陳慶洋，龐秋，胡志力,1b，劉祥

7075高強(qiáng)鋁合金構(gòu)件冷成形強(qiáng)化機(jī)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(xué) a.現(xiàn)代汽車(chē)零部件技術(shù)湖北省重點(diǎn)實(shí)驗(yàn)室 b.汽車(chē)零部件技術(shù)湖北省協(xié)同創(chuàng)新中心 武漢 430070；2.武漢東湖學(xué)院 機(jī)電工程學(xué)院，武漢 430212；3.東實(shí)（武漢）實(shí)業(yè)有限公司，武漢 430000）

針對(duì)7075高強(qiáng)鋁合金構(gòu)件在固溶-淬火-時(shí)效處理過(guò)程中成形精度低的問(wèn)題，提出了7075高強(qiáng)鋁合金預(yù)強(qiáng)化冷成形工藝，研究7075高強(qiáng)鋁合金構(gòu)件冷成形強(qiáng)化機(jī)制?；诟邚?qiáng)鋁合金短流程高性能成形技術(shù)，經(jīng)過(guò)固溶-時(shí)效處理，獲得預(yù)強(qiáng)化處理的7075鋁合金板料，使用預(yù)強(qiáng)化處理的7075鋁合金板料冷成形試制帽形梁。通過(guò)拉伸試驗(yàn)、杯突試驗(yàn)測(cè)試預(yù)強(qiáng)化處理的7075鋁合金板料及帽形梁力學(xué)性能，并通過(guò)透射電子顯微鏡試驗(yàn)解釋7075高強(qiáng)鋁合金構(gòu)件冷成形強(qiáng)化機(jī)制。預(yù)強(qiáng)化處理的7075鋁合金板料抗拉強(qiáng)度為540 MPa，延伸率為19.3%，強(qiáng)度接近7075鋁合金T6態(tài)強(qiáng)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(dá)到7075鋁合金O態(tài)的87%。使用預(yù)強(qiáng)化處理的7075鋁合金板料冷成形試制的帽形梁表面質(zhì)量良好，無(wú)破裂等情況。經(jīng)過(guò)烤漆工藝后，帽形梁抗拉強(qiáng)度為（560±5）MPa，屈服強(qiáng)度為（480±5）MPa，與7075高強(qiáng)鋁合金T6態(tài)強(qiáng)度相當(dāng)。預(yù)強(qiáng)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(qiáng)鋁合金的強(qiáng)度和塑性。使用預(yù)強(qiáng)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(shí)，基體中部分GP Ⅱ區(qū)會(huì)轉(zhuǎn)變?yōu)棣?相，析出相的轉(zhuǎn)變和加工硬化的結(jié)合提高了成形構(gòu)件的強(qiáng)度，使其強(qiáng)度可以達(dá)到7075高強(qiáng)鋁合金T6態(tài)強(qiáng)度水平。

7075鋁合金；預(yù)強(qiáng)化成形工藝；冷成形；析出相轉(zhuǎn)變；加工硬化

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

目前關(guān)于7075高強(qiáng)鋁合金成形方法研究最多的是由帝國(guó)理工大學(xué)林建國(guó)院士提出的熱成形-淬火一體化技術(shù)，該技術(shù)的應(yīng)用大大提高了高強(qiáng)度鋁合金的成形性能。但是其固溶、時(shí)效工藝處理時(shí)間較長(zhǎng)，導(dǎo)致其工藝流程較長(zhǎng)，生產(chǎn)效率較低[12-14]。Sonar等[15]研究發(fā)現(xiàn)，經(jīng)過(guò)固溶-淬火處理后，7075鋁合金結(jié)構(gòu)部件內(nèi)部存在較高的殘余應(yīng)力，在后續(xù)的自然時(shí)效過(guò)程中結(jié)構(gòu)部件會(huì)發(fā)生嚴(yán)重的塑性變形。Zbigniew等[16]在研究7075鋁合金W態(tài)成形B柱時(shí)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(yīng)變積累。Jaskiewicz等[17]研究了7075鋁合金板材在100 ℃和150 ℃成形時(shí)的U形原件，研究發(fā)現(xiàn)，雖然成形后的U形原件抗拉強(qiáng)度達(dá)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(huì)有回彈現(xiàn)象，并且其成形后的構(gòu)件強(qiáng)度較低，往往需要后續(xù)的熱處理提高強(qiáng)度[19]。

