Meaning
Mechanical surface treatments that introduce compressive stresses through controlled particle impact enhance the fatigue life of battery cooling systems subjected to thermal cycling. Residual stress peening involves bombarding the surface of a metal part with small spheres of steel, glass or ceramic at high velocities. Each impact creates a tiny dimple and causes the material beneath the surface to push back, creating a layer of compressive stress.
This layer acts as a barrier that prevents the initiation and growth of cracks caused by the repeated expansion and contraction of the battery pack. The process is effective until the compressive layer is worn away or the part is subjected to temperatures high enough to relieve the stress.
Stress Profile
Depth and magnitude of the compressive layer depend on the size, hardness and speed of the peening media. Residual stress peening aims to create a profile where the maximum compression is just below the surface to counteract the tensile stresses that occur during use. This profile is measured using x ray diffraction or hole drilling techniques to ensure it meets the design specifications.
If the peening is too intense, it can cause surface damage that actually shortens the life of the part.
Fatigue Improvement
Cracks are unable to open or propagate in a region that is under compression, which significantly extends the number of cycles a component can survive. Residual stress peening is particularly beneficial for the welded joints of cooling plates where the thermal stresses are highest. This treatment allows for the use of thinner materials while still maintaining the required durability for a ten year vehicle life.
It also improves the resistance of the metal to stress corrosion cracking in humid environments.
Process Parameter
Consistency of the result is managed by controlling the coverage of the surface and the intensity of the media stream. Residual stress peening requires a clean and uniform surface to achieve the best results, so parts are often degreased before treatment. The angle of impact and the duration of the cycle are optimized for the specific geometry of the battery housing.
If the media becomes broken or contaminated, it can introduce sharp edges that create new stress concentrators. This data ensures that the manufacturing process is stable and that every part provides the same level of protection against fatigue failure.
