Mr Raghvendra Gopal
Founder & Director, Materials info consultancy private limited
Quality Control Aluminum Alloy Casting: Failure Investigation and Root Cause Analysis
Testing failure of housing assembly was observed at client end, as leakage was seen from tube joint during vibration testing. Similar leakage observed at customer test after approx. 390 h. Tube was found to come out of the mixer body in some cases.
The assembly was found to fail during leakage test conducted after the temperature conditioning at 150 °C for 48 hours , though o.k. initially at room temperature. Once failure was noticed during the leakage test, even after cooling to the room temperature still leakage was observed from the joint when tested.
- 1. Based on literature analysis, since aluminum and silicon have different coefficients of thermal expansion, as the molten mass solidifies during cooling, besides the usual macroscopic residual stress, additional residual stresses develop at the microscopic level
- 2. These residual stresses may get relieved during subsequent process or any other test condition, thus affecting the dimensional stability.
- 3. To confirm the same, one local casting was subjected to the stabilization temperature of 200 °C for 48 hours including heating, time by which casting achieves the temperature, soaking and then cooling as per the test arrangement provided by the client. Only the temperature has been kept 200 °C.
- 4. The casting bore was analyzed before the stabilization cycle for CMM.
- 5. After the stabilization treatment, again the bore was checked for CMM and was found to be more. This indicates that there is an increase in bore ID after the stabilization cycle.
- 6. The microstructure was analyzed for the local casting, imported casting, local casting after stabilization and local casting found OK and failed in test.
See the Microstructure Test Results in this Image Gallery:
- The microstructure of these casting alloys contains α-Al dendrites as the main constituent, which is present with eutectic Si particles and many intermetallic phases such as Al2Cu, Mg2Si, Fe-bearing phases etc.
- The size, morphology and distribution of microstructural features govern the mechanical properties of these alloys. It is well-known that a refined microstructure results in improved tensile properties.
- The microstructure of the Al-Si cast alloys primarily consists of a primary phase (α-Al) and eutectic mixture of Al-Si. The amount of eutectic mixture in the microstructure depends on the level Si. The eutectic mixture contains soft Al as matrix containing Si particles.
- Secondary Dendrite Arm Spacing (SDAS) is one of the most important microstructural features in dendritic solidification of alloys (for e.g. Al-Si alloys) during the casting process. SDAS has a significant influence on the mechanical behavior of the cast aluminum components.
- As we can see microstructure, imported components show a finer Alpha alumnium or finer secondary dendritic arm spacing than local components.
- The stabilization treatment makes the structure finer for Alpha aluminum . This is close to the imported component.
- The structure is coarser in a component failed during leakage test. The passed component also has a coarser structure.
- Very little work is done on stabilization of high pressure die cast aluminum silicon alloys which was attempted in the present work. This was done in order to relieve the residual stresses.
- The stabilization treatment resulted in increase in bore diameter which was studied using CMM. This could be the reason for the failure during leakage test after conditioning.
- The microstructure shows refined primary Alpha after the stabilization treatment which is closer to the imported component.
- The level of residual stress in imported component could be less than locally cast components. This can be further verified using residual stress measurements if required.
Recommendations and Further Work:
- 1. Treatment at 200 °C for 48 hours before final machining and bore grinding.
- 2. This exercise shall be taken up at supplier end followed by the testing for leakage and validation.
- 3. Once validated, the same shall be incorporated in the component drawing and supplier process documents.