7.1 Conclusions
A component-level study has been performed to investigate the effect of warm forming on formability and springback behavior on aluminum alloy brazing sheet widely used in the automotive heat exchanger industry. The brazing sheet consisted of AA3003 core and AA4045 clad layers and manufactured to O-, H22- and H24-temper conditions. The following conclusions were derived from the results of this research:
1. Vickers hardness tests were performed after heating samples blanks to temperatures ranging from 25 to 350⁰C for pre-selected heating time of 40 seconds. The results show that there was no permanent reduction in strength (hardness) with increasing temperature.
2. Twist compression tests were performed to characterize the tribological behavior at elevated temperatures. It was found that the coefficient of friction (COF) is a strong function of temperature. For dry tests, the core side (AA3003) had significantly higher COF compared to the clad side (AA4045). Little difference was observed between the two layers using lubricant.
3. The extended Nadai model [32] was able to capture the negative hardening effect observed for harder temper materials at elevated temperatures.
4. Four different tooling configurations (TC1 to TC4) were developed to improve the formability and wrinkling behavior in the warm forming process.
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b. A two-stage forming operation was implemented for TC2 to pre-form the I/O port region to promote a more uniform strain distribution to prevent strain localization in the second stage. The part displayed severe wrinkling.
c. TC3 included the optimized I/O port geometry and channel geometry in the first stage to increase formability and reduce wrinkling. Formability was improved but the design was ineffective in terms of reducing wrinkling behavior. An undesirable snap-through buckling effect was also introduced.
d. The TC4 (single-stage forming) introduced geometric modifications to the I/O port- cooling channel intersection and a developed blank was used to optimize the material flow into the I/O port feature while still maintaining a sufficient flange area for brazing. These changes significantly improved formability.
e. Multi-stage forming was quite effective in increasing formability but introduced part defects such as wrinkling and snap-through buckling.
f. Use of lubrication was essential to promote a successful forming operation, especially at elevated temperatures due to increase in COF
g. The punch velocity had minor adverse effects on formability
h. The benefit of warm forming on formability was not clearly observed as fracture and localization occurred at elevated temperatures
5. The springback behavior was characterized by studying a wide range of experimental parameters. Overall, warm forming was very effective in reducing springback. The results show that:
a. Harder temper conditions exhibit higher springback. H24 springback was more than two-fold greater than that of O-temper.
b. No beneficial effect of temperature was observed in terms of reducing springback for O-temper parts.
c. Significant reduction in springback was observed for H22 and H24 with increases in temperature. The maximum reductions occurred at 325⁰C (51% and 68% respectively). d. The use of lubricant increased springback slightly (up to 15% at 350⁰C).
e. An increase in punch speed reduced the benefit of warm forming. f. Initial coil set did not have an effect on springback.
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g. Sheet direction did not have an effect on springback.
h. Sheet orientation had considerable effect on springback due to the strength and thermal expansion differences between the two layers. The inverted sheet orientation had 29% lower springback compared to the conventional sheet orientation (clad layer up). i. Punch load increase from 10 kN to 80 kN and holding time (2 to 30 seconds) had
significant impact on springback (up to 88% reduction observed at 325⁰C for 30 second holding time and 54% at 250C for 2 second holding time for H24-temper).
6. Springback simulations that take the thermal expansion and strength gradient and through- thickness compression into account were developed. This was accomplished by discretizing the mesh with a combination of solid and shell elements. The simulation results successfully predicted the qualitative trend in the experiment with satisfactory match to the measured data in regards to the temperature effects. The model was not able to accurately capture the effect of lubricant and punch load.
7.2 Recommendations
The following recommendations are made for future work:
1. Material characterization should be expanded to include higher temperature conditions (up to 350⁰C). More advanced material models that considers texture-based anisotropy and kinematic hardening (Bauschinger effect) should be considered.
2. The effect of through-thickness compression was underestimated in the numerical simulations. The cause for the underestimation should be identified to accurately capture the compression effect that has significant impact on springback. The blank mesh should be refined to contain more elements in the through-thickness direction to better capture the residual stress gradient.
3. Sheet thinning should be measured at critical locations to correlate forming temperature, punch load, and lubrication to formability quantitatively.
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4. The effect of warm forming and lubricants on brazeability should be assessed to ensure the brazing performance is not hindered. Furthermore, the maximum allowable springback for the parts to be brazed successfully should be established.
5. The reduction in die clearance at elevated temperatures due to thermal expansion could have negative impact on warm formability; thus further investigation is needed.
6. Different friction characterization tests capable of measuring the coefficient of friction at very low sliding distance should be explored.
7. The mechanical behavior of the clad layer should be obtained with more reliable method such as tensile testing.
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