KR100571522B1 - Solid state joining method of fine grain metal plate by surface moving friction welding method - Google Patents

Abstract

The present invention relates to a surface moving friction welding method for joining a thin plate-like welding member with frictional heat, and more particularly, to a surface moving friction welding method capable of joining a welding member having a fine grain structure without changing its size. will be.

The present invention provides a surface friction friction welding method for joining the micro-grain metal sheet while maintaining or reducing the size of the crystal grains by utilizing the strong plastic deformation generated between the probe and the joining member at the surface portion, and also thick micro-grain plate Provides a surface transfer friction welding method that once again performs welding on the opposite side of the primary joint surface during welding, and also double-pass welding by installing a round bar probe symmetrically when welding thick fine plate material. Provide friction welding method.

According to the present invention, when the butt joints of the fine grain metal sheets are bonded to each other by using the surface moving friction welding method, the size of the fine grains can be maintained as it is, or by making them smaller.

Surface moving friction welding, grain size, fine grain, plate

Description

Method for solid phase welding the metal plates having ultrafine grains by surface friction welding process

1 is a schematic view of a welding apparatus and process of the surface movement friction welding method,

2 is a process schematic diagram of double pass welding;

3 is a process schematic diagram of simultaneous double pass welding;

Figure 4 is a cross-sectional photograph and grain size of each part after the surface movement friction welding of 1050 aluminum fine grain plate of 1.0mm thickness,

5 is a cross-sectional photograph of the 1050 aluminum fine grain plate having a thickness of 1.0 mm and the orientation difference distribution between the grains according to the site after friction welding;

Figure 6 is a cross-sectional photograph and grain size after the TIG welding of 1050 aluminum fine grain plate of 1.0mm thickness.

Explanation of symbols on the main parts of the drawings

1, 2: joining members 3, 3 ', 7, 7': round bar probe

4 welding part 6 plastic area

The present invention relates to a surface moving friction welding method for joining a thin plate-like welding member with frictional heat, and more particularly, to a surface moving friction welding method capable of joining a welding member having a fine grain structure without changing its size. will be.

The friction welding method is a welding method in which an appropriate amount of heat is generated by controlling friction using a principle that heat is generated by friction for a member to be welded, and plastic plastic flow is generated in an area adjacent to the friction part to be joined. This friction welding method has been researched and developed for decades and has the advantage of solid phase welding compared to conventional melt welding. However, one welding member must be axisymmetric and used for structures requiring continuous welding in a specific direction. It also has the drawback of not being able to.

As a method of improving the conventional simple friction welding method, friction stir welding (WO93 / 10935, WO95 / 26254) introducing a third hard probe and a probe pin has been developed. The principle of this friction stir welding is to insert the probe pin made of a hard rod-shaped hard material with a special screw-shaped protrusion along the butt face of the joining member joint at high speed, and then the friction between the probe pin and the joining member. Heat is generated. By the frictional heat, the surrounding material is thermally softened and mixed in a forced plastic flow by the rotation of the probe pin to form a joint. However, in friction stir welding, it is possible to apply only the thickness of the joining member to at least 1.2 mm or more because of the existence of the probe pin, and the material to fill the empty space caused by the insertion of the probe pin at the welding end is known. There is a disadvantage that the key hole is caused because it no longer exists in the.

Unlike the friction welding described above, a surface-moving friction welding method (Korean Patent Application No. 10-2003-68113), which enables thin sheet welding of 1.2 mm or less and enables continuous welding without leaving a welding defect at the welding end, is developed by the present inventors. It has been.

On the other hand, in recent years, many techniques have been developed for miniaturizing grains for the production of metal sheets having excellent strength and ductility without changing alloying components or dimensions. However, in the usual melt welding method, the metal at the junction portion is melted and solidified once, thereby making it impossible to maintain the fine grain structure of the material. Moreover, due to the formation of coagulation tissue, the mechanical properties are greatly deteriorated. Therefore, in order to utilize the refined metal sheet material in a real product, it is essential to maintain the size of the refined crystal grains intact or to make the size of the fine metal sheet material bonded without the microstructure loss of the material.

The present invention provides a surface-moving friction welding method in the butt joint to a fine metal sheet having a fine grain in order to achieve the above problems, the grain size of the joining portion at the time of joining to maintain the grain size of the metal sheet Its purpose is to.

