KR20210097251A - A dissimilar material spot welding method - Google Patents

Abstract

The present invention discloses a dissimilar material spot welding method for aluminum alloy and stainless steel plate, which belongs to the technical field of metal material welding. According to the present invention, a spherical surface electrode with a diameter of 30 to 90 mm is used for the aluminum alloy side, and a flat end surface electrode with a diameter of 8 to 20 mm is used for the stainless steel plate side.

Description

Dissimilar material spot welding method {A DISSIMILAR MATERIAL SPOT WELDING METHOD}

The present invention relates to the technical field of metal material welding, in particular to a dissimilar material spot welding method for aluminum alloy and stainless steel sheet.

In an emergency situation of resource scarcity and environmental pollution, automobile weight reduction is one of the measures to achieve energy saving and emission reduction in the field of automobile industry. Methods for achieving automobile weight reduction mainly include the use of lightweight materials, optimized design of automobile structures, and application of advanced manufacturing techniques. Increasing the proportion of lightweight materials such as aluminum alloys and high-strength steel in automobile bodywork is the most direct and effective way to achieve automobile weight reduction. The main idea/technical approach to achieve vehicle weight reduction in the automotive industry is to construct a steel/aluminum composite body frame structure by adding aluminum alloy parts to the high-strength steel vehicle body structure. Therefore, dissimilar material connection (welding) between aluminum alloy and high-strength steel has become an urgent problem to be solved in the automobile manufacturing process.

The weldability of dissimilar materials between aluminum alloys and steels is poor due to the significant dissimilar material differences between aluminum and steel in thermophysical properties such as melting point, specific heat capacity, thermal conductivity, electrical resistance and coefficient of linear expansion, and the low solid solubility of Fe in Al. do. Conventional fusion welding (eg, TIG electric arc welding, MIG/MAG electric arc welding, CO ₂ gas shield welding, etc.) and resistance welding (eg, power frequency AC spot welding, etc.) are used. In this case, many defects such as brittle intermetallic compounds, shrinkage cavities, cracks, etc. mainly occur in welded joints, which seriously degrade the performance of welded joints, making it difficult to meet service requirements. Therefore, conducting an in-depth study on the dissimilar material weldability properties of aluminum alloy and high-strength steel to develop a practical and efficient welding technique is one of the main problems and technical difficulties for large-scale application of aluminum alloy and high-strength steel in the automotive industry. became one

If a high-quality and high-efficiency connection between aluminum alloy and high-strength steel can be achieved using the resistance spot welding method, which is one of the most widely used welding methods in the automobile manufacturing process, the usage of aluminum alloy and high-strength steel will be greatly improved. It will provide the technological foundation necessary to realize the weight reduction of automobiles.

It is an object of the present invention to provide a dissimilar material spot welding method for an aluminum alloy and a stainless steel sheet that can be used to perform dissimilar material welding between an aluminum alloy for automobiles and a stainless steel sheet and is simple to implement. This method can prevent splashing during welding, effectively control the growth behavior of intermetallic compounds at the interface of aluminum alloy/stainless steel sheet, thereby reducing the occurrence of brittle intermetallic compounds, and It can improve the performance of dissimilar material spot-welded joint of alloy and stainless steel sheet.

The technical solution of the present invention is as follows.

A dissimilar material spot welding method for an aluminum alloy and a stainless steel sheet includes performing welding to an aluminum alloy and a stainless steel sheet using a spot welding electrode to obtain a dissimilar material spot-welded joint of the aluminum alloy and the stainless steel sheet. The spot welding electrode has an optimized special end surface topography, a spherical surface electrode with a diameter of 30 to 90 mm is used for the aluminum alloy side, and a flat end surface electrode with a diameter of 8 to 20 mm is used for the stainless steel plate side. The aluminum alloy is a 6000 series aluminum alloy sheet, and the stainless steel sheet is a hot-galvanized high-strength IF steel sheet. The spot welding electrode is formed of a copper-chromium (CuCr) alloy. The spherical surface electrode and the flat end surface electrode have a surface roughness (Ra) value of less than 0.8 μm.

Spot welding splash does not occur during spot welding, and after welding by adopting optimized process parameters, adhesion and alloying between the workpiece and the electrode on the surface where the dissimilar material spot-welded joint of the aluminum alloy and stainless steel sheet is formed No reaction occurs, and the nugget of the spot-welded joint on the aluminum alloy side has a size of at most 10.1 mm. At the interface between the aluminum alloy and the stainless steel sheet, the intermetallic compound has _{a phase composition of Fe 2} Al ₅ and Fe ₄ Al ₁₃ and has a thickness of less than 4.0 μm. In addition, the surface indentation depth of the aluminum alloy side is 17% of the thickness of the aluminum alloy plate, and the spot-welded joint has a tensile shear force of up to 5400N.

The dissimilar material spot-welded joint of the aluminum alloy and stainless steel sheet has a fracture mode of button fracture during the tensile shear test, and the crack extends in the aluminum alloy base material along the periphery of the spot-welded joint, No fracture of the bonding surface occurs.

