7075高強鋁合金構件冷成形強化機制研究

陳慶洋，龐秋，胡志力,1b，劉祥

7075高強鋁合金構件冷成形強化機制研究

陳慶洋1a，龐秋2*，胡志力1a,1b，劉祥3

（1.武漢理工大學 a.現(xiàn)代汽車零部件技術湖北省重點實驗室 b.汽車零部件技術湖北省協(xié)同創(chuàng)新中心 武漢 430070；2.武漢東湖學院 機電工程學院，武漢 430212；3.東實（武漢）實業(yè)有限公司，武漢 430000）

針對7075高強鋁合金構件在固溶-淬火-時效處理過程中成形精度低的問題，提出了7075高強鋁合金預強化冷成形工藝，研究7075高強鋁合金構件冷成形強化機制?；诟邚婁X合金短流程高性能成形技術，經(jīng)過固溶-時效處理，獲得預強化處理的7075鋁合金板料，使用預強化處理的7075鋁合金板料冷成形試制帽形梁。通過拉伸試驗、杯突試驗測試預強化處理的7075鋁合金板料及帽形梁力學性能，并通過透射電子顯微鏡試驗解釋7075高強鋁合金構件冷成形強化機制。預強化處理的7075鋁合金板料抗拉強度為540 MPa，延伸率為19.3%，強度接近7075鋁合金T6態(tài)強度水平，塑性接近7075鋁合金O態(tài)塑性水平。杯突值為16.6 mm，達到7075鋁合金O態(tài)的87%。使用預強化處理的7075鋁合金板料冷成形試制的帽形梁表面質量良好，無破裂等情況。經(jīng)過烤漆工藝后，帽形梁抗拉強度為（560±5）MPa，屈服強度為（480±5）MPa，與7075高強鋁合金T6態(tài)強度相當。預強化處理的7075鋁合金板料基體內(nèi)部存在大量GP Ⅱ區(qū)組織，這有助于提高7075高強鋁合金的強度和塑性。使用預強化處理的7075鋁合金板料冷成形試制的帽形梁在烤漆工藝處理時，基體中部分GP Ⅱ區(qū)會轉變?yōu)棣?相，析出相的轉變和加工硬化的結合提高了成形構件的強度，使其強度可以達到7075高強鋁合金T6態(tài)強度水平。

7075鋁合金；預強化成形工藝；冷成形；析出相轉變；加工硬化

7075高強鋁合金因其卓越的強度重量比、耐腐蝕性、熱處理性能而被廣泛應用于航空航天工業(yè)和汽車工業(yè)等多個領域[1-3]。7075高強鋁合金構件大多應用于強度要求較高的位置，因此常常需要固溶-時效處理以提高成形構件的強度和硬度[4-5]。在7075高強鋁合金沖壓成形過程中常常會引入殘余應力，固溶、時效等熱處理工藝有助于釋放殘余應力，防止成形構件出現(xiàn)開裂等情況[6-7]。然而，對7075高強鋁合金構件進行熱處理不可避免地會增加工藝流程，導致生產(chǎn)成本增高和生產(chǎn)效率降低，并且熱處理可能導致成形構件的尺寸變化，使成形精度降低[8-11]。

目前關于7075高強鋁合金成形方法研究最多的是由帝國理工大學林建國院士提出的熱成形-淬火一體化技術，該技術的應用大大提高了高強度鋁合金的成形性能。但是其固溶、時效工藝處理時間較長，導致其工藝流程較長，生產(chǎn)效率較低[12-14]。Sonar等[15]研究發(fā)現(xiàn)，經(jīng)過固溶-淬火處理后，7075鋁合金結構部件內(nèi)部存在較高的殘余應力，在后續(xù)的自然時效過程中結構部件會發(fā)生嚴重的塑性變形。Zbigniew等[16]在研究7075鋁合金W態(tài)成形B柱時發(fā)現(xiàn)，在回彈和減薄情況下，在B柱的底部出現(xiàn)了最大的減薄和應變積累。Jaskiewicz等[17]研究了7075鋁合金板材在100 ℃和150 ℃成形時的U形原件，研究發(fā)現(xiàn)，雖然成形后的U形原件抗拉強度達到540 MPa，但是其幾何形狀偏差很大?；敉麍D等[18]研究了7000系鋁合金成形工藝，研究發(fā)現(xiàn)，W態(tài)和O態(tài)的7000系鋁合金板料相較于T6態(tài)的7×××系鋁合金板料具備更優(yōu)良的室溫成形性。雖然W態(tài)和O態(tài)板料等軟態(tài)成形可以保證室溫成形性的要求，但是成形后會有回彈現(xiàn)象，并且其成形后的構件強度較低，往往需要后續(xù)的熱處理提高強度[19]。

