Why Did the Hardness of a Zinc-Aluminum Alloy Drop After Aging? Missing Solution Treatment and Overaging

A company developed a zinc-aluminum alloy metal cover for phones with a post-heat-treatment hardness requirement of 145 HV or higher. The part received no solution treatment; instead, it was artificially aged at 120–220°C for 12 h. The as-cast hardness was 147–148 HV0.2, already above the lower limit, but after 188°C×12 h the hardness fell to 89.5–96.5 HV0.2 and the batch was rejected.

The hardness drop is only the symptom. To find the root cause, it was necessary to confirm the alloy composition, compare microstructures before and after heat treatment, and check the heat-treatment response of the alloy grade. The work followed hardness testing, chemical analysis, metallography, microscopic morphology, and heat-treatment verification.

1. Hardness Testing

Method: Sections cut from parts before and after heat treatment were tested for hardness.

Findings:

• Before heat treatment: 147–148 HV0.2, slightly above the 145 HV lower limit.
• After 188°C×12 h: 89.5–96.5 HV0.2, below the lower limit.

Conclusion: The difference is large and cannot be attributed to single-point scatter. However, low hardness is only the result; whether the material itself is abnormal needs further confirmation.

2. Chemical Composition Analysis

Method: A sulfur/carbon analyzer and ICP-OES were used to determine the composition, which was compared with the standard composition of a ZA50 zinc-aluminum alloy.

Findings: The main elements were aluminum, zinc, copper, silicon, and iron. No carbon or sulfur was detected. The composition was basically consistent with the ZA50 series.

Conclusion: The material belongs to the ZA50 zinc-aluminum alloy family. Because the composition matches a mature grade, the existing microstructure and heat-treatment rules of this system can be used to explain the behavior without starting from scratch.

3. Metallographic Examination

Method: Samples before and after heat treatment were examined metallographically and the microstructures were compared.

Findings:

• Before heat treatment: dendritic cast structure was visible. The structure consisted of primary α phase, (α+β) eutectoid between α phases, and secondary phase particles. Many granular eutectoid structures had precipitated inside the primary α phase.
• After heat treatment (188°C×12 h): dendritic morphology disappeared, precipitates increased and changed shape, and particles agglomerated and began to coarsen.

Conclusion: The abundant granular eutectoid before heat treatment indicates a slow casting cooling rate. The microstructure after heat treatment is a typical overaged structure. The problem most likely lies in excessive heat treatment rather than insufficient heat treatment.

4. SEM and EDS Analysis

Method: Scanning electron microscopy (SEM) was used to observe the morphology of samples in both conditions, and energy-dispersive spectroscopy (EDS) was performed on characteristic micro-regions to identify phase compositions.

Findings:

• Before heat treatment: lamellar eutectoid structures precipitated at grain boundaries. EDS showed that gray and black micro-regions were zinc-rich aluminum solid solutions, while white micro-regions were eutectoid structures precipitated at grain boundaries and inside primary α grains, consistent with metallographic results.
• After heat treatment: lamellar eutectoid structures were no longer visible, and white particles at grain boundaries agglomerated and grew. Eutectoid structures changed from fine lamellae to coarse particles. Fluorine in the spectra came from HF in the metallographic etchant and is not part of the material composition.

Conclusion: As strengthening elements such as silicon, copper, and manganese precipitated and alloy phases agglomerated, matrix hardness dropped and overaging occurred. Iron precipitated in needle form; at low levels it has little effect on properties.

5. Heat-Treatment Performance Analysis

Method: The test results were compared with the metallurgical rules of the ZA50 system.

Findings: The product was a ZA50-type alloy with slightly increased aluminum and silicon. Higher aluminum improves mechanical properties; adding 2–8% silicon greatly improves wear resistance and long-term dimensional stability. The as-cast hardness in a metal mold was already 147–148 HV, above the lower limit.

Conclusion: Proper heat treatment can stabilize the microstructure and improve dimensional stability. However, under overaging conditions, strength and hardness decrease as aging temperature and time increase, while elongation after fracture and impact toughness increase.

6. Heat-Treatment Verification Tests

Method: Samples without heat treatment and with different heat treatments were tested. Five process groups were run, two samples per group, and hardness was measured 48 h after heat treatment.

Findings:

• Aging only without solution treatment: 200°C×1.5 h gave 90–106.2 HV0.2, still below 145 HV.
• Solution treatment + water quench: 179.3 HV0.2 and 184.2 HV0.2.
• Solution treatment + air cool: 174.9 HV0.2 and 175.3 HV0.2.
• Solution treatment + water quench + 100°C×50 min: peak hardness of 191.6 HV0.2.
• Solution treatment + water quench + 100°C×70 min: hardness fell back to 184.7 HV0.2.

Conclusion: Artificial aging for 12 h without solution treatment produces overaging even at 120–188°C, causing a significant hardness drop. Both temperature and time are critical aging parameters.

7. Metallographic Re-check After Verification

Method: Samples from the verification tests were examined metallographically again to correlate microstructural changes with hardness data.

Findings:

• Solution treated + water quenched, unaged: few precipitates at grain boundaries; primary α phase was grayish-white with little intragranular precipitation.
• Solution treated + air cooled, unaged: primary α phase was gray; grain-boundary precipitates did not increase obviously, similar to the water-quenched condition.
• Solution treated + water quenched + 100°C×50 min: same grain-boundary precipitates as the unaged water-quenched condition; primary α phase was grayish-white with fine intragranular precipitates.
• Solution treated + water quenched + 100°C×70 min: black secondary phases at grain boundaries began to agglomerate, and intragranular precipitates increased noticeably.

Conclusion: Hardness and microstructure match: peak hardness corresponds to fine intragranular precipitation, while the hardness drop corresponds to secondary-phase agglomeration. Phenomenon, data, and microstructure are mutually consistent.

Root Cause and Recommendations

The composition matched the ZA50 system and the as-cast hardness of 147–148 HV0.2 was already above the requirement, so material selection was not the cause. The real reason was twofold: first, solution treatment was omitted, so strengthening elements did not fully dissolve into the matrix and the amount of strengthening phase available for precipitation during aging was limited; second, the production aging parameters of 120–220°C×12 h far exceeded the peak window for this alloy and pushed it directly into the overaging region. Precipitates agglomerated, grew, and coarsened, dispersion strengthening failed, and hardness dropped. The peak hardness window for this alloy is solution treatment followed by 100°C×50 min, reaching 191.6 HV0.2.

For production recovery and future control:

• Map the aging curve with small samples first. This alloy has a low-temperature, short-time peak, which is the opposite of the conventional “high temperature, long time” rule for non-ferrous metals. Run several groups to locate the peak and the allowable process window.
• Do not skip solution treatment. Without it, adjusting aging parameters alone cannot make up the hardness gap. Cooling method can also be verified: in this case, solution treatment + water quench gave about 179–184 HV0.2, and air cool gave about 175 HV0.2.
• Clarify the purpose of heat treatment. As-cast hardness was already ≥145 HV, so heat treatment is for microstructure and property control, not “higher temperature and longer time always give higher hardness.”
• Rejected batches can be reprocessed. Parts below 145 HV can be re-treated by solution treatment plus artificial aging. When the solution microstructure is good, aging time can be extended to improve overall properties while maintaining hardness.

Disclaimer: This case has been desensitized. All data come from real laboratory analysis. Reproduction without permission is prohibited.