Key technology of multi-material body self-locking riveting

Self-locking riveting technology principle and process

Self-piercing riveting (SPR), also known as self-piercing riveting, was proposed by Fuhrmeister in 1978. The SPR process is powered by a hydraulic cylinder or a servo motor to directly press the rivet into the plate to be riveted. Under the pressure, the rivet is plastically deformed, and after filling, it is filled in the rivet mold, and the rivet legs are turned around to form a mechanical interlocking cold forming connection process. SPR is a stable plate joining technology.

In the car body connection, it is necessary to consider the connection static strength and fatigue strength and the weight of the car body. Therefore, most car manufacturers choose to apply the semi-tubular rivet self-pierce riveting technology to the assembly of lightweight car body sheets. Here we mainly introduce the semi-tubular rivet self-piercing riveting process.

1. Semi-tubular rivet self-pierce riveting process

Schematic diagram of the connection process of the self-piercing riveting process of semi-tubular rivets

It is divided into four stages:

1) Clamping stage: semi-tubular rivet self-piercing riveting is to press the plate to be connected by rivet nose;

2) Puncture stage: the punch moves downwards, pushing the rivet to force it to pierce the upper sheet, while the rivet also drives the lower sheet to the die, plastic deformation occurs, and the semi-tubular rivet is pressed against the rivet. Under the action, pierce the upper layer to be connected to the sheet;

3) Expansion stage: As the riveting process progresses, the rivet legs gradually open, and the lower sheet material is plastically deformed and gradually filled into the concave mold. Under the joint action of the punch and the concave mold boss, the rivet legs are expanded to the periphery. Embedding the lower layer to form a mechanical interlocking structure between the rivet and the sheet;

4) Punching and riveting: When the punch presses the rivet down until the rivet head is in close contact with the upper surface of the upper layer and is flush, it can be considered that the riveting is completed. At this time, the crimping ring releases the blanking force, and the punch will return to the initial work. Bit, the end of the riveting.

2. Self-pierce riveting process features

1) The process cycle is short and the noise is low, usually a riveting process cycle is only 1 to 4 seconds;

2) For the connection between the aluminum plates, the fatigue resistance and tensile strength of the self-piercing riveted joints are better than the welded parts;

3) For the connection between ordinary steel plates, the tensile strength of the welding is slightly better than the self-piercing riveting, and the fatigue resistance is slightly worse than the self-piercing riveting;

4) It is possible to connect plates of various materials: high-strength steel plates, aluminum alloy plates, plastic plates, composite sandwich plates, etc.;

5) The multi-layer sheet set can be riveted.

At present, the self-piercing riveting method has been widely used in the manufacture of aluminum alloy bodies of Audi, BMW, Jaguar, Volvo, General Motors, Ford and Daimler-Chrysler, and the joint fatigue strength is up to 2 times that of resistance spot welding.

Domestic research on self-piercing riveting started late. In recent years, Tianjin University, East China Jiaotong University, Shanghai Jiaotong University and Tsinghua University have done a lot of theoretical and experimental research around aluminum alloy.

Shanghai Jiaotong University studied the influence of process elements such as die and rivet size on the quality of aluminum-steel self-piercing riveting for aluminum and steel heterogeneous joints. All in all, SPR has become the most promising multi-material body connection method due to the lack of heat input.

3. Semi-tubular rivet self-pierce riveting process application

Self-piercing riveting is widely used in automobile bodies due to its simple process and low cost.

Compared with nailless riveting, SPR joints have good static mechanical properties and high fatigue life. They have been widely used in GM, Audi, BMW, Jaguar, Mercedes-Benz and Volkswagen for heterogeneous materials such as aluminum and steel, as well as multilayers. The board connections are shown in the table below.

SPR usage statistics in a typical multi-material hybrid body

Two self-locking riveting process parameters and selection

The mechanical characteristics of the SPR process determine that the quality of the riveting is closely related to factors such as rivets, molds, plates, and stamping equipment.

The diameter and length of the rivet legs, the diameter of the die and the height of the boss, the strength and arrangement of the plates, the stamping load and the speed have a great influence on the joint quality and the joint strength.

1. Selection of main process parameters of self-locking riveting

1.1 Principle of selection of rivet size

For the self-locking riveting process, the design of the shape and size of the rivet directly determines the joint form of the joint. Therefore, in the self-locking riveting process parameters, the shape and size of the rivet must first be determined. The type of rivet has been standardized (see GB/T1013 and GB/T875), and the rivet models selected for materials of different plasticity, strength and thickness are also different.

