Magnesium alloy motorcycle handlebar tube technology of superplastic bulging

 

(Henan University of Science and Technology, Luoyang 471003)

 

Abstract: The design of a set of magnesium alloy tube bulging shape forming device for magnesium alloy motorcycle handlebar tube superplastic bulging shape forming studied experimentally and confirmed by experiments of magnesium alloy tube superplastic bulging forming process. The main process parameters are: molding temperature 340 ~ 370 ℃, bulging initial pressure 2.0 ~ 2.2MPa, axial pressure 1.0 ~ 1.5 MPa, with bulging carried bulging pressure phased loaded to 10 MPa. In this condition, the tubes forming a high dimensional accuracy, uniform thickness, no thinning phenomenon. Superplastic bulging with technology to improve the ability of plastic forming of magnesium alloy, and to achieve a motorcycle handlebar tube precision forming, improve material utilization and reduce costs.

 

Keywords: pipe; bulging; magnesium alloy; superplasticity

 

At present, the research and application of magnesium alloys attracted world attention, magnesium alloy consumption was sustained rapid growth in the global trend. Magnesium alloy has a density, specific strength and specific stiffness, damping resistance, machinability, good thermal conductivity, electromagnetic shielding ability, good casting properties and dimensional stability, etc., while magnesium resources are very rich, and magnesium alloy products easy to recycle, environmentally friendly features. Therefore, the magnesium alloy as the "21st century green engineering materials," in recent years in the automotive, aerospace, electronics and other fields rapidly increasing development prospects [1-2].

 

Magnesium alloy is the close packed hexagonal structure, less slip systems, and the temperature of its lower ductility, formability is poor. So most of the products are made of magnesium alloy casting (casting, casting, etc.) forming method [3-5], but with extrusion, rolling, forging and other plastic forming method [6] processed magnesium alloy products rarely. Magnesium alloy motorcycle handlebars with light weight, good vibration absorption, increased riding comfort, but like a certain model motorcycle handlebar tube products such as magnesium alloy, because of its complex shape, there is no method of using plastic production. At present the domestic motorcycle handlebar basically made using traditional pipe bending, there is a large mass, vibration absorption and poor shortcomings.

 

Using superplastic bulging technology at a certain temperature to obtain excellent plastic forming ability, thus forming the complex structure of magnesium alloy parts [7]. This paper describes the use of methods to achieve superplastic bulging magnesium alloy motorcycle handlebar tube precision plastic forming, pipe cross-sectional dimension along the axial direction of the gradual change in shape in the mold once. Which is characterized by simple equipment, without large dedicated equipment, forming high precision, high utilization of materials, easy operation and low cost.

 

An experiment

 

1.1 Experimental equipment and materials

 

Experimental equipment used YX32-315 hydraulic press (nominal pressure of 3 150 kN), magnesium alloy tube superplastic bulging experimental apparatus and magnesium alloy billet preheating furnace each set. Experimental magnesium alloy tube as AZ91D, diameter 14 mm, diameter 22 mm, length 965 mm. After keeping the thickness of the tube bulging basically unchanged, the maximum outer diameter of the middle section, is 28 mm, the maximum diameter and uniform across the diameter transition.

 

1.2 superplastic bulging bulging experimental apparatus and method

 

The experimental device consists of mold, tube sealing mechanism, heating mechanism, pneumatic and hydraulic auxiliary bulging bodies institutions. Die by the upper mold 1, the lower mold 5, the main provider of bulging cavity tube, the tube Precision molded into a desired shape; tube sealing mechanism controlled by the plug and plug 2,7 3,8 composed of hydraulic cylinders , providing for sealing the tube when the tube bulging desired axial feeding pressure; heating means 6 by the heating rods, the upper and lower mold of the tube 3, to provide heat required for heating the tube; pressure bulging mechanism consists of a pneumatic cylinder composed by hydraulic cylinder to cylinder filled compressed air required to achieve a certain pressure bulging pressure; hydraulic auxiliary mechanism is mainly fuel tank, hydraulic pumps, hydraulic valves and other components of the controlled circuit, in accordance with the need to complete certain actions. The specific operation: the first step, the mold is heated to a specific temperature 1,5, then preheated in a furnace magnesium alloy tube 6 into the mold cavity, the mold and given a certain pressure F. The second step in the hydraulic cylinder plug 2,7 3,8 driven forward, so that the pipe ends and plugs sealing the tube ends to apply some pressure. The third step, the pneumatic cylinder 4 is filled, up to a certain pressure, holding a certain time, the tube bulging and fully fit the mold cavity. The fourth step is relieved pneumatic cylinder pressure, plug back, remove the tube, placed under a preheated tube, start a new bulging process.

 

1.3 process parameters

 

1.3.1 bulging temperature

 

Magnesium hcp crystal structure, magnesium matrix less independent slip systems, it is only basal slip sliding systems and conical twin of five slip systems, namely three geometric and two independent slip systems slip systems, poor ductility at room temperature and low temperature, but when deformation temperature is higher than 200 ℃, can slip systems increased magnesium alloy, magnesium alloy plastic processing capacity has increased significantly. On the deformation of magnesium alloys have a high strain rate sensitivity, while high-speed deformation, can easily lead to cracking of magnesium alloys. Therefore, to ensure the yield of bulging, it is necessary to superplastic forming.

