引言(yan)
TC4(Ti-6Al-4V)钛合金(jin)综合(he)性能优异,已(yi)广(guang)泛(fan)应用(yong)于(yu)航(hang)空(kong)航(hang)天(tian)、海洋工程(cheng)和(he)生(sheng)物(wu)医疗(liao)等(deng)领域,是(shi)目(mu)前应(ying)用(yong)最(zui)广泛的(de)钛(tai)合(he)金[1-3],其占钛(tai)合金(jin)应(ying)用(yong)总(zong)量(liang)的(de)50%以(yi)上[4]。TC4钛(tai)合(he)金属(shu)于(yu)典型(xing)的难加工(gong)材(cai)料(liao),其(qi)成形(xing)困难(nan),常(chang)采(cai)用热(re)加工方(fang)式(shi)获(huo)取半(ban)成(cheng)品(pin)或(huo)成品(pin)[5-6]。常(chang)见的(de)钛(tai)合金(jin)热加(jia)工(gong)方(fang)式(shi)包(bao)括(kuo):锻造(zao)、轧(ya)制及挤(ji)压等(deng),其(qi)中(zhong)轧(ya)制是高效(xiao)生(sheng)产(chan)钛合(he)金(jin)板(ban)、带、管(guan)及棒(bang)等的主(zhu)要(yao)方(fang)式(shi)。当(dang)前(qian),较多学者(zhe)基于理论(lun)分析、实验研究及(ji)有(you)限(xian)元模拟(ni)研究了TC4钛(tai)合金(jin)热变形(xing)行(xing)为、组织演变及工(gong)艺参(can)数影响,但(dan)研(yan)究(jiu)工(gong)艺多(duo)集(ji)中(zhong)在锻造领域。针(zhen)对TC4钛(tai)合(he)金热轧(ya)制的研(yan)究较(jiao)离(li)散(san),关(guan)于(yu)TC4钛合(he)金(jin)热轧(ya)制工艺对(dui)轧(ya)件组织(zhi)、性能及(ji)缺(que)陷(xian)等(deng)研(yan)究也亟需系统(tong)性(xing)的梳(shu)理。
因(yin)此(ci),本(ben)文以TC4钛(tai)合金(jin)为(wei)研(yan)究对象(xiang),综合评述(shu)了(le)该(gai)合(he)金在(zai)热(re)轧(ya)过(guo)程(cheng)中(zhong)组(zu)织(zhi)演变(bian)及(ji)性能(neng)控制机(ji)理(li),展望了(le)该(gai)合(he)金热(re)轧(ya)制技(ji)术(shu)的发(fa)展(zhan)方向(xiang)。首先基于TC4钛合金轧件特(te)点,分别阐(chan)述了板(ban)带箔及(ji)管(guan)棒(bang)材(cai)轧制的工艺(yi)特(te)点。归(gui)纳了该(gai)合(he)金(jin)热(re)轧(ya)时(shi)组(zu)织演(yan)变机理,考(kao)虑(lv)了(le)轧(ya)制工(gong)艺(yi)参(can)数(shu)对(dui)组织和性(xing)能(neng)的显(xian)著(zhu)影响(xiang),同(tong)时关注了TC4钛(tai)合(he)金(jin)热(re)轧时(shi)缺(que)陷形成机(ji)理,进而论(lun)述(shu)了(le)数值(zhi)模(mo)拟在热轧(ya)工(gong)艺(yi)研究中(zhong)的重(zhong)要(yao)应(ying)用(yong)。
1、TC4钛合金(jin)热(re)轧件(jian)分类及加(jia)工(gong)工艺
基于(yu)TC4钛(tai)合(he)金热轧(ya)件特(te)点(dian),将(jiang)该合(he)金(jin)半(ban)成(cheng)品(pin)或(huo)成(cheng)品(pin)分为(wei)两大类(lei):板带箔(bo)材和(he)管(guan)棒材,分别归(gui)纳两大类轧(ya)件(jian)的(de)热(re)轧(ya)工艺(yi)技术特(te)点和应用(yong)优势。
1.1板带(dai)箔轧制
TC4钛合金(jin)热(re)轧加(jia)工可以(yi)获(huo)得各种板(ban)带箔材(cai)产品,以下介绍几种(zhong)常(chang)见的板带箔热轧(ya)工艺:单向(xiang)轧(ya)制、交(jiao)叉(cha)轧(ya)制、异步(bu)轧制和包(bao)覆(fu)叠(die)轧。
1.1.1单(dan)向(xiang)轧(ya)制和(he)交(jiao)叉轧(ya)制
常见的(de)单(dan)向轧(ya)制(zhi)(图1a)是沿着(zhe)板材(cai)的(de)一(yi)个方(fang)向完(wan)成多道(dao)次(ci)轧(ya)制工(gong)艺,单(dan)向(xiang)轧制(zhi)的轧件(jian)由(you)于(yu)晶粒的择(ze)优(you)取(qu)向(xiang)容易(yi)形成基(ji)面(mian)织(zhi)构,导(dao)致板材呈(cheng)现(xian)强(qiang)烈(lie)的各(ge)向异(yi)性[7-8]。交(jiao)叉(cha)轧制(zhi)如(ru)图(tu)1b所示,在(zai)每(mei)道次(ci)轧(ya)制完(wan)之(zhi)后(hou)改(gai)变轧(ya)向(xiang)90°进(jin)行间隔(ge)轧制,或是(shi)在(zai)完(wan)成(cheng)一(yi)个方向的多(duo)道(dao)次轧制后转(zhuan)向(xiang)90°再进行(xing)一轮(lun)轧制[9]。多项(xiang)实验(yan)[10-12]证(zheng)明交叉(cha)轧(ya)制(zhi)工艺相(xiang)较于(yu)单向(xiang)轧制能(neng)够(gou)有(you)效得(de)弱(ruo)化(hua)钛材轧制织构(gou),显著改(gai)善(shan)合(he)金的各向(xiang)异性(xing)。