A. Selection of rivet diameter: mainly based on the thickness of the riveted sheet. Generally, the head diameter of the rivet is D1=D2+(2~3) mm.

1 The larger the thickness of the sheet, the larger the diameter of the rivet head; for the two-layer sheet with a thickness of 2 mm, the rivet of Ø5.3 mm diameter is used; the single-layer sheet with a thickness of 1 mm is riveted with Ø3. 3 mm diameter rivet;

2 In order to improve the fatigue life, thick rivets are also used to rive the thick sheets.

B. Selection of rivet length: mainly based on rivet diameter and riveted sheet thickness. L = (T + C).

â‘  For large diameter rivet, the rivet relative standard results obtained by one to the next.

2 For small diameter rivets, the results are up one step above the standard rivet.

Note: where: T is the total thickness of the riveted sheet, T = t1 + t2; C is the length of the rivet beyond the thickness of the plate. According to the statistical analysis of the literature, the expression of C is C =0. 617D2 - 0.1246 where: D2 is the diameter of the rivet leg. L', T, C, and D2 are all in mm. The rivet parameters are mainly as shown:

Rivet head diameter D1, leg diameter D2, length L

2. The design method of the mold

After the rivet size is determined, the mold is selected or designed. The core of the mold design is the design of the cavity. The following is a selection of several core sizes.

A. The depth of the mold h: the depth is related to the length of the rivet extending plate thickness, generally 2. 17 mm, or smaller, but should be greater than 1. 5 mm, the main parameters of the self-locking riveting die are mainly as shown;

Cavity inner wall bottom diameter D, cavity depth h

B. Inner wall bottom diameter D2: mainly based on the thickness of the lower layer of the riveted. D2 =Da +0. 8t2 where t2 is the thickness of the lower sheet and Da is the upper diameter of the rivet after expansion

C. Conical bottom diameter D1: In general, (D2 - D1 ) /2 = 1.5 to 2 mm, ie D1 = D2 - (3 ~ 4) mm

3. The order of the sheets follows the following principles:

1) When riveting two layers of the same material, place the thicker sheet on the lower layer.

2) When riveting two layers of different materials, place the plastic sheet in the lower layer.

3) When riveting metal and non-metal sheets, place the metal on the lower layer.

The above briefly introduces the basic principles of process parameters selection for conventional semi-tubular rivet self-locking riveting. However, with the development of lightweight vehicles, the body materials become stronger, thinner and lighter, and the material combination is more diversified, making the SPR process face new The challenge.

The main problems faced by self-locking riveting under lightweight background

Lightweight background: The use of lightweight materials such as aluminum alloys, magnesium alloys, lightweight high-strength steels, and composite materials to reduce the weight of automobiles has gained widespread attention in the automotive industry and has become the main technology for lightweight vehicles.

The current application ratio of high-strength steel in automobiles

Automotive lightweight development trend

Multi-material design effects of the EU's ultralight vehicles

Considering the cost and light weight effect, the multi-material body design will represent the future development trend of the car body structure. This will cause the traditional SPR process to face enormous challenges and even eliminate the traditional SPR process.

1. The main problem facing self-locking riveting

(1) For aluminum/steel connection: It is difficult to deform ultra-high-strength steel of 800 MPa or higher. It is difficult to form self-locking between rivet and lower steel plate from aluminum riveting to steel, and cavity is easily formed in rivet from steel riveting to aluminum. The lower plate will be pressed through, and the rivet will be severely pierced or even cracked due to insufficient strength when piercing the upper steel plate, and the joint quality is difficult to ensure.

Aluminum alloy 6061-T6/DP980 SPR joint high-power detection

(2) When the thickness of the lower layer is not more than 1 mm, the material of the lower layer is too thin, and the fluctuation of the process or material properties easily leads to the riveting of the lower layer, which causes the risk of corrosion.

High-power detection of 6061-T6SPR joints with a plate thickness of 1 mm

(3) For low elongation materials such as magnesium alloys, cracking is likely to occur inside or at the bottom of the joint during plastic deformation.

Magnesium alloy/aluminum alloy SPR joint

SPR is a typical double-point connection process. It is necessary to cooperate with the die to realize the connection of the plates, and it is impossible to realize the single-side connection of the closed structural parts such as the aluminum alloy profiles.