通過武漢理工大學華林教授團隊提出的高強鋁合金短流程高性能成形技術（Pre-hardening Forming，PHF）可以獲得一種高性能預強化鋁合金[20-21]，該合金同時具備優(yōu)良的塑性和強度。通過該技術溫成形得到的鋁合金構件無須后續(xù)熱處理，強度即可達到T6態(tài)水平，能夠在保證成形精度的同時，大大提高生產(chǎn)效率。通過該技術得到的預強化鋁合金大多應用于溫成形和熱成形上，而在冷成形上的應用研究較少。因此，本文以7075高強鋁合金為研究對象，將PHF技術應用到7075高強鋁合金冷成形工藝中，研究7075高強鋁合金預強化冷成形工藝，以期為鋁合金冷成形工藝的優(yōu)化和應用提供參考。

1 試驗

選取1.5 mm厚的7075鋁合金板料為研究對象，其化學成分如表1所示。

表1 7075鋁合金化學成分

Tab.1 Chemical composition of 7075 aluminum alloy wt.%

裁取合適尺寸的7075鋁合金板料，經(jīng)過固溶-時效處理后，獲得預強化處理的7075鋁合金板料，其工藝示意圖如圖1a所示。沿著預強化處理的7075鋁合金板料軋制方向選取3個拉伸試樣和3個杯突試樣，拉伸試樣尺寸如圖1b所示。為了進行對照，分別對7075鋁合金進行退火和T6態(tài)處理，以獲得O態(tài)和T6態(tài)的7075鋁合金板料，同樣選取拉伸試樣和杯突試樣。拉伸試驗在MMS-200型萬能試驗機上進行，其試驗速率為0.001 s?1。杯突試驗在Erichsen試驗機上進行，其壓邊力為250 kN，試驗速率為5 mm/min。拉伸試驗和杯突試驗結果均取平均值。

帽形梁是汽車B柱中最復雜的結構，為了驗證7075鋁合金預強化冷成形工藝的可行性，使用預強化處理的7075鋁合金板料冷沖壓試制帽形梁，冷沖壓試驗在DP36-250H型沖壓機上進行，其凹、凸模結構如圖2所示。對成形后的帽形梁進行烤漆工藝處理以模擬汽車零部件生產(chǎn)過程，并測試烤漆后帽形梁的結構強度。

2 結果與分析

2.1 預強化處理的7075鋁合金力學性能

不同狀態(tài)下板料的應力-應變曲線如圖3a所示。預強化（Pre-hardening，PH）處理的7075鋁合金板料在室溫條件下具備優(yōu)良的強度和塑性，其抗拉強度可達540 MPa，接近7075高強鋁合金T6態(tài)強度水平，遠高于7075高強鋁合金O態(tài)強度。預強化處理的7075鋁合金板料的延伸率可達19.3%，遠高于7075高強鋁合金T6態(tài)的12%，僅比O態(tài)的延伸率低2.4%。杯突試驗結果如圖3b所示，可知，預強化處理的7075鋁合金板料杯突值相當于7075高強鋁合金O態(tài)的87%，而T6態(tài)的杯突值僅為O態(tài)的53%。預強化處理的7075鋁合金在保證接近7075高強鋁合金T6態(tài)強度的同時，其室溫成形性遠高于T6態(tài)的。

圖1 7075鋁合金預強化冷成形工藝示意圖（a）和拉伸試樣（b）

圖2 帽形梁沖壓模具結構

圖3 不同狀態(tài)下板料的力學性能和成形性

2.2 成形構件力學性能

上述試驗結果表明，預強化處理的7075鋁合金在室溫下具備優(yōu)良的強度和塑性，為了進一步驗證7075鋁合金預強化冷成形工藝在實際生產(chǎn)中的可行性，本文通過預強化處理的7075鋁合金板料冷沖壓試驗，成功試制了汽車B柱中最復雜的部分——帽形梁，其表面質量良好，無任何裂紋等缺陷，如圖4所示。

