Q2. What are the drawbacks of the MIG welding process?
The MIG welding process is largely used in joining metals, though it presents several drawbacks related to porosity, hot cracking, strength reduction, distortion and residual stresses, particularly in welding aluminium alloys.
Q3. What is the effect of the FS on the welds?
The fatigue behaviour of MIG welds and FS post-processed welds is largely affected by residual stresses in the transversal direction because they are applied in the same direction as fatigue loading.
Q4. How many passes were used to process the MIG fusion zone?
In order to avoid many passes for processing the entire MIG fusion zone, the overlap ratio (OR), as defined by equation 1 [13], was set to 0 with a pin of 4 mm diameter, larger than the tool diameter used for the MIG-R+FSP series.pd lOR −=1 (1)Where dp is the pin diameter and l is the distance between the pin axis in two successive passes.
Q5. What is the main reason why the weld toes are critical?
Although the weld toes are critical zones in terms of fatigue strength, due to stress concentration, the lack of wetting may not be particularly detrimental if parallel to loading direction.
Q6. Why was the pin depth set to 2.9 mm?
Due to the finite stiffness of the Cincinnati milling machine, the apparent depth of pin penetration was set to 2.9 mm in order to obtain an actual depth of 2.5 mm.
Q7. What is the main mechanism of hardening for this alloy?
The completion of post-processing does not alter the hardness of MIG welds significantly, because the main mechanism of hardening for this aluminium alloy is not grain refinement or plastic deformation, but the presence of hardening precipitates, already dissolved by MIG welding.
Q8. How can the microstructure of the processed zone be controlled?
the microstructure and mechanical properties of the processed zone can be controlled by optimizing the tool design, FSP parameters and active cooling/heating [5,6].
Q9. What is the nomenclature for MIG welds without reinforcement?
The nomenclature MIG_R+FSP applies to the MIG welds with Reinforcement and post-processed by FSP, while the nomenclature MIG_NR+FSP applies to the MIG welds without reinforcement (NonReinforced) and post-processed by FSP.
Q10. What is the reason for the increase in fatigue strength?
it can be concluded that the increase of about 25% observed in fatigue strength was largely due to the improvement of microstructure, the reduction or removal of MIG welding defects, and a possible decrease in the residual stress field.
Q11. What is the effect of the FS processing on the weld strength?
The increase in fatigue strength produced by FS processing can be explained by the following: i) change of geometry, especially due to the increase of curvature radius; ii) microstructure modification or porosity elimination; iii) an possible decrease of residual stress.
Q12. What is the average grain size of the welds in the zone?
MIG welds in that zone displayed an average grain size of approximately 100 m,while the grain size after processing was only 5-8 µm.
Q13. What is the effect of the FS processing on the fatigue strength of the welds?
the residual stresses measured at the toes of both MIG reinforced welds (MIG_R) and MIG reinforced welds submitted to FS processing (MIG_R + FSP) were approximately zero (σt = 0), which means they have no influence on the improvement in fatigue behaviour obtained in this study.