In the present invention, there is no need to additionally add an electrode plate to prevent the electrode from adhering to the product during welding, and the aluminum alloy and stainless steel plate to reduce the thickness of the intermetallic compound layer at the aluminum/steel interface There is no need to add an aluminum/steel clad plate as an interlayer to aid spot welding of Therefore, the present invention has the advantages of the above-mentioned spot-welded joint of simple process, high production efficiency, low cost and high quality compared to the methods reported in related studies.

The present invention uses a spot welding electrode having a special end surface shape optimized for performing spot welding to obtain a high quality spot-welded joint of aluminum alloy and stainless steel sheet. The present invention is specifically embodied as follows.

First, the surface of the stainless steel sheet to be welded is washed with acetone to remove oil contaminants, and then air-dried. Second, the aluminum alloy plate to be welded is mechanically polished to remove the dense aluminum oxide film on the surface (either manually polished using 800# SiC abrasive paper or polished using a polishing machine with an abrasive wheel polishing head). The spot welding electrode is made of a copper-chromium (CuCr) alloy, machined by a digitally controlled lathe or dedicated electrode cap finisher to provide an optimized end surface and size, and a surface roughness (Ra) value of less than 0.8 μm in diameter. and a spherical surface electrode of 30 to 90 mm and a flat end surface electrode of 8 to 20 mm in diameter. A medium frequency direct current resistance spot welding device is selected and operated with the technical indicators to be described later. Primary voltage 380 V, primary frequency 50 Hz, secondary frequency 1 kHz, secondary maximum short-circuit current 30 kA and maximum electrode pressure of 9 kN. The spherical surface electrode and the flat end surface electrode are mounted and fixed on the electrode holder of the medium frequency direct current resistance spot welding apparatus, the flat end surface electrode disposed at the fixed end of the electrode holder, the spherical surface electrode being the electrode It is disposed at the moving end of the holder, and due to this arrangement, the normality of the overlapping structure can be ensured when the workpiece to be welded interacts with the electrode. The stainless steel plate and the aluminum alloy plate are superposed on the flat end surface electrode and positioned on the flat end surface electrode, in which case the flat end surface electrode is positioned at the center of the overlapping region of the stainless steel plate surface. According to the set spot welding optimization process parameters (spot welding current 10 to 26 kA, welding time 0.05 to 0.4 s, electrode pressure 1.0 to 5.0 kN, pre-pressing time 0.2 to 0.4 s, and holding time 0.3 to 0.4 s), the procedure is programmed in the human-machine interface of the spot welding device. Spot welding is performed on the overlap joint of the stainless steel plate and the aluminum alloy plate, wherein the welding process parameters are automatically controlled by a pre-programmed procedure command to perform pre-compression, welding and maintenance steps. During the welding process, the aluminum alloy is melted at the interface of the stainless steel plate/aluminum alloy plate to form a spot-welded nugget, whereas melting does not occur on the stainless steel plate side. In addition, the molten aluminum alloy rapidly spreads and precipitates on the surface of the stainless steel sheet, and forms a metallurgical joint under the action of atomic interdiffusion and interfacial reaction to form a final dissimilar material spot-welded joint between the stainless steel sheet and the aluminum alloy sheet. provides

Hereinafter, the present invention will be described in detail with reference to specific examples and comparative examples.

Example 1

A stainless steel plate was wire-cut to a size of 90 mm x 25 mm x 1 mm. The aluminum alloy plate was wire cut to a size of 90 mm x 25 mm x 1.5 mm. The spot welding method of the present invention was used to perform welding, the spot welding electrode was made of a copper-chromium (CuCr) alloy, a spherical surface electrode with a diameter of 70 mm was used on the aluminum alloy side, and a diameter of 10 mm A flat end surface electrode was used on the stainless steel plate side, and both electrodes had a surface roughness (Ra) value of 0.8 μm. The spot welding process parameters were selected as follows. Spot welding current 22 kA, welding time 0.3 s, electrode pressure 3.5 kN, pre-pressing time 0.4 s and holding time 0.4 s.

As a result of the test, the surface shape of the dissimilar material spot-welded joint of the aluminum alloy/stainless steel sheet was good, and no alloying reaction occurred between the surface of the stainless steel sheet side and the flat end surface electrode, and the aluminum alloy side A circular indentation appeared on the surface, and the center of the welded joint did not adhere to the spherical surface electrode. The dissimilar material spot weld joint of the aluminum alloy/stainless steel sheet was formed by the wetting and diffusion action of the molten aluminum alloy on the surface of the solid high-strength steel accompanied by atomic diffusion and interfacial reaction, thus molten-brazing It was a weld joint.