通過(guò)武漢理工大學(xué)華林教授團(tuán)隊(duì)提出的高強(qiáng)鋁合金短流程高性能成形技術(shù)（Pre-hardening Forming，PHF）可以獲得一種高性能預(yù)強(qiáng)化鋁合金[20-21]，該合金同時(shí)具備優(yōu)良的塑性和強(qiáng)度。通過(guò)該技術(shù)溫成形得到的鋁合金構(gòu)件無(wú)須后續(xù)熱處理，強(qiáng)度即可達(dá)到T6態(tài)水平，能夠在保證成形精度的同時(shí)，大大提高生產(chǎn)效率。通過(guò)該技術(shù)得到的預(yù)強(qiáng)化鋁合金大多應(yīng)用于溫成形和熱成形上，而在冷成形上的應(yīng)用研究較少。因此，本文以7075高強(qiáng)鋁合金為研究對(duì)象，將PHF技術(shù)應(yīng)用到7075高強(qiáng)鋁合金冷成形工藝中，研究7075高強(qiáng)鋁合金預(yù)強(qiáng)化冷成形工藝，以期為鋁合金冷成形工藝的優(yōu)化和應(yīng)用提供參考。

1 試驗(yàn)

選取1.5 mm厚的7075鋁合金板料為研究對(duì)象，其化學(xué)成分如表1所示。

表1 7075鋁合金化學(xué)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(guò)固溶-時(shí)效處理后，獲得預(yù)強(qiáng)化處理的7075鋁合金板料，其工藝示意圖如圖1a所示。沿著預(yù)強(qiáng)化處理的7075鋁合金板料軋制方向選取3個(gè)拉伸試樣和3個(gè)杯突試樣，拉伸試樣尺寸如圖1b所示。為了進(jìn)行對(duì)照，分別對(duì)7075鋁合金進(jìn)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(yàn)在MMS-200型萬(wàn)能試驗(yàn)機(jī)上進(jìn)行，其試驗(yàn)速率為0.001 s?1。杯突試驗(yàn)在Erichsen試驗(yàn)機(jī)上進(jìn)行，其壓邊力為250 kN，試驗(yàn)速率為5 mm/min。拉伸試驗(yàn)和杯突試驗(yàn)結(jié)果均取平均值。

帽形梁是汽車(chē)B柱中最復(fù)雜的結(jié)構(gòu)，為了驗(yàn)證7075鋁合金預(yù)強(qiáng)化冷成形工藝的可行性，使用預(yù)強(qiáng)化處理的7075鋁合金板料冷沖壓試制帽形梁，冷沖壓試驗(yàn)在DP36-250H型沖壓機(jī)上進(jìn)行，其凹、凸模結(jié)構(gòu)如圖2所示。對(duì)成形后的帽形梁進(jìn)行烤漆工藝處理以模擬汽車(chē)零部件生產(chǎn)過(guò)程，并測(cè)試烤漆后帽形梁的結(jié)構(gòu)強(qiáng)度。

2 結(jié)果與分析

2.1 預(yù)強(qiáng)化處理的7075鋁合金力學(xué)性能

不同狀態(tài)下板料的應(yīng)力-應(yīng)變曲線如圖3a所示。預(yù)強(qiáng)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(qiáng)度和塑性，其抗拉強(qiáng)度可達(dá)540 MPa，接近7075高強(qiáng)鋁合金T6態(tài)強(qiáng)度水平，遠(yuǎn)高于7075高強(qiáng)鋁合金O態(tài)強(qiáng)度。預(yù)強(qiáng)化處理的7075鋁合金板料的延伸率可達(dá)19.3%，遠(yuǎn)高于7075高強(qiáng)鋁合金T6態(tài)的12%，僅比O態(tài)的延伸率低2.4%。杯突試驗(yàn)結(jié)果如圖3b所示，可知，預(yù)強(qiáng)化處理的7075鋁合金板料杯突值相當(dāng)于7075高強(qiáng)鋁合金O態(tài)的87%，而T6態(tài)的杯突值僅為O態(tài)的53%。預(yù)強(qiáng)化處理的7075鋁合金在保證接近7075高強(qiáng)鋁合金T6態(tài)強(qiáng)度的同時(shí)，其室溫成形性遠(yuǎn)高于T6態(tài)的。

圖1 7075鋁合金預(yù)強(qiáng)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(jié)構(gòu)

圖3 不同狀態(tài)下板料的力學(xué)性能和成形性

2.2 成形構(gòu)件力學(xué)性能

上述試驗(yàn)結(jié)果表明，預(yù)強(qiáng)化處理的7075鋁合金在室溫下具備優(yōu)良的強(qiáng)度和塑性，為了進(jìn)一步驗(yàn)證7075鋁合金預(yù)強(qiáng)化冷成形工藝在實(shí)際生產(chǎn)中的可行性，本文通過(guò)預(yù)強(qiáng)化處理的7075鋁合金板料冷沖壓試驗(yàn)，成功試制了汽車(chē)B柱中最復(fù)雜的部分——帽形梁，其表面質(zhì)量良好，無(wú)任何裂紋等缺陷，如圖4所示。