The present invention utilizes a strong plastic deformation generated between the probe and the bonding member in the surface portion in order to achieve the above-described technical problem surface transfer friction welding method for joining the fine grain metal sheet material while maintaining or reducing the size of the crystal grains as it is; to provide.

In addition, the present invention provides a surface movement friction welding method of performing one welding again on the opposite side of the primary bonding surface when welding the thick fine grain plate material.

In addition, the present invention provides a surface movement friction welding method for performing double pass welding by symmetrically installing the probe of the round bar when welding the thick fine plate material.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail with reference to drawings.

1 is a view showing a schematic diagram of a friction welding method according to the present invention.

In the butt welding of two plate-shaped joining members 1 and 2 having a fine grain structure, the annular probe 3 coupled to the rotational power source rotates at a high pressure at a constant pressure to generate proper frictional heat. As the joint request line 5 is continuously passed through the horizontal motion, the weld part 4 having a width of approximately the probe diameter is formed. The plastic region 6 generated due to the rotation of the round bar-shaped probe 3 is softened due to frictional heat generated directly under the probe 3, that is, at the surface of the joining members 1 and 2, and due to the processing heat generated by plastic deformation. Bonded due to extreme plastic flow. When applying this method, the size of the grains of the plastic region 6 is similar or smaller depending on the size of the grains of the joining members 1 and 2 according to the rotational speed and the linear movement speed of the round bar probe. The shape of the crystal grains at the side part is schematically shown in.

       The extreme plastic flows caused by the bonding process can lead to the formation of dislocation 쎌 structures or continuous or discontinuous dynamic recrystallization, thus ensuring the connection of fine grains with high grain boundaries. In this case, plastic deformation at low temperature is known to be advantageous to obtain microcrystalline grains. However, too much probe rotation speed or too large probe radius during bonding will cause excessive plastic deformation and increase the temperature of the junction to an area well above the recrystallization temperature of the material. In this case, the grains generated in the joining member are grain grown and can no longer maintain a fine state. On the other hand, it is impossible to secure the amount of temperature rise or plastic deformation required for bonding at a low rotation speed of the probe below the critical size or too small a probe radius. Accordingly, in the present invention, in butt bonding to a fine metal sheet having fine grains, the present invention seeks to provide a surface-moving friction welding method in which the grain size of the bonding portion can be bonded while maintaining the grain size of the metal sheet.

The junction temperature required for solid-state bonding should be at least 70% of the melting temperature (T _m , unit: absolute temperature) of the material, with reference to the recrystallization temperature of the material to be joined. On the other hand, in order for the bonded material to maintain fine grains, it is difficult for the temperature of the material to exceed 90% of the melting temperature. Therefore, the temperature range of the joint considering the bonding conditions of the material and the retention conditions of the fine grains is expressed by Equation 1.

[Equation 1]

On the other hand, the relationship between the calorific value of heat generated from the material during the joining in the steady state and the amount of heat escaping to the outside is shown in Equation 2.

[Equation 2]

는 소성일이 열로 변환되는 정도를 표현하는 계수로 대개 0.9에서 1사이의 값을 갖는다.

는 재료의 전단유동응력으로 온도와 전단변형율속도의 함수이다.

는 접합부에 야기되는 평균 전단변형율 속도이며 프로브의 회전속도와 프로브의 직경에 의존한다. D와 t는 각각 프로브의 직경과 접합부재의 두께이다. k와 h는 접합부재의 열전도도와 프로브와 접합부재 사이의 열전달 계수이며 온도의 함수이다. T_e는 주위 온도이며, T와 T_e의 단위는 절대온도(K) 이다.

는 접합부 경계에서의 거리에 따른 온도구배이다. here,

Is a coefficient representing the degree to which the plastic work is converted into heat, and is usually between 0.9 and 1.

Is the shear flow stress of the material, a function of temperature and shear strain rate.

Is the average shear strain velocity at the joint and depends on the probe rotation speed and the probe diameter. D and t are the diameter of the probe and the thickness of the joining member, respectively. k and h are the thermal conductivity of the joint and the heat transfer coefficient between the probe and the joint and are a function of temperature. T _e is the ambient temperature, and the unit of T and T _e is the absolute temperature (K).

Is the temperature gradient over distance at the junction boundary.