Defects such as shrinkage cavities, cracks, etc. did not occur at the aluminum/steel interface of the spot-welded joint, the diameter of the aluminum alloy nugget of the spot-welded joint was 10.1 mm, and the surface indentation depth on the aluminum alloy side was 17% of the thickness of the aluminum alloy plate. An intermetallic compound layer occurred at the interface of the aluminum alloy/stainless steel sheet of the spot-welded joint, and had a maximum thickness (4.0 μm) at the center of the joint. Micro X-ray diffraction analysis showed that the intermetallic compound was composed _{of Fe 2} Al ₅ and Fe ₄ Al _{13 .}

Example 2

The stainless steel sheet to be welded and the aluminum alloy sheet had the same composition and product size as in Example 1. The spot welding method of the present invention was used to perform welding, the spot welding electrode was made of a copper-chromium (CuCr) alloy, a spherical surface electrode with a diameter of 70 mm was used on the aluminum alloy side, and a diameter of 12 mm A flat end surface electrode of phosphorus was used on the stainless steel plate side, and both electrodes had a surface roughness (Ra) value of 0.8 μm. The spot welding process parameters were selected as follows. Spot welding current 24 kA, welding time 0.35 s, electrode pressure 3.5 kN, pre-pressing time 0.4 s and holding time 0.4 s.

As a result of the test, the surface shape of the dissimilar material spot-welded joint of the aluminum alloy/stainless steel sheet was good, and the spherical surface electrode did not adhere to the surface of the stainless steel sheet side and the aluminum alloy side. Defects such as shrinkage cavities, cracks, etc. did not occur at the aluminum/steel interface of the spot-welded joint, the diameter of the aluminum alloy nugget was 9.5 mm, and the surface indentation depth on the aluminum alloy side was the thickness of the aluminum alloy plate was 16% of _{An intermetallic compound layer (Fe 2} Al ₅ and Fe ₄ Al ₁₃ ) occurred at the interface of the aluminum alloy/stainless steel sheet of the spot-welded joint, and had the maximum thickness (3.8 μm) at the center of the joint.

Example 3

The stainless steel sheet to be welded and the aluminum alloy sheet had the same composition and product size as in Example 1. The spot welding method of the present invention was used to perform welding, the spot welding electrode was made of a copper-chromium (CuCr) alloy, and a spherical surface electrode with a diameter of 90 mm was used on the aluminum alloy side, and a diameter of 10 mm A flat end surface electrode of phosphorus was used on the stainless steel plate side, and both electrodes had a surface roughness (Ra) value of 0.8 μm. The spot welding process parameters were selected as follows. Spot welding current 25 kA, welding time 0.4 s, electrode pressure 4 kN, pre-pressing time 0.4 s and holding time 0.4 s.

As a result of the test, the surface shape of the dissimilar material spot-welded joint of the aluminum alloy/stainless steel sheet was good, and the spherical surface electrode did not adhere to the surface of the stainless steel sheet side and the aluminum alloy side. Defects such as shrinkage cavities, cracks, etc. did not occur at the aluminum/steel interface of the spot-welded joint, the diameter of the aluminum alloy nugget was 9.2mm, and the surface indentation depth on the aluminum alloy side was the thickness of the aluminum alloy plate of 13.6%.

comparative example

The stainless steel sheet to be welded and the aluminum alloy sheet had the same composition and product size as in Example 1. A conventional F-type electrode spot welding method was used to perform welding. The optimized spot welding process parameters were selected as follows. Spot welding current 9 kA, welding time 0.3 s, electrode pressure 3 kN, pre-pressing time 0.4 s and holding time 0.4 s.

As a result, the surface shape of the dissimilar material spot-welded joint of the aluminum alloy/stainless steel sheet deteriorates, and an alloying reaction occurs between the surface of the stainless steel sheet side and the electrode, and the surface of the aluminum alloy side and the electrode adhesion This happened. The diameter of the aluminum alloy nugget of the spot-welded joint was 5.8 mm, and an intermetallic layer of up to 13 μm occurred at the aluminum/steel interface of the spot-welded joint. Micro X-ray diffraction analysis showed that the intermetallic compound was composed _{of Fe 2} Al ₅ and Fe ₄ Al _{13 .} Therefore, the welding quality of the aluminum alloy and stainless steel sheet obtained by the spot welding method of the present invention is significantly superior to that of the conventional F-type electrode spot welding method.

Claims (4)

performing welding to an aluminum alloy and a stainless steel sheet using a spot welding electrode to obtain a dissimilar material spot-welded joint of the aluminum alloy and the stainless steel sheet;
A spherical surface electrode having a diameter of 30 to 90 mm is used for the aluminum alloy side, and a flat end surface electrode having a diameter of 8 to 20 mm is used for the stainless steel plate side.
The method according to claim 1, wherein the spot welding electrode is formed of a copper-chromium (CuCr) alloy.
The method of claim 1 , wherein the spherical surface electrode and the flat end surface electrode have a surface roughness (Ra) value of less than 0.8 μm.
3. The method according to claim 2,
removing the dense aluminum oxide film by mechanically polishing the aluminum alloy surface;
removing oil contaminants by washing the surface of the stainless steel sheet with acetone;
mounting the spot welding electrode on an electrode holder of a medium frequency direct current resistance spot welding apparatus; and
performing welding according to set spot welding parameters to obtain the dissimilar material spot-welded joint for the aluminum alloy and stainless steel sheet;
wherein the flat end surface electrode is disposed at the fixed end of the electrode holder and the spherical surface electrode is disposed at the moving end of the electrode holder.