Four new self-locking riveting technology

1. Preheating self-locking riveting

Laser preheating self-locking riveting: For the low elongation magnesium alloy material, it is easy to crack during the SPR process. DURANDET et al. and EASTON proposed pre-heating the magnesium alloy sheet by laser before the SPR process to improve its ductility. It can effectively eliminate the cracks generated during the SPR process of low elongation materials such as magnesium alloy and cast aluminum.

(a) SPR connector (b) Preheated SPR connector

At present, the laser preheating process can be combined with the SPR equipment produced by Henrob to realize the automatic preheating SPR process, but the heating by laser is costly and difficult to apply on a large scale.

Electric heating plate pre-heating self-locking riveting: Professor Wang of Zhengzhou University and others pre-heated the magnesium alloy by electric heating plate before the SPR process, and studied the influence of different pre-heating temperature on the connection of SPR joint. Studies have shown that a perfect defect-free joint can be obtained at 180 °C, as shown in the figure below.

(a) Room temperature (b) 150 °C (c) 180 °C

Induction heating self-locking riveting: In order to achieve self-piercing riveting of ultra-high strength steel DP980 and aluminum alloy, Shanghai Jiaotong University proposed Thermal assisted self-piercing riveting (TSPR).

The process arranges a mosquito-repellent coil on the side of the steel plate, and generates eddy current in the steel plate by excitation of high-frequency alternating current, and rapidly heats the portion to be riveted of the steel plate.

The induction heating method has high heating efficiency and can heat the steel plate to the required temperature within 1 s. The TA-SPR process can effectively improve the small amount of joint undercut and the crack at the bottom of the joint caused by the large deformation resistance of the ultra-high strength steel such as DP980 during the SPR process.

2. Electric auxiliary self-locking riveting

In order to improve the riveability of high-strength steel materials, Electrical assisted self-piercing riveting (EA-SPR) was developed.

Electric auxiliary heating process schematic

Principle: In the process of SPR joint formation, external high-density current is applied to high-strength steel material in real time, and electro-plastic effect and Joule heating effect are utilized to improve the shape deformation ability of high-strength steel and reduce its deformation resistance, thereby improving the riveting quality.

Effect of electric auxiliary heating on riveting force

Effect of electric auxiliary SPR on quality improvement of high strength steel-aluminum alloy joint

Compared to the traditional SPR process:

1) Effectively reduce the riveting force;

2) When riveting from high-strength steel to aluminum alloy, with the deformation resistance of high-strength steel decreasing, the inner cavity height of the rivet is reduced, the amount of undercut is increased, the rough condition of the rivet pier is obviously relieved, and the aluminum alloy crushing at the bottom of the joint is also Effective suppression has been obtained.

3, self-flushing friction welding process

For the low-expansion materials such as magnesium alloys and cast aluminum, the joints are prone to cracking when self-piercing and riveting. The automakers began to use the friction self-piercing riveting (F-SPR) process for magnesium alloys.

Schematic diagram of F-SPR process

Principle: The process is based on SPR. The inner or outer drive is used to drive the semi-tubular rivets to rotate at the same time as the axial feed. The whole process is completed by two stages: rotary rivet and in-situ stirring.

In the riveting stage, the frictional heat softening metal generated by the high-speed rotation of the rivet solves the problem of difficult deformation of the material with low elongation in the SPR process, realizes the mechanical connection of the material with low elongation rate, and matches the mixing time and the stirring speed in the in-situ stirring stage. Precisely control the frictional heat and temperature at the interface of the heterogeneous material to achieve a solid phase connection between the rivet and the surrounding material.

Effect of F-SPR on crack suppression

Process characteristics: 1) F-SPR process can effectively inhibit the cracks inside and under the joint of aluminum alloy and magnesium alloy, and improve the mechanical properties of the joint.

2) The introduction of the solid phase connection doubled the mechanical properties of the final F-SPR joint.

3) The F-SPR process does not require pre-fabrication or pre-heating of the sheet. The process is highly integrated and can be used not only for the connection of difficult-to-deform metal materials, but also for the joining of composite materials.

However, the F-SPR process places high demands on equipment and rivets. During high-speed rotation and feed, it is necessary to ensure that the axis of rotation of the rivet and the geometric axis of symmetry and the geometric axis of the die maintain a high degree of concentricity. Avoid creating a gap between the rivet in the joint and the surrounding material.