The average shear strain rate at the joint is given by

[Equation 3]

Where S is the rotational speed of the probe. Solving equation 3 into equation 2 solves the temperature of the junction at steady state. The diameter and rotation speed of the probe should be selected so that the temperature of the joint thus obtained satisfies Equation 1, so that welding can be performed while maintaining the grain size of the grain having a small grain size.

In addition, in the present invention, the plastic flow due to the surface friction has to penetrate into the inside of the material, so welding of a thick plate is impossible in a single pass. In this case, double pass welding is performed once again on the opposite side of the primary joint surface. The process schematic diagram is shown in FIG.

In this case, the diameter of the probe 3 ′ may be changed according to the thickness of the plates 1 and 2 to be joined.

In another method of FIG. 3, two round bar probes 7 and 7 ′ are installed in a symmetrical manner, and a double pass welding is performed at the same time, thereby enabling the joining of thicker plates through simultaneous double pass welding. It is possible to miniaturize or maintain the size of the fine initial grains as it is.

Example

A 1.0 mm thick 1050 aluminum alloy sheet with a grain size of about 2.17 μm was welded using the surface-moving friction welding method proposed by the present inventors under conditions of a probe diameter of 5 mm, a rotation speed of 1600 rpm, and a welding speed of 100 mm / min. . For these welding conditions, the temperature of the joint is calculated using Equations 2 and 3 to obtain 803K (530 ^o C), which is 0.86 for 933K (660 ^o C), the melting temperature of a typical aluminum alloy. Since it corresponds to T _m , Equation 1 is satisfied. Therefore, in this welding condition, the joining can be performed while maintaining the grain size with a small grain size of the joint.

FIG. 4 is a cross-sectional photograph of a welded portion of the material and a grain size of the base material and the welded portion are observed by EBSD (Electron Back Scattered Diffraction) method. The shape of the plastic zone is the largest on the surface and decreases toward the inside. In the welding conditions, the grain size of the surface portion (b) with the highest plastic deformation was 1.1 µm, which was the smallest, and it was found that (c) was somewhat larger, but it was similar to that of the substrate (a) at about 2.04 µm. It could be confirmed that further refinement.

5 shows the distribution of the orientation difference of the grains in the welded portion. As shown in FIG. 5, it can be seen that there are many distributions of diagonal grain boundaries having sufficiently large azimuth differences of the grains in the welded portion, and that the amount of the diagonal grain boundaries is increased in the upper portion (b) of the weld portion rather than the base material portion (a). The weld center (c) also shows a diagonal grain boundary similar to that of the base material (a). Through this, it can be seen that stable grains can be obtained by using the surface moving friction welding method.

FIG. 6 shows the results of observing the grain size by performing the TIG welding, which is a conventional welding method, in joining the same aluminum alloy plate as the comparative example. Due to the temperature increase due to the large amount of heat input during welding, the grain size of the thermally-heat-directed portion (a) was increased to about 60㎛, and the grain size of the welded portion (c) and the boundary portion (b) was considerably coarse to about 25㎛. have. Through this, it can be seen that the method using the surface moving friction welding method proposed in the present invention can effectively bond the fine metal sheet material.

According to the present invention, when the butt joints of the fine grain metal sheets are bonded to each other by using the surface moving friction welding method, the size of the fine grains can be maintained as it is, or even smaller.

Claims (4)

Equation 2 below Equation 2:

(here,

Is a coefficient representing the degree to which the plastic work is converted into heat,

Is the shear flow stress of the material,

Is the average shear strain velocity at the joint, D is the diameter of the probe, t is the thickness of the joint, k is the thermal conductivity of the joint, h is the heat transfer coefficient between the probe and the joint, Te is the ambient temperature,

Is the temperature gradient according to the distance from the junction boundary.) Equation 3 below Equation 3:

Where S is the rotational speed of the probe Calculate the temperature (T) of the junction at steady state using the corresponding diameter (D) of the probe whose calculated temperature (T) satisfies 0.7Tm <T <0.9Tm of the melting temperature (Tm) of the molten member. And a surface friction friction welding method for joining the fine grain metal sheet material while maintaining or reducing the size of the crystal grains by utilizing the strong plastic deformation generated between the probe and the joining member at the surface portion by the rotation speed (S). delete The method according to claim 1, Surface-friction welding method in which a thick fine grain plate is welded once again on the opposite side of the primary joint surface. The method according to claim 1, Surface transfer friction welding method in which double-pass welding is performed by symmetrically installing the round bar probe up and down when welding thick fine grain material.

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