 

To determine the magnesium alloy extruded tube superplastic temperature range, along the tube axis line cutting out "I" shaped thin tensile specimens at 280 ~ 400 ℃ between v = 6 mm / min at a constant rate of do superplastic tensile test, each specimen holding time 20min, temperature interval is 30 ℃, Figure 2 shows the tensile strength after stretching - elongation curve.

 

As can be seen from the figure, as the temperature rises, AZ91D magnesium alloy elongation increased from 310 ℃ start AZ91D magnesium alloy exhibits superplasticity, continue to heat up to 340 ℃, maximum elongation reached 161% . When the temperature continues to increase to 370 ℃ and 400 ℃, high temperature oxidation, AZ91D magnesium alloy by reduced elongation of 115% and 105% elongation with temperature indicates that this non-monotonic AZ91D magnesium is 340 ℃ The optimum temperature alloy superplastic deformation. In the experiment, take 310 ~ 370 ℃ as magnesium alloy tube bulging temperatures.

 

1.3.2 Select the initial pressure

 

The initial internal pressure superplastic bulging shape forming process of the important parameters, the value selected is appropriate bulging directly determines the success of the process. The magnesium alloy tube for this bulging, the initial radial bulging without restraint, in order to ensure the tube will not be burst, yielding in the case of a deformable tube, the selection of the initial internal pressure should be lower.

 

1.3.3 Axial feeding pressure

 

Axial feeding tube bulging process is another important factor to consider. According to design requirements, bulging of the pipe wall thickness requirements after essentially the same, in order to ensure no bulging rear wall thinning must be given within a reasonable axial pressure. Given axial pressure pipe ends need to consider the friction with the mold and maintain the required pipe plastic deformation axis pressure, but also to meet the sealing requirements.

 

1.3.4 Seal

 

Tube ends sealed in bulging bulging process is to maintain the pressure of the key. Seals are used cone-shaped punch is pressed into the method, this sealing method has good sealing effect, high production efficiency, easy operation and so on. Taking into account the present study, bulging pressure medium is a gas, the sealing has more stringent requirements, the need to do some work on the sealing plug hard chrome plated and polished. During the experiment also proved that this method is very effective sealing.

 

2 Experimental results and analysis

 

After repeated experiments, and ultimately get the appropriate process parameters: Bulging temperature 340 ~ 370 ℃, bulging initial pressure 2.0 ~ 2.2MPa, axial pressure 1.0 ~ 1.5 MPa. In this process parameters to produce a qualified product (see Figure 3).

 

Pressure and other process parameters have an impact on product quality. Temperature is too low, bulging more difficult, causing bulging initial pressure increases, the pressure becomes large initial deformation, deformation process is not ideal because uniform deformation, partial tube will be excessive bulging, ruptured; temperature is too high , magnesium alloy oxidation severe and lead to instability or bursting. Bulging bulging initial pressure is the key, the pressure is too small, bulging very slow, even inflation does not move; too much pressure, causing uneven deformation, causing rupture. Inappropriate choice of axial pressure on product quality also have important implications, wall thinning caused by too small even cause Mifengbuyan leak; much instability is mainly caused by tube, forming a spiral twist.

 

Bulging pressure is loaded in stages. The first step, given the initial pressure of 2.0 MPa, holding pressure 10 min, the middle part of the mold has been posted mold view full, only the middle and both ends of the transition portion bulging is not enough; Second, given pressure 4 MPa, holding pressure 5 min, at this time bulging area extending from the middle to the ends, but not completely; the third step, a given pressure 6 MPa, holding pressure 3 min, bulging at this time completely, the basic shape; fourth step, given pressure of 10 MPa, this step is to ensure that the wall is completely attached to the main mode, to prevent bulging is not sufficient, because this time bulging in the previous step has been completed, so even if a given pressure is relatively large does not burst.

 

3 Conclusions

 

(1) The magnesium alloy is poor low temperature ductility, plastic processing difficulties, by raising the temperature, the superplastic state where the plastic working, forming part of a complex.

 

(2) in the magnesium alloy tube superplastic bulging shape forming, a good control of reasonable temperature, pressure and axial find the best bulging with the relationship between the pressure, you can get qualified products.

 

(3) In this experiment, bulging temperature 340 ~ 370 ℃, bulging initial pressure 2.0 ~ 2.2 MPa, axial pressure 1.0 ~ 1.5MPa, with bulging carried bulging pressure load in stages to 10 MPa. Forming out of the product uniform thickness, no thinning phenomenon, to punch more fully, precise dimensions, the yield is relatively high.

 

(4) In the initial stages of bulging, bulging axial pressure to improve the quality of a significant effect, bulging subsequent stage, axial compression of the role will be significantly reduced, during the bulging pressure load is carried out in stages.