赵(zhao)帅(shuai)[11]研(yan)究了(le)单(dan)向轧制(zhi)和交叉轧制(zhi)两种(zhong)热轧(ya)方(fang)式(shi)对(dui)TC4钛合(he)金(jin)显(xian)微组(zu)织(zhi)和(he)力学性(xing)能的影(ying)响。结(jie)果表明(ming),单向轧(ya)制(图(tu)2a)下钛(tai)合(he)金(jin)板材(cai)在(zai)<0001>方向(xiang)呈(cheng)现择优(you)取(qu)向(xiang),表(biao)现(xian)出(chu)强烈(lie)的各(ge)向(xiang)异性(xing);交(jiao)叉(cha)轧制(zhi)(图2b)后(hou)板材晶(jing)粒(li)取(qu)向(xiang)更均匀,其(qi)各(ge)向(xiang)异(yi)性明显降(jiang)低(di)。虽然交叉一(yi)次(ci)后TC4钛(tai)合(he)金力(li)学(xue)性能略(lve)有下(xia)降(jiang),但当交(jiao)叉次数(shu)达到两(liang)次时,该合金性(xing)能得到(dao)改(gai)善。RD方向(xiang)的抗拉(la)强(qiang)度较(jiao)接近(jin)TD方向,而(er)TD方向的(de)屈服(fu)强(qiang)度(du)明(ming)显高(gao)于RD方(fang)向(xiang)。虽(sui)然文献[11]中反映(ying)了交叉轧制和单向轧(ya)制的(de)异同点,但其忽(hu)略了实(shi)际(ji)生产中(zhong),尤(you)其是多火次(ci)轧(ya)制成形(xing)时(shi),交(jiao)叉次数(shu)多高(gao)于(yu)两(liang)次(ci),应(ying)当(dang)进一步提升(sheng)交叉(cha)换(huan)向的(de)次数(shu),研(yan)究其组(zu)织和性能演(yan)变的(de)规律(lv)。
1.1.2异(yi)步轧(ya)制
轧制(zhi)力(li)不对等是异(yi)步(bu)轧制最显(xian)著的(de)特(te)征(zheng)[13],上(shang)下工(gong)作(zuo)辊(gun)表面(mian)线速(su)度(du)或(huo)辊(gun)径(jing)不(bu)等,在变(bian)形区(qu)会形(xing)成(cheng)搓轧区,从(cong)而(er)增(zeng)加变(bian)形(xing)区域的剪(jian)切变(bian)形(xing)量。根(gen)据穿带形(xing)式(shi)的(de)不同(tong),异(yi)步(bu)轧制(zhi)常分(fen)为4种(zhong),如图(tu)3所示。
异(yi)步轧(ya)制所需(xu)要的(de)轧制(zhi)力(li)明显(xian)低于同步(bu)轧(ya)制[14],在轧制精度(du)上(shang)也(ye)远(yuan)远高(gao)于同(tong)步(bu)轧(ya)制,适(shi)用于(yu)板(ban)带(dai),尤其是极薄(bao)带材的生(sheng)产(chan)。异步轧(ya)制(zhi)中(zhong)的(de)剪(jian)切应变更(geng)容易激活TC4钛合金(jin)潜在的(de)滑(hua)移系(xi),从而有利(li)于改(gai)善合(he)金(jin)的(de)塑(su)性(xing)变形(xing)能(neng)力,同(tong)时(shi)剪切应(ying)变(bian)有利于晶(jing)粒破碎(sui)获(huo)得细(xi)晶粒(li),从(cong)而(er)通过晶(jing)粒(li)细化的方式提(ti)高合金(jin)的力学(xue)性能(neng)[15]。
LIJP等(deng)[12]研究(jiu)了(le)不同(tong)辊速比(bi)的异(yi)步(bu)热(re)轧对(dui)TC4钛合(he)金组(zu)织和力(li)学(xue)性能的(de)影(ying)响(xiang),结(jie)果表(biao)明辊(gun)速(su)比(bi)为(wei)1.0时,轧后(hou)板(ban)材(cai)晶粒(li)呈(cheng)带状(zhuang),并平行于(yu)轧(ya)制(zhi)方向(xiang),如图4a所示(shi);当辊(gun)速(su)比(bi)提(ti)高至1.1和(he)1.2时(shi),晶粒呈(cheng)等轴状(zhuang),为(wei)典型(xing)的再(zai)结晶晶粒形(xing)貌(mao),如图(tu)4b和4c所示。此(ci)外(wai),白小雷等(deng)[16]采用(yong)与(yu)LIJP等[12]相同的辊(gun)速比进行(xing)异步轧制,发(fa)现随着辊(gun)速(su)比的(de)增加,晶粒变(bian)得更加均(jun)匀(yun)细小(xiao),性能显(xian)著增(zeng)加,如(ru)图4d所示(shi)。综(zong)合(he)考(kao)虑,TC4钛合金采用辊速(su)比(bi)为1.1时的(de)异步轧制(zhi),其组(zu)织(zhi)与性能(neng)匹配相(xiang)对(dui)最优(you)。
