A few days ago, when I was producing titanium chopping boards for a customer, after the laser cutting was completed, the titanium chopping boards were all bent after being left overnight. When I looked at these bent titanium chopping boards, my first reaction was residual stress.
What is residual stress? The official definition of residual stress is: the self-balanced internal stress that remains in the object after the external force or uneven temperature field is eliminated. Mechanical processing and strengthening processes can cause residual stress. For example, cold drawing, laser cutting, cutting, rolling, shot peening, casting, forging, welding and metal heat treatment, etc., may cause residual stress due to uneven plastic deformation or phase change.
Is this definition not very clear? Let's explain it in common language. It's not very precise, but it's very vivid. Just like when we use rubber bands. When we let go of the stretched rubber band, it will shrink back, and when it shrinks back, it will twist. When metal materials are forced to deform during processing, "resistance force" will also be generated inside. Even if the external force disappears, this "resistance force" will still remain inside the material, forming an invisible "stress field".
Where does residual stress come from? There are three main reasons:
1. Mechanical processing: When the turning tool cuts the metal, the part that is cut off releases stress, but the remaining part is forced to "twist".
2. The "ice and fire" of alternating hot and cold during the processing. For example: the high temperature zone and low temperature zone of the weld during welding. Like the road surface expanded by the sun, the surrounding low temperature zone is like a water pipe that shrinks in the cold winter, and the competition between hot and cold produces stress.
3. The "transformation sequelae" caused by the phase change stress of metal materials. When the metal cools, the organization changes, and the volume changes are not synchronized, causing deformation.
There are three main reasons for the destructive power of residual stress:
1. It causes changes in the size and shape of objects.
2. It shortens the service life of parts.
3. It reduces the corrosion resistance and fatigue strength of metals.
How can residual stress be eliminated? There are three most commonly used methods:
1. Natural aging: placing metal workpieces in a natural environment, and releasing internal stress naturally through temperature changes and climate effects. It is mainly used to improve the dimensional stability of materials.
2. Heat treatment: It is to eliminate or reduce residual stress by using the thermal relaxation effect of residual stress. It is generally treated by annealing, tempering, etc.
3. Vibration aging: It refers to a method commonly used to eliminate residual internal stress in engineering materials. It is through vibration that the vector sum of the residual internal stress and the additional vibration stress in the workpiece exceeds the yield strength of the material, causing the material to undergo a slight plastic deformation, thereby relaxing and reducing the internal stress in the material.
Residual stress is an inevitable phenomenon in mechanical manufacturing, which has an important impact on the size, life, plasticity and corrosion resistance of the product. Therefore, how to reduce the residual stress of the product in the process of mechanical manufacturing requires attention from the entire process of product design, manufacturing and production, and take corresponding measures to control and eliminate it. Mastering the law of residual stress is the basic requirement for producing a high-quality product.
Nicole
Company: Baoji Jimiyun Dynamic Co., Ltd
Country:China
Add:Baoti road,Jintai,Baoji city,Shaanxi,China
Cel:+86 13369210920
Gmail:nicole@jmyunti.com
Website:www.jm-titanium.com
