Application and technology development of spring

2、 Development of spring materials
The development of spring application technology puts forward higher requirements for materials. It is mainly to improve the fatigue life and anti relaxation performance under high stress; secondly, it is required to have corrosion resistance, non-magnetic, electrical conductivity, wear resistance, heat resistance and so on according to different working methods. Therefore, in addition to the development of new spring materials, in addition to strictly controlling the chemical composition, reducing non-metallic inclusions, improving the surface quality and dimensional accuracy of spring materials have also achieved beneficial results.

1. With the development of alloy steel, Si Cr steel has been widely used in valve spring and suspension spring. In order to improve the fatigue life and anti relaxation property, V and mo were added into Si Cr steel. At the same time, Si CR drawing steel wire has been developed, which has better anti relaxation property than piano steel wire when working at high temperature. With the rapid miniaturization of engine, Ti alloy with good flutter resistance, light weight and small elastic modulus has been widely used, and its strength can reach 2000 MPa.

2. Development of stainless steel wire
1) The strength of austenitic stainless steel wire is better than that of ferrite structure, and its corrosion resistance is better than that of martensite structure.
2) Low temperature drawing or low temperature nitriding drawing can improve the strength of steel wire. The microstructure of martensite is unstable when heated, but the cryptoacicular martensite can be formed by drawing in low temperature liquid nitrogen, which can obtain high strength in hot state. This kind of steel wire has been widely used in the United States and Japan, but it can only handle steel wire less than 1 mm.
3) The precision spring in electronic equipment is required to be non-magnetic, and the hidden needle martensite can not be formed during the drawing process of this kind of steel wire. Therefore, N, Mn, Ni and other elements should be added. In order to meet this demand, the United States developed aus205 (0.15c-17cr-1ni-15mn-0.3n) and Yus (0.17c-21cr-5ni-10mn-0.3n). Due to the increase of Mn content, cryptoacicular martensite will not be formed during processing. After solution treatment, the strength can reach 2000mPa and the fatigue property is higher than SUS304.

3. Improve the purity of materials. For high strength materials, strictly control the inclusion and improve the purity to ensure its performance. For example, the oxygen content of valve spring material has reached 20 × 10 ¯ 6.

4. Improving the surface quality of materials has a great influence on the fatigue performance. In order to ensure the surface quality, the surface layer of 0.1 mm is peeled for materials with special requirements. Eddy current inspection is used for defects with depth of 0.5mm. The surface roughness can be reduced to RA = 6.5 ~ 3.4 μ m by electrolytic grinding.

5. The development of electroplated steel wire in special circumstances, in addition to spring characteristics, it also requires additional properties such as corrosion resistance and conductivity, which are mostly solved by electroplating process.
The corrosion resistance of some stainless steel wires and Piano steel wires is equivalent to that of zinc plating. If another layer of ZnAl (5%) alloy is plated, the corrosion resistance can be increased by about 3 times.
For stainless steel wire or piano steel wire with resistance performance requirements, copper plating can be carried out for steel wire with diameter less than 0.4mm, copper can be used for steel wire larger than 0.4mm, and stainless steel material can be used outside. Generally, the conductivity of piano steel wire can be improved by plating 5 μ m Ni.
Generally speaking, the fatigue strength can be improved by the process of surface hardening to form residual stress, such as shot peening and surface nitriding. At present, non electrolytic Ni plating is being studied. 7% P can be precipitated as PNI by heating (300-500 ℃), and Vickers hardness can be increased to HV500. After shot peening, the hardness can be increased by 10% if Ni is heated below 300 ℃.

6. Development of shape memory alloy (SMA), 50ti-50ni is the most promising one in spring. The spring made of shape memory alloy can stretch and stretch under the action of temperature. It is mainly used in the control system of constant temperature, constant load and constant deformation. Because the actuator is driven by spring expansion and contraction, the working stress of spring changes greatly.

7. The application of ceramics, which has high elastic modulus and low fracture strength, is suitable for places with little change. At present, we are developing ceramics with heat resistance, wear resistance and good insulation, and superplastic zinc alloy (SPZ) with high strength at room temperature. In addition, there is high strength silicon nitride, which can withstand high temperature, up to 1000 ℃. However, ceramic spring is not suitable for working under impact load.

8. Application of fiber reinforced plastics in springs glass fiber reinforced plastic (GFRP) plate springs have been widely used in Britain, the United States and Japan. Besides the transverse suspension, it can also be used in special light vehicles, such as the longitudinal suspension of racing cars. At present, carbon fiber reinforced plastic (CFRP) suspension spring has been successfully developed, which is 20% lighter than metal leaf spring.

3、 Development of spring processing technology
At present, the processing equipment and production line of mechanical spring are developing to the depth and breadth of numerical control (NC) and computer control (CNC). However, with the change of spring material and geometry, the processing technology has also developed.

1) The suspension spring with variable outer diameter, variable pitch and variable steel wire diameter (three variable) can be machined without molding. Since the development of three variable spring, tapered steel bar has been used for winding on CNC lathe, but the price of finished product is not ideal. Now it is necessary to control the roll speed and drawing force to obtain the required cone shape through the spring coiling machine under the heating state, and quench it with the processing waste heat.
2) The hollow stable spring rod is made of low carbon boron steel plate, rolled and welded.
3) The torsion bar is made of 45 steel with high purity and high frequency quenching to obtain high hardness and large residual compressive stress on the surface, so as to improve the fatigue life and anti relaxation ability.
4) The leaf spring widely used in electronic products is basically formed by stamping and automatic bending. At present, the joint technology of composite materials is mainly developed.