1.1.3包覆(fu)叠轧(ya)
包(bao)覆(fu)叠(die)轧(ya)是(shi)将(jiang)单层或多(duo)层轧(ya)制(zhi)板(ban)材(cai)用(yong)同种(zhong)或(huo)其(qi)他金(jin)属(shu)包覆(图(tu)5),通过热(re)轧(ya)加工(gong)成(cheng)薄(bao)板(ban)的轧(ya)制(zhi)工艺[17]。该(gai)轧(ya)制方(fang)式可以(yi)有(you)效改善(shan)钛合金板材加(jia)工(gong)时(shi)受力(li)不均的现(xian)象,提高轧件(jian)的火(huo)次(ci)加工效(xiao)率(lv)和(he)组(zu)织(zhi)性(xing)能(neng)[18]。TC4钛合金在一火(huo)或(huo)多(duo)火(huo)大压(ya)下(xia)量热(re)轧(ya)时(shi),易出现(xian)边(bian)裂等质(zhi)量(liang)缺(que)陷,而(er)采(cai)用包覆(fu)叠(die)轧(ya)工(gong)艺能(neng)有效(xiao)减(jian)弱边(bian)裂(lie)现象(xiang),从(cong)而(er)提高(gao)产(chan)品质量(liang)。通常(chang),采(cai)用(yong)钢板或纯钛(tai)板作(zuo)为(wei)TC4钛合(he)金(jin)包覆叠轧(ya)的(de)包(bao)覆(fu)层(ceng)[19]。
张(zhang)国霞(xia)等[20]通(tong)过(guo)3种不(bu)同轧制方式获得(de)TC4钛(tai)合金(jin)薄板,并(bing)研(yan)究了(le)其室(shi)/高温性能。与另(ling)外(wai)两(liang)种轧(ya)制方式(shi)对比(bi),经(jing)包覆(fu)钢板(ban)多(duo)片(pian)叠轧(ya)的板(ban)材组(zu)织更(geng)加细匀(yun),如图(tu)6所(suo)示(shi),其(qi)室(shi)/高(gao)温性能(neng)明(ming)显优(you)异(yi)。尤其(qi)是,该方式下(xia)所获得(de)的(de)TC4钛(tai)合(he)金(jin)板材在(zai)920℃下进行(xing)高温拉伸时(shi),可(ke)进入(ru)超塑(su)性(xing)状态(tai),伸(shen)长(zhang)率(lv)可(ke)达1120%。因(yin)此(ci),实际(ji)生(sheng)产中(zhong)可以(yi)尝(chang)试(shi)借(jie)鉴超(chao)塑(su)性成形(xing)生(sheng)产(chan)TC4钛合(he)金薄板,不(bu)仅可避(bi)免(mian)钛(tai)合(he)金板材(cai)开裂(lie)情(qing)况(kuang),而(er)且能(neng)获得组织(zhi)和性能匹配性较(jiao)优的产(chan)品。
1.2管(guan)棒(bang)材(cai)轧(ya)制
与钢铁等(deng)金属(shu)材料相比,钛(tai)合(he)金耐蚀性(xing)较(jiao)强,在石油开(kai)采(cai)、盐碱(jian)工业(ye)、海洋工(gong)程(cheng)等(deng)领域已(yi)受到(dao)广泛(fan)应用[21]。考虑(lv)产(chan)品(pin)服役周期(qi)和(he)经济(ji)价(jia)值,钛(tai)合金(jin)管具(ju)有(you)更(geng)高的应(ying)用(yong)价值,如国内外已(yi)在(zai)石油(you)开(kai)采(cai)等(deng)领(ling)域(yu)使用(yong)TC4钛(tai)合(he)金(jin)无(wu)缝(feng)管(guan)。钻孔(kong)挤(ji)压(ya)和斜(xie)轧穿孔(kong)是生产钛合(he)金无缝管(guan)的主(zhu)要(yao)方(fang)式(shi)[22]。李(li)宝(bao)霞(xia)等(deng)[23]通过(guo)斜轧穿(chuan)孔+热(re)轧的方式生(sheng)产大规(gui)格TC4钛(tai)合金管材,如(ru)图7所(suo)示(shi)。其(qi)中,观(guan)察距离外(wai)管壁20~25mm处(chu)纵(zong)向和(he)横向断面(mian)组(zu)织,发(fa)现管材不同(tong)方(fang)向组织均(jun)匀,热轧组织是网篮(lan)组(zu)织与(yu)魏氏体结合(he)体(ti)。通常,相较(jiao)于(yu)挤压(ya)+机械加工(gong)生产大规格钛(tai)合金(jin)管(guan)材(cai),采(cai)用斜轧穿孔(kong)+热轧的方式可以大(da)幅(fu)度提(ti)高材料(liao)的(de)利(li)用率。