圖4 帽形梁結(jié)構(gòu)

在汽車(chē)實(shí)際生產(chǎn)過(guò)程中，烤漆工藝有助于提高汽車(chē)部件的整體質(zhì)量、耐久性等，是必不可少的一個(gè)關(guān)鍵步驟。為模擬實(shí)際生產(chǎn)過(guò)程，對(duì)冷成形的帽形梁進(jìn)行烤漆工藝處理，并在帽形梁不同位置選取拉伸試樣測(cè)試其結(jié)構(gòu)強(qiáng)度?？酒峁に囂幚砗蟮拿毙瘟翰煌课坏氖覝乩旖Y(jié)果如圖5所示。

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

圖5 烤漆后帽形梁不同部位的室溫拉伸曲線

2.3 7075高強(qiáng)鋁合金冷成形強(qiáng)化機(jī)制

7075鋁合金進(jìn)行T6處理會(huì)增加η'相等強(qiáng)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(qiáng)度，但同時(shí)析出相作為位錯(cuò)運(yùn)動(dòng)的障礙，限制了基體的塑性變形，導(dǎo)致T6態(tài)7075鋁合金在室溫下強(qiáng)度高，但室溫成形性差。而經(jīng)過(guò)預(yù)強(qiáng)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(guò)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(cuò)運(yùn)動(dòng)，它與位錯(cuò)的相互作用阻礙了塑性變形的進(jìn)行，從而提高了預(yù)強(qiáng)化處理的7075鋁合金強(qiáng)度。同時(shí)，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(cuò)運(yùn)動(dòng)。因此，相較于η'相等強(qiáng)化相對(duì)位錯(cuò)的阻礙作用，GP Ⅱ區(qū)的阻礙作用較小，這使得預(yù)強(qiáng)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(gòu)件烘烤后的透射電鏡圖如圖7所示。預(yù)強(qiáng)化處理的7075鋁合金板料內(nèi)部主要強(qiáng)化相為GP Ⅱ區(qū)，由圖7可知，經(jīng)過(guò)短時(shí)烘烤的帽形梁的內(nèi)部出現(xiàn)了強(qiáng)化相η′相[24-25]，這說(shuō)明在烘烤過(guò)程中，部分GP Ⅱ區(qū)轉(zhuǎn)變?yōu)棣恰湎?，析出相的轉(zhuǎn)變和加工硬化的結(jié)合使烤漆后的帽形梁具備與7075鋁合金T6態(tài)相當(dāng)?shù)膹?qiáng)度[26]。因其冷沖壓成形過(guò)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(cuò)密度，但是在短時(shí)烘烤過(guò)程中，其內(nèi)部的細(xì)小析出物尺寸變大，導(dǎo)致其阻礙位錯(cuò)的能力降低，因此，這些大變形量位置的強(qiáng)度和小變形位置的強(qiáng)度相當(dāng)。

圖7 成形構(gòu)件烘烤后的透射電鏡圖

3 結(jié)論

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

2）使用預(yù)強(qiáng)化處理的7075鋁合金板料冷成形試制的帽形梁的表面質(zhì)量良好，無(wú)任何破裂情況。經(jīng)過(guò)烤漆工藝處理后，帽形梁的抗拉強(qiáng)度達(dá)到（560±5）MPa，屈服強(qiáng)度達(dá)到（480±5）MPa，這與7075高強(qiáng)鋁合金T6態(tài)強(qiáng)度水平相當(dāng)。

3）經(jīng)過(guò)烤漆處理后，帽形梁的內(nèi)部部分強(qiáng)化相從GP Ⅱ區(qū)轉(zhuǎn)變?yōu)棣恰湎?，析出相的轉(zhuǎn)變和加工硬化的結(jié)合使帽形梁具備與7075高強(qiáng)鋁合金T6態(tài)相當(dāng)?shù)膹?qiáng)度。

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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(guó)家自然科學(xué)基金（52075400，52275368）；湖北省重點(diǎn)研發(fā)計(jì)劃（2021BAA200）；湖北省科技重大專(zhuān)項(xiàng)（2022AAA 001）；湖北省自然科學(xué)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(qiáng)鋁合金構(gòu)件冷成形強(qiáng)化機(jī)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）