4. Electrical blocking riveting or resistance element welding (REW)

For the challenge of SPR process for ultra-high strength steel and thin plate applications, the Rivet plug welding process has been proposed abroad.

The process realizes the connection of the heterogeneous metal by "locking" the upper layer between the rivet cover and the lower layer by welding between the solid rivet and the lower layer which are homogenous to the lower layer, thereby avoiding the hardening of the dissimilar metal when directly welding. Brittle phase problem.

Schematic diagram of electric blocking welding or resistance unit welding

Luo Zhen and others from Tianjin University conducted research on resistance unit welding of boron steel and aluminum alloy: traditional resistance spot welding can hardly connect boron steel and aluminum alloy, and resistance unit welding can complete its reliable connection and maximum tensile shear force of joint. It reaches 7142 N and the plasticity is also very good.

Process engineering

Currently, Volkswagen has applied the REW process to the manufacture of the new Passat aluminum/steel hybrid.

5, Flow drill screw (FDS)

Flow drilling and riveting technology: The plate to be joined is softened by the high-speed rotation of the screw, and is pressed and screwed into the plate to be joined under the action of huge axial pressure, and finally a threaded connection is formed between the plate and the screw.

crafting process

Advantages of the process: high-quality connection of aluminum alloy plates can be realized, and high-quality connection between steel screws and homogenous plates can be realized, which overcomes the low strength of stud welding and the inability to achieve spot welding of special parts.

With the optimized connection parameters, high-quality structural plate joints can be obtained, and at the same time, high-quality joints between the multi-layer boards which cannot be realized by welding methods such as spot welding can be realized.

The FDS process also has its obvious disadvantages in the application process:

(1) For the thinner plate combination of the lower plate, since the lower plate is only 1 to 2 turns of thread in contact with the FDS screw, the positive tensile strength of the joint is low.

(2) When the upper plate is a high-strength material, it is necessary to pre-form the plate because it is difficult to be penetrated by the FDS screw. How to achieve precise positioning and alignment of these prefabricated holes poses a significant challenge to the FDS process when joining multiple layers of sheet metal combinations.

(3) For the matching of the plates requiring pre-made holes, when the FDS process is combined with the structural adhesive, the structural adhesive will be extruded from the preformed holes, causing contamination of the rivet gun, residual structural adhesive on the surface of the joined plate, and structural rubber impeding the screws. Movement causes the connection to fail.

(4) Since the lower plate is penetrated, the FDS screw is exposed to the air, and the gap between the lower plate and the FDS screw easily causes the corrosive medium to enter, making the joint susceptible to electrochemical corrosion problems.

FDS is mostly used in the interior panel of the door, the floor of the car body, the inner panel of the rear panel, and the trunk lid.

Application of flow drill screw technology on Audi A8

6, rubber riveting connection

The bonding process and the SPR process are combined, that is, the Rivet bonding process is a process in which a structural adhesive is applied at a joint position of a connected member, and then an SPR connection is performed.

Application of rubber riveting composite technology in BMW 5 Series body

The figure above shows the effect of structural adhesive on the shear properties and cross tensile properties of SPR joints.

It shows that: 1) The shear performance of the rubber-rive composite joint is significantly better than that of pure rubber joint and pure riveting, which is more than twice that of the pure SPR joint, and the energy absorption capacity is also greatly improved (Fig. a).

2) For the cross tensile test (Fig. b), the force mode determines that the rubber layer first tears along the periphery, and then the riveted joint fails again, thus lacking the combined lifting effect, even the amount of undercut due to the introduction of structural glue is slightly reduced. The cross-stretching strength of the rubber-rive composite joint is slightly lower than that of the pure SPR joint, but it is still significantly higher than that of the pure rubber joint.

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Lightweight is an important means of energy saving and emission reduction of automobiles. Considering the cost and effect of lightweighting, multi-material mixing is an inevitable trend in the development of lightweight technology for automobile bodies.

The mechanical connection technology represented by self-pierce riveting can improve the main problems faced by multi-material body connection, and thus becomes an important means for the connection of heterogeneous materials at this stage.

However, with the increase of the lightweight level of automobiles, the application of multi-material body is more and more widely, the traditional SPR technology can not fully adapt, and new SPR technology must be developed, such as the pre-heated SPR and the electric auxiliary SPR mentioned above. Friction stir SPR and so on.

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