传(chuan)统横列(lie)式(shi)轧机生(sheng)产的(de)钛(tai)合金(jin)棒材成(cheng)品质(zhi)量和(he)规格(ge)有限,生产(chan)效率偏(pian)低(di)[24]。为(wei)了稳定(ding)产(chan)品质(zhi)量和(he)加(jia)速(su)产(chan)品(pin)生产(chan)周(zhou)期(qi),国(guo)内(nei)外目前普遍(bian)采(cai)用(yong)热(re)连轧(ya)方式(shi)来生(sheng)产(chan)小规格钛(tai)合(he)金棒(bang)材(cai),采(cai)用(yong)大(da)变(bian)形量(liang)加工(gong)技术制备钛(tai)及(ji)钛合金(jin)棒(bang)材(cai)也(ye)是(shi)实(shi)际生(sheng)产中(zhong)采(cai)用(yong)最(zui)多(duo)的(de)办法(fa),在保障材料力(li)学(xue)性(xing)能(neng)的(de)前(qian)提(ti)下,尽(jin)可(ke)能(neng)采用(yong)大(da)的(de)道次(ci)变形量(liang)制(zhi)备(bei)钛的(de)棒材(cai),从(cong)而(er)减(jian)少加工道次,提高材(cai)料利(li)用(yong)率,降低钛(tai)合金棒(bang)材的(de)加(jia)工(gong)成本(ben)。黄帆等(deng)[24]对(dui)热连轧机组中的(de)预精轧(ya)机组(zu)孔型(xing)系统(tong)进(jin)行了(le)优化设计,优(you)化(hua)孔(kong)型较原(yuan)孔型(xing)生产的TC4钛合金(jin)热(re)连轧棒(bang)材显(xian)微组织(zhi)更加(jia)均(jun)匀、晶(jing)粒尺度更(geng)细(xi)小(xiao),力(li)学性能(neng)优势(shi)明(ming)显。LOPATINNV[25]采用(yong)有限(xian)元(yuan)方法(fa)模(mo)拟和实验(yan)分析,研究了(le)螺旋(xuan)轧制(zhi)对TC4钛(tai)合金显(xian)微(wei)组(zu)织演(yan)变(bian)的影(ying)响,结(jie)果(guo)表(biao)明(ming)球状(zhuang)晶(jing)粒(li)的(de)尺(chi)寸在(zai)表(biao)面点和(he)轴心(xin)点(dian)分别为10和(he)13μm左右,加工(gong)后的棒(bang)材中(zhong)晶(jing)粒尺寸(cun)明显增加,球(qiu)化(hua)过(guo)程(cheng)与(yu)晶(jing)粒增(zeng)长同时(shi)发(fa)生(图8)。热(re)连轧(ya)技(ji)术相对(dui)于(yu)传(chuan)统(tong)横(heng)列(lie)式(shi)轧机具(ju)有(you)多方(fang)面显著优(you)势,包(bao)括提(ti)高(gao)生产(chan)效率,改(gai)善(shan)产(chan)品质量,减(jian)少(shao)加(jia)工道次,适应多(duo)样(yang)化(hua)需(xu)求以及(ji)节(jie)能(neng)环保。连(lian)续性(xing)的轧制工(gong)艺不(bu)仅(jin)可以缩(suo)短(duan)生产周期(qi),降(jiang)低成(cheng)本(ben),而(er)且可(ke)以(yi)保(bao)持(chi)产品一(yi)致性,减(jian)少(shao)废(fei)品(pin)率,使(shi)其(qi)成(cheng)为制备(bei)小(xiao)规(gui)格钛(tai)合金(jin)棒(bang)材(cai)等领域的首选方(fang)法。
2、TC4钛(tai)合金在(zai)热轧过(guo)程(cheng)中(zhong)的组(zu)织演变(bian)
在TC4钛(tai)合金的热(re)轧(ya)工(gong)艺(yi)中,组织结构(gou)的质量(liang)直接(jie)决定了材(cai)料(liao)的(de)力学性能、耐腐(fu)蚀性(xing)及疲劳寿(shou)命。
因此,深(shen)入(ru)研(yan)究TC4钛(tai)合金(jin)在(zai)热轧(ya)过程(cheng)中(zhong)的(de)组织演(yan)变规(gui)律(lv)尤(you)为重(zhong)要,涵(han)盖了(le)晶粒(li)生(sheng)长、取(qu)向分布和相变等(deng)多个(ge)方面。为了在(zai)热轧(ya)过(guo)程中(zhong)获(huo)得更(geng)高(gao)质(zhi)量(liang)的TC4钛(tai)合(he)金轧件,仍(reng)需(xu)进(jin)一(yi)步(bu)探(tan)索如(ru)何(he)实(shi)现(xian)晶(jing)粒细(xi)化、提(ti)升(sheng)力学(xue)性(xing)能以及有(you)效(xiao)控制相变。
2.1晶(jing)粒细化
