper and other impurities, AA6022 alloy and AZ31 (2 to 3 weight percent magnesium) After butt welding an aluminum 1% by weight, zinc) alloy, the weld was subjected to a 90 degree bending test to evaluate whether a crack occurred.

Friction stir welding was performed with the rotational speed of 2450rpm and the welding speed of 1.5mm / s optimized for the joining of aluminum-magnesium alloy plate, and the welding specimen thickness was 1mm which is known as the limit of friction stir welding.

The rotating body used tool steel + magnesium coating (PVD) as an example and tool steel as a comparative example.

Table 1 shows the brittleness of the weld to the coating of the welding device rotor according to the Examples and Comparative Examples.

The fraction of Al ₁₂ Mg ₁₇ intermetallic compound was measured by an optical microscope and an image analyzer at ± 5 mm (total length 10 mm) at the center of the weld.

Through the above embodiment, by suppressing the formation of the Al ₁₂ Mg ₁₇ intermetallic compound in the welded part to reduce the brittleness of the welded part, it is possible to prevent the occurrence of cracks during the 90 degree bending test.

In the case of welding through a comparative example, it was found that the fraction of the intermetallic compound reached 52% and the brittleness was strong.

When the thickness of the magnesium coating layer is less than 20㎛ has the effect of lowering the fraction (11%) of the intermetallic compound, but the desired effect was not obtained due to the limit of the amount introduced.

As described above, according to the method for preventing brittleness of the welded portion during friction stir welding of the automobile body according to the present invention, when joining the AA6XXX aluminum-magnesium alloy sheet for automobiles, 20 μm or more of magnesium is PVD on the surface of the welding device rotor. By friction stir welding, the fraction of Al ₁₂ Mg ₁₇ intermetallic compound in the weld zone can be lowered to 9 or less, thereby reducing the brittleness, thereby improving the weld quality of the dissimilar metal sheet of aluminum-magnesium alloy. Has an effect of improving the elongation, fatigue resistance and the like.

Claims (3)

In the friction stir welding of aluminum-magnesium alloy plate (FSW), magnesium is coated on the surface of the rotating body of the welding apparatus including the shoulder part and the pin part by physical vapor deposition (PVD) to perform a welding process. Method for preventing brittleness of the welded part during friction stir welding of the vehicle body. delete The method according to claim 1, The magnesium is a brittle prevention method of the welded portion during friction stir welding of the vehicle body, characterized in that the coating is coated with a thickness of 20㎛ or more.

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