圖4 帽形梁結構

在汽車實際生產(chǎn)過程中，烤漆工藝有助于提高汽車部件的整體質量、耐久性等，是必不可少的一個關鍵步驟。為模擬實際生產(chǎn)過程，對冷成形的帽形梁進行烤漆工藝處理，并在帽形梁不同位置選取拉伸試樣測試其結構強度。烤漆工藝處理后的帽形梁不同部位的室溫拉伸結果如圖5所示。

從室溫拉伸結果可知，經(jīng)過烤漆工藝處理的帽形梁各部位抗拉強度達到（560±5）MPa，屈服強度可達（480±5）MPa，這與7075高強鋁合金T6態(tài)的強度水平相當。

圖5 烤漆后帽形梁不同部位的室溫拉伸曲線

2.3 7075高強鋁合金冷成形強化機制

7075鋁合金進行T6處理會增加η'相等強化相在基體中的析出[22]，如圖6a所示，析出相提高了鋁合金強度，但同時析出相作為位錯運動的障礙，限制了基體的塑性變形，導致T6態(tài)7075鋁合金在室溫下強度高，但室溫成形性差。而經(jīng)過預強化處理的7075鋁合金內(nèi)部組織主要為GP Ⅱ區(qū)，如圖6b所示，GP Ⅱ區(qū)是鋁合金析出相成核過程中的中間階段。GP Ⅱ區(qū)可在一定程度上阻礙晶體的位錯運動，它與位錯的相互作用阻礙了塑性變形的進行，從而提高了預強化處理的7075鋁合金強度。同時，由于GP Ⅱ區(qū)是析出相成核的中間階段，它可以允許一定程度的位錯運動。因此，相較于η'相等強化相對位錯的阻礙作用，GP Ⅱ區(qū)的阻礙作用較小，這使得預強化處理的7075鋁合金具備良好的塑性[23]。

圖6 透射電鏡圖

成形構件烘烤后的透射電鏡圖如圖7所示。預強化處理的7075鋁合金板料內(nèi)部主要強化相為GP Ⅱ區(qū)，由圖7可知，經(jīng)過短時烘烤的帽形梁的內(nèi)部出現(xiàn)了強化相η′相[24-25]，這說明在烘烤過程中，部分GP Ⅱ區(qū)轉變?yōu)棣恰湎?，析出相的轉變和加工硬化的結合使烤漆后的帽形梁具備與7075鋁合金T6態(tài)相當?shù)膹姸萚26]。因其冷沖壓成形過程中的變形量較大，帽形梁2、3、5、6位置的塑性較低。雖然這些位置因其變形量大而具備較大的位錯密度，但是在短時烘烤過程中，其內(nèi)部的細小析出物尺寸變大，導致其阻礙位錯的能力降低，因此，這些大變形量位置的強度和小變形位置的強度相當。

圖7 成形構件烘烤后的透射電鏡圖

3 結論

1）經(jīng)過預強化處理的7075鋁合金板料抗拉強度為540 MPa，延伸率為19.3%，強度接近7075高強鋁合金T6態(tài)水平，延伸率接近7075高強鋁合金O態(tài)水平，并且其杯突值可達7075高強鋁合金O態(tài)的87%。預強化處理的7075鋁合金板料內(nèi)部強化相主要為GP Ⅱ區(qū)，這使其具有良好的力學性能。

2）使用預強化處理的7075鋁合金板料冷成形試制的帽形梁的表面質量良好，無任何破裂情況。經(jīng)過烤漆工藝處理后，帽形梁的抗拉強度達到（560±5）MPa，屈服強度達到（480±5）MPa，這與7075高強鋁合金T6態(tài)強度水平相當。

3）經(jīng)過烤漆處理后，帽形梁的內(nèi)部部分強化相從GP Ⅱ區(qū)轉變?yōu)棣恰湎啵龀鱿嗟霓D變和加工硬化的結合使帽形梁具備與7075高強鋁合金T6態(tài)相當?shù)膹姸取?/p>