金(jin)属(shu)和合(he)金的(de)微观(guan)结构(gou)细(xi)化(hua)是提(ti)高(gao)材(cai)料(liao)综合(he)性(xing)能的(de)有效方法(fa)[26],晶粒尺寸较小的(de)金属(shu)和(he)合(he)金比(bi)粗晶(jing)粒(li)对(dui)应(ying)物(wu)在强(qiang)度和(he)塑(su)韧(ren)性(xing)方面具有(you)明显(xian)优(you)势(shi)[27]。在(zai)超(chao)细晶(jing)粒TC4钛(tai)合金的(de)研究中,剧烈(lie)塑性(xing)变(bian)形(xing)(SeverePlasticDeformation,SPD)技(ji)术已经(jing)成为近十(shi)多(duo)年(nian)的研究热点,但高压扭转[28]、等通(tong)道(dao)角(jiao)挤(ji)压(ya)[29]、多(duo)向锻(duan)造[30]和(he)异步轧制等(deng)SPD技术在(zai)工业(ye)化量产TC4钛合(he)金成(cheng)品件(jian)方(fang)面(mian)仍存(cun)在诸(zhu)多技术挑(tiao)战(zhan)。目前,大(da)变形(xing)轧制仍(reng)然是生(sheng)产超细(xi)晶粒(li)TC4钛(tai)合(he)金(jin)的(de)有效(xiao)工(gong)业(ye)化量(liang)产(chan)手(shou)段(duan)[31],大(da)变形(xing)热轧工艺(yi)在塑(su)造(zao)钛(tai)材(cai)形状的同时(shi),也能有(you)效地细化(hua)晶(jing)粒,研究(jiu)表明(ming)[32]动态再(zai)结晶(DynamicRecrystallization,DRX)在(zai)TC4钛合(he)金晶(jing)粒(li)细化(hua)中发挥(hui)着(zhe)重要(yao)作(zuo)用(yong),动(dong)态(tai)加工过(guo)程(cheng)中,大(da)量(liang)位错在晶(jing)界(jie)处缠(chan)结形(xing)成(cheng)亚晶(jing),从而形(xing)成新的(de)细小晶粒,而连(lian)续动态(tai)再(zai)结晶在(zai)晶(jing)胞内(nei)部通(tong)过形核(he)长(zhang)大(da)形(xing)成(cheng)新(xin)的(de)细(xi)小晶粒,从而(er)达(da)到(dao)细晶强化的(de)效(xiao)果(guo)。WANGX等[33]研(yan)究(jiu)了(le)TC4钛合金(jin)热(re)加工过(guo)程(cheng)中动(dong)态再结晶(jing)对组(zu)织细(xi)化的(de)作(zuo)用。如图9所示(shi),当应变(bian)小于1.01时,α相的形貌(mao)没(mei)有(you)太大变(bian)化(hua),而当应(ying)变(bian)增加(jia)到(dao)2.43时,残留的原(yuan)始α相颗粒呈(cheng)现拉长(zhang)状,再(zai)结(jie)晶(jing)晶粒(li)增(zeng)多(duo)。这(zhe)表明(ming)单一的变形方(fang)式(shi)下,TC4钛合金(jin)虽然(ran)会(hui)发(fa)生(sheng)动态再(zai)结晶(jing),但(dan)微(wei)观(guan)组织(zhi)仍处于(yu)不均匀状(zhuang)态。因(yin)此(ci),在考(kao)虑实(shi)际(ji)生产(chan)时(shi),复杂(za)路(lu)径(jing)下的变(bian)形方(fang)式可能(neng)是促(cu)进晶(jing)粒(li)均(jun)匀细化(hua)的(de)有(you)效方(fang)式。
2.2织构(gou)演(yan)变(bian)
织构特(te)征能(neng)定量反(fan)映(ying)材料在塑性(xing)变形过(guo)程中(zhong)显(xian)微(wei)组织(zhi)结(jie)构(gou)的演(yan)变(bian)规律[34],与(yu)铝(lv)和(he)钢(gang)不同,钛(tai)合(he)金具有(you)明(ming)显的(de)各(ge)向(xiang)异性(xing)[35]。室温下(xia),TC4钛(tai)合金(jin)大部(bu)分(fen)由α相(xiang)(密排六(liu)方(fang),HCP结构(gou))组成(cheng),因而TC4钛(tai)合金各(ge)向异(yi)性(xing)显著(zhu),在(zai)变形(xing)过程中易形成(cheng)织(zhi)构[36],最(zui)常见(jian)的(de)两(liang)类织(zhi)构是(shi)α晶(jing)粒的(de)c轴(zhou)(晶(jing)向(xiang)<0001>)集中(zhong)趋向平(ping)行于(yu)轧(ya)制(zhi)板材横向TD的(de)T型(xing)织(zhi)构(gou)和α晶粒的c轴(zhou)集(ji)中(zhong)趋(qu)向(xiang)平(ping)行(xing)于(yu)轧制(zhi)板(ban)材(cai)法(fa)向(xiang)ND的(de)B型(xing)织构[37]。在(zai)微(wei)观上,当(dang)单独的(de)α晶(jing)粒c轴(zhou)与应(ying)力轴平(ping)行或垂直时(shi),晶(jing)粒(li)内部可(ke)动滑(hua)移系(xi)的施(shi)密特因(yin)子(zi)较小(xiao),此(ci)时晶粒的(de)变形难度(du)最大(da),因(yin)而(er)TC4钛合(he)金在(zai)轧制(zhi)过(guo)程中(zhong)容(rong)易于(yu)形成T型和(he)B型织构(gou)(图10)。
