高強(qiáng)7050鋁合金超低溫大變形加工與組織、性能調(diào)控

高強(qiáng)7050鋁合金超低溫大變形加工與組織、性能調(diào)控Cryogenic Processing High-Strength 7050 Aluminum Alloy and the Controlling of the Microstructures and Mechanical Properties

通過低溫冷卻+軋制變形的方法研究了高強(qiáng)7050鋁合金的低溫塑性變形及其對合金組織性能的影響。結(jié)果表明，高強(qiáng)7050鋁合金經(jīng)液氮冷卻預(yù)處理后可實(shí)現(xiàn)與溫/熱軋相比較高的軋制變形加工量，并產(chǎn)生大量亞結(jié)構(gòu)和高密度位錯，使合金顯著強(qiáng)化，其中低溫下的高變形能力主要與合金在低溫下具有高的加工硬化能力密切相關(guān)，而強(qiáng)度提升主要來自于固溶強(qiáng)化和變形位錯強(qiáng)化的貢獻(xiàn)。雖然超低溫變形能夠明顯加快淬火態(tài)高強(qiáng)7050鋁合金的時效進(jìn)程，但直接時效處理可使超低溫變形態(tài)7050鋁合金保有較高的強(qiáng)度和一定延伸率，其中析出和位錯強(qiáng)化是其強(qiáng)化主因，而時效引起的回復(fù)和強(qiáng)化相析出共同促進(jìn)延伸率的改善。淬火態(tài)高強(qiáng)7050鋁合金在室溫變形過程中，形變熱引起基體中析出的溶質(zhì)原子團(tuán)簇（或GP區(qū)）和η′相與變形位錯發(fā)生交互作用，導(dǎo)致大量剪切帶（失穩(wěn)區(qū)）形成，從而易引發(fā)軋板開裂或邊裂，而超低溫變形過程中溶質(zhì)擴(kuò)散受阻以致強(qiáng)化相析出被抑制，從而明顯降低了剪切失穩(wěn)區(qū)的發(fā)生，使合金能夠獲得均勻、穩(wěn)定的塑性變形或良好的加工硬化，確保獲得較高質(zhì)量的超低溫軋板。高強(qiáng)鋁合金在低溫下所表現(xiàn)出來的優(yōu)異塑性變形和加工能力有望成為改善高強(qiáng)鋁合金難變形加工的有效途徑。

The high strength or flow stress as well as low plastic deformability of 7000 series Al alloys makes it difficult to improve their microstructures and mechanical properties by cold processing. But advanced alloying methods and processing technologies are continually developed to obtain higher mechanical properties and acceptable elongation for these high-strength Al alloys. The present studies used the cryogenic deformation (rolling) to processing high-strength 7050 Al alloys and studied its effects on the microstructures and mechanical properties. It is found that after the pre-cooling with liquid nitrogen, the quenched high-strength 7050 Al alloy can obtain much higher rolling reduction, similar to that under warm or hot rolling, and great number of substructures and high-density dislocations are formed that greatly increased the strength. The higher cryogenic deformability would be mainly related with the higher work-hardening ability at low temperature while the strength enhancement would be largely contributed by the solution and dislocation strengthening. The cryogenic deformation can obviously stimulate the ageing process of the quenched 7050 Al alloy, but the direct aging of the cryogenic-rolling 7050 Al alloy can assure higher strength and acceptable elongation, which would be greatly contributed by the precipitation and dislocation strengthening while the recovery and ageing-induced precipitates help improve the elongation. During room-temperature rolling the quenched 7050 Al alloy, the formation of GP zones and η′ phases caused by the heats transformed from the deformation as well as their interaction with dislocations leads to the appearance of amounts of shear bands (instability areas), which will easily cause the cracking or edge-cracking of the rolling sheets. However, the cryogenic rolling with distinctly impeding the solute diffusion results in the suppression of the precipitation of the strengthening phases so as to decrease the occurrence of the shear instability areas, and uniform and stable plastic deformation or good work-hardening as well as high-quality rolling sheets are obtained. Thus, the excellent plastic deformability of high-strength Al alloys at cryogenic temperatures could be suggested as an effective way to improve the processing of high-strength Al alloys.

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