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Cold Forming Strengthening Mechanism of 7075 High-strength Aluminum Alloy Components

CHEN Qingyang1a, PANG Qiu2*, HU Zhili1a,1b, LIU Xiang3

(1.a. Hubei Key Laboratory of Advanced Technology of Automobile Components, b. Hubei Collaborative Innovation Center for Automotive Components Technology, Wuhan University of Technology, Wuhan 430070, China; 2. School of Mechanical and Electrical Engineering, Wuhan Donghu University, Wuhan 430212, China; 3. Dongshi (Wuhan) Auto Parts Co., Ltd., Wuhan 430000, China)

High-strength aluminum alloy components, such as those made of 7075 alloy, typically undergo solution treatment, quenching, and aging to enhance their strength and other properties. In response to the challenge of reduced forming precision during the heat treatment process for 7075 high-strength aluminum alloy components, a pre-strengthening cold forming process for 7075 high-strength aluminum alloy is proposed. The work aims to study the strengthening mechanisms involved in the cold forming of 7075 high-strength aluminum alloy components. By pre-hardening forming technique for high-strength aluminum alloys, the pre-hardened 7075 aluminum alloy sheets were obtained after solution treatment and aging. A cap-shaped beam was fabricated by the pre-hardening cold forming process for 7075 high-strength aluminum alloy. Mechanical properties of the pre-hardened 7075 aluminum alloy sheets and the cap-shaped beam were evaluated through tensile tests and cupping tests. Additionally, the strengthening mechanisms involved in the cold forming of 7075 high-strength aluminum alloy components were elucidated based on the results from transmission electron microscopy experiments. The pre-hardened 7075 aluminum alloy sheet exhibited a tensile strength of 540 MPa and an elongation percentage of 19.3%, with strength levels close to those of 7075 aluminum alloy in the T6 temper, and plasticity comparable to that of 7075 aluminum alloy in the O temper. The cupping value was measured at 16.6 mm, reaching 87% of the cupping value for 7075 aluminum alloy in the O temper. The cap-shaped beam produced by the pre-hardening cold forming process for 7075 aluminum alloy showed excellent surface quality, with no signs of fractures. After the painting process, the cap-shaped beam demonstrated a tensile strength of (560±5) MPa and a yield strength of (480±5) MPa, comparable to the strength of 7075 high-strength aluminum alloy in the T6 temper. The pre-hardened 7075 aluminum alloy sheet contains a significant amount of GP Ⅱ zone structure within the matrix. This structure contributes to enhancing the strength and plasticity of 7075 high-strength aluminum alloy. The cap-shaped beam fabricated by the pre-hardening cold forming process for 7075 high-strength aluminum alloy exhibits a transformation of some GP Ⅱ zones within the matrix into η' phase when treated in painting process. The combination of phase precipitation transformation and work hardening elevates the strength of the formed component, allowing it to achieve strength levels equivalent to 7075 high-strength aluminum alloy in the T6 temper.

7075 aluminum alloy; pre-hardening forming process; cold forming; phase precipitation transformation; work hardening

10.3969/j.issn.1674-6457.2024.03.016

TG166.3

A

1674-6457(2024)03-0152-07

2024-01-07

2024-01-07

國家自然科學基金（52075400，52275368）；湖北省重點研發(fā)計劃（2021BAA200）；湖北省科技重大專項（2022AAA 001）；湖北省自然科學基金（2023AFA069）

The National Natural Science Foundation of China (52075400, 52275368); The Key Research and Development Program of Hubei Province (2021BAA200); Major Program of Hubei Province (2022AAA001); Hubei Provincial Natural Science Foundation of China (2023AFA069)

陳慶洋, 龐秋, 胡志力, 等. 7075高強鋁合金構件冷成形強化機制研究[J]. 精密成形工程, 2024, 16(3): 152-158.

CHEN Qingyang, PANG Qiu, HU Zhili, et al. Cold Forming Strengthening Mechanism of 7075 High-strength Aluminum Alloy Components[J]. Journal of Netshape Forming Engineering, 2024, 16(3): 152-158.

（Corresponding author）