OBASIG等[38]分别在800和950℃温度下对(dui)TC4钛合(he)金(jin)进(jin)行(xing)热轧实(shi)验(yan),研究(jiu)发现(xian)在(zai)这两个温度下(xia)单(dan)向热制过(guo)程(cheng)中(zhong)形成(cheng)的晶体织(zhi)构通常(chang)会产生两种主要(yao)类型:在(zai)800℃轧制(zhi)条(tiao)件下,形成(cheng)Β/T型织构(gou),而(er)950℃时转(zhuan)向T型(xing)织(zhi)构,这(zhe)主要与(yu)两(liang)种(zhong)温(wen)度(du)下TC4钛合(he)金的(de)α和(he)β组(zu)织(zhi)比例不(bu)同有关(guan)。王(wang)伟等(deng)[39]观(guan)察(cha)到类(lei)似(shi)现(xian)象(xiang),当变(bian)形量(liang)增(zeng)加到90%时,TC4钛(tai)合金(jin)随着(zhe)变形(xing)温(wen)度(du)的升高,显(xian)微(wei)组织(zhi)由(you)B织构转(zhuan)化为T织(zhi)构(gou)和(he)锥面织(zhi)构(gou),塑(su)性(xing)变形(xing)由(you)基(ji)面(mian)滑移(yi)转化(hua)为(wei)柱面(mian)滑移,显(xian)微(wei)组织(zhi)中α相(xiang)尺(chi)寸减(jian)小而β相(xiang)含(han)量(liang)增大(da),合(he)金的(de)抗(kang)拉(la)强度(du)和(he)伸(shen)长(zhang)率均(jun)增(zeng)大。SABATR等(deng)[40]通(tong)过研(yan)究(jiu)TC4板材在(zai)600和(he)800℃温(wen)度轧(ya)制过程中(zhong)织构(gou)的演(yan)变机(ji)理,实验将(jiang)4mm厚(hou)的(de)钛板(ban)经过(guo)3次(ci)轧制,变形(xing)量分(fen)别为(wei)50%、70%和90%,图(tu)11中(zhong)ODF图的φ2=0°和(he)30°截(jie)面表明(ming)最(zui)大强(qiang)度(du)的纹(wen)理分量接近(jin)基(ji)底(di)纤(xian)维区域(yu),这些(xie)纹(wen)理(li)沿φ1部(bu)分(fen)延展。在(zai)压下率(lv)为90%时(shi),两种轧制温度(du)下都观察(cha)到(dao)占(zhan)优(you)势的基础(chu)纤(xian)维。然(ran)而,观(guan)察(cha)到(dao)基(ji)础纤维的强度(du)在(zai)较低的轧(ya)制(zhi)温度下(xia)比(bi)在(zai)较(jiao)高的(de)轧(ya)制温度下更高(gao)。一般情(qing)况下,轧(ya)制温(wen)度(du)越低(di),TC4钛合金出现织(zhi)构(gou)聚(ju)集的现象(xiang)越(yue)严(yan)重,而(er)考虑(lv)到(dao)在(zai)实际生产时,终轧(ya)温(wen)度(du)常(chang)超过(guo)800℃,故研究(jiu)高温(wen)状(zhuang)态(tai)下(xia)织(zhi)构演(yan)变(bian)具有(you)更强(qiang)的(de)实际意义。
2.3相(xiang)组分再分(fen)配
TC4钛(tai)合(he)金(jin)热(re)轧(ya)时处于(yu)复(fu)杂的温(wen)度场中,随(sui)着(zhe)轧(ya)制(zhi)的(de)进(jin)行(xing),板(ban)材温度呈降(jiang)低趋势,该(gai)过程(cheng)中(zhong)会从(cong)基(ji)体中(zhong)持续(xu)不断(duan)地析出(chu)次(ci)生(sheng)α相[41],导致相(xiang)组分一(yi)直(zhi)处(chu)于(yu)再(zai)分(fen)配(pei)状(zhuang)态。与体(ti)心(xin)立方结构(gou)的(de)β相(xiang)相比(bi),α相(xiang)是密排六(liu)方结构(gou),塑(su)性(xing)及(ji)变(bian)形(xing)能力较(jiao)弱。故轧(ya)制(zhi)过(guo)程(cheng)更(geng)加困难(nan),容(rong)易(yi)导致(zhi)应(ying)力集(ji)中(zhong)和(he)变形不(bu)均(jun)匀(yun),从(cong)而(er)引发(fa)裂(lie)纹(wen)和其(qi)他(ta)缺陷的(de)形成。TC4钛合(he)金(jin)作(zuo)为双相合(he)金,相(xiang)组(zu)分再分(fen)配对(dui)热(re)轧工(gong)艺过程中合(he)金(jin)组织演变(bian)和(he)力学性(xing)能有着(zhe)重要(yao)的(de)影响。
李(li)瑞等[42]研究了(le)3种不(bu)同(tong)温度下(xia)热(re)轧工艺对TC4ELI钛(tai)合金(jin)板(ban)材(cai)显微组织的影响(xiang)。轧(ya)制温度(du)由高到(dao)低(di)时(shi)TC4ELI钛合(he)金板材(cai)显(xian)微组(zu)织(zhi)分(fen)别是魏氏(shi)组(zu)织、网篮(lan)组织和等(deng)轴组(zu)织,这是(shi)由于(yu)轧制温(wen)度(du)靠近相(xiang)变(bian)点(dian),显(xian)微(wei)组织发(fa)生(sheng)了相(xiang)组(zu)分(fen)再分(fen)配(pei)。SUNSD等(deng)[43]研究(jiu)发现(xian)热加工温度对相组(zu)成存在明(ming)显影(ying)响(xiang)。
从(cong)图12a中(zhong)可以(yi)看出650℃时相(xiang)界仍旧清晰,750℃时位错(cuo)明显(xian)增(zeng)多(duo),在高(gao)温(wen)(950℃)下,通过XRD检测(ce)到TC4钛合(he)金中(zhong)β相含(han)量(liang)较高(gao),表明(ming)TC4钛合金(jin)中(zhong)α相(xiang)中(zhong)元素转(zhuan)移到(dao)β相(xiang)中(zhong),发(fa)生(sheng)相组(zu)分再(zai)分(fen)配。相(xiang)组(zu)分再分(fen)配(pei)的(de)规律(lv)对(dui)生产(chan)TC4钛(tai)合金板材十分重要,尤(you)其是考虑跨(kua)相(xiang)变(bian)点轧制,不(bu)仅(jin)能(neng)获得更加(jia)细小(xiao)的(de)理(li)想组(zu)织(zhi),而(er)且(qie)能(neng)破碎(sui)原始组(zu)织(zhi)。
3、热(re)轧(ya)工艺参数(shu)对TC4钛合(he)金(jin)组(zu)织和(he)性(xing)能的影(ying)响(xiang)
TC4钛(tai)合金(jin)通(tong)过采用(yong)合适(shi)的热轧工艺来控(kong)制(zhi)轧件组(zu)织结构,使轧件达(da)到(dao)理(li)想的(de)强度-塑性-韧性(xing)要(yao)求。通常(chang),由(you)于(yu)热(re)轧温(wen)度(du)、应(ying)变量、应(ying)变速率和冷却速率等(deng)工(gong)艺参数的复(fu)杂(za)影(ying)响(xiang),热(re)轧(ya)过程(cheng)中(zhong)存(cun)在(zai)的加(jia)工硬化(hua)和动态(tai)软(ruan)化(hua)过程(cheng),轧(ya)件(jian)的(de)织构、组(zu)织(zhi)和力学(xue)性(xing)能会发生相应的(de)变(bian)化。
3.1热轧(ya)温(wen)度
从(cong)微观角度来(lai)看(kan),热轧温(wen)度对(dui)TC4钛合(he)金的(de)显微(wei)组织(zhi)具有显(xian)著(zhu)的影响(xiang)[44],热轧(ya)过(guo)程(cheng)中高温促(cu)使(shi)的(de)动态软化(hua)是典(dian)型(xing)的(de)热激(ji)活(huo)过(guo)程(cheng),合金(jin)中(zhong)原子(zi)运动频率(lv)随着(zhe)热轧温(wen)度(du)的(de)升高(gao)逐渐(jian)加(jia)剧,原子扩(kuo)散(san)更加容(rong)易(yi),位错滑移的临(lin)界(jie)分切应(ying)力(li)降(jiang)低,可开动的(de)滑(hua)移(yi)系增多(duo),从(cong)而(er)使晶粒(li)之间的变形协调(diao)性增(zeng)加(jia),动态(tai)软(ruan)化效应得(de)以加强。从(cong)工艺(yi)角度(du)来(lai)说(shuo),钛(tai)及钛合(he)金比(bi)热容(rong)小(xiao),高温活(huo)性强(qiang),轧制(zhi)温(wen)度(du)过(guo)高会(hui)导致(zhi)轧件中心(xin)与表层温度分布不(bu)均,轧件表面开(kai)裂、氧(yang)化(hua)和析(xi)氢(qing)腐蚀等缺陷(xian)问(wen)题(ti)[45];热轧温度(du)过(guo)低,则(ze)会由于(yu)TC钛合金(jin)塑(su)性(xing)加(jia)工(gong)性能差(cha),增大轧(ya)制(zhi)难(nan)度(du),因而(er)热(re)轧(ya)温(wen)度(du)的(de)选(xuan)择(ze)在(zai)钛(tai)及(ji)钛(tai)合金热(re)轧生(sheng)产(chan)过(guo)程显得尤为(wei)关(guan)键(jian)。
TC4钛(tai)合(he)金属(shu)于(yu)双(shuang)相钛合金(jin),轧(ya)制过(guo)程中(zhong)涉(she)及到固(gu)态(tai)相(xiang)变(bian),工程上(shang)TC4钛(tai)合(he)金(jin)的(de)初轧(ya)温度(du)通(tong)常在相(xiang)变(bian)点(dian)以(yi)上,二(er)火次(ci)及后(hou)续(xu)的火次(ci)在(zai)相(xiang)变点(dian)以下。在(zai)轧(ya)制温(wen)度(du)对(dui)TC4钛合金中(zhong)厚板显(xian)微组(zu)织和力学(xue)性能(neng)的影响规律研(yan)究中(zhong),任万(wan)波[46]研(yan)究结(jie)果表明在(α+β)两(liang)相区(qu),随(sui)着轧(ya)制温度(du)降低,TC4钛(tai)合(he)金(jin)中厚板(ban)的晶粒(li)尺寸不断(duan)减(jian)小(xiao),强(qiang)度(du)、塑性和韧(ren)性不断增加(jia),但(dan)过低(di)的(de)轧(ya)制温(wen)度常(chang)常(chang)伴(ban)随(sui)着轧件的质量问(wen)题(ti),如(ru)板形(xing)不良和边部(bu)裂(lie)纹(wen)等。LUOYM等[45]将(jiang)锻(duan)造(zao)的等轴(zhou)TC4钛合金分(fen)别在(zai)840、870、900和(he)930℃(图13a~图(tu)13d)下进行轧制,热(re)轧温度对(dui)显微(wei)组织(zhi)的(de)影(ying)响(xiang)可(ke)描述(shu)为(wei):轧(ya)制(zhi)温度低于(yu)900℃时,α晶(jing)粒沿轧(ya)制(zhi)方向(xiang)伸长(zhang),没(mei)有(you)观(guan)察(cha)到明显的(de)动(dong)态(tai)再(zai)结(jie)晶(jing)和(he)相变行(xing)为;当(dang)轧制温度(du)大于900℃时,晶(jing)粒发(fa)生再结晶和(he)相(xiang)变(bian),拉(la)长(zhang)的初始α晶(jing)粒的比例(li)随(sui)着(zhe)轧(ya)制(zhi)温(wen)度的(de)升高而降低,而再(zai)结晶(jing)的(de)等轴(zhou)α晶(jing)粒和(he)层状(zhuang)αs+β组(zu)合的比例增(zeng)加(jia);当(dang)轧(ya)制(zhi)温度(du)达到(dao)930℃时(shi),由(you)于动(dong)态再结(jie)晶(jing)和相(xiang)变(bian)行为(wei),轧件呈现(xian)典(dian)型(xing)的(de)双(shuang)峰(feng)组织结(jie)构(gou)。图(tu)13e~图(tu)13g表明(ming)轧(ya)制温度对TC4钛合(he)金的(de)动(dong)态(tai)力(li)学性能(neng)也有很大(da)影(ying)响(xiang),动态力(li)学性能(neng)各(ge)向异性(xing)的(de)趋(qu)势随轧(ya)制(zhi)温(wen)度(du)变化(hua)而变化,RD方向(xiang)上热轧(ya)温(wen)度(du)对(dui)TC4轧件(jian)的(de)伸长(zhang)率(lv)影响明显(xian),TD方(fang)向上热(re)轧温(wen)度为(wei)930℃时(shi)流变(bian)应力明显增高(gao)。
合适的(de)轧制(zhi)温(wen)度的(de)选择(ze)不仅会影(ying)响TC4钛(tai)合金板材的组(zu)织和(he)性(xing)能,还(hai)会涉(she)及(ji)到加(jia)工(gong)工序的简(jian)易(yi)程(cheng)度。
3.2变形量
TC4钛合(he)金变形抗(kang)力大、热加工区(qu)间(jian)窄,针对不同(tong)轧(ya)件产(chan)品,轧(ya)制时的(de)应(ying)变(bian)量(liang)(压下量(liang))控(kong)制(zhi)十分重(zhong)要[47]。当(dang)应变(bian)量较(jiao)小(xiao)时,晶粒(li)破(po)碎不(bu)完全[48]。
然而(er),由于(yu)钛(tai)合(he)金的变形抗(kang)力(li)较(jiao)大(da),如(ru)果在(zai)较(jiao)大的变形(xing)量下进(jin)行(xing)轧(ya)制实验(yan),虽(sui)然会促进晶(jing)粒(li)破(po)碎(sui)完全,细(xi)化(hua)组(zu)织(zhi),增强轧件力(li)学(xue)性(xing)能,但是也(ye)容易引(yin)起(qi)轧(ya)件内(nei)部(bu)应(ying)力(li)集(ji)中,从(cong)而(er)增(zeng)加(jia)了(le)裂纹(wen)形(xing)成的(de)风(feng)险(xian)并且(qie)变(bian)形量较(jiao)大,也(ye)需要更大的轧制力,这也影响轧(ya)辊的(de)使(shi)用寿(shou)命。
赵冰等(deng)[49]在(zai)800~950℃范围(wei)内,将(jiang)60%、70%、80%和(he)90%共4种(zhong)应(ying)变(bian)量(liang)作(zuo)为变量(liang)条件(jian),研究TC4钛(tai)合(he)金热轧(ya)过(guo)程(cheng)中(zhong)显微组(zu)织的演变,结(jie)果表(biao)明(ming)低(di)变(bian)形量(liang)下(xia)合金组织(zhi)以(yi)网篮状为(wei)主(zhu),存(cun)在少量的再(zai)结(jie)晶等(deng)轴(zhou)组(zu)织。但当变形量(liang)达到(dao)90%时,网篮状(zhuang)组(zu)织厚度明(ming)显减(jian)小,片(pian)层的(de)排(pai)布(bu)呈(cheng)现(xian)一定的规(gui)律性(xing),等轴组(zu)织的晶粒规格也显(xian)著减(jian)小(xiao)。姚学峰等(deng)[48]研究发(fa)现(xian)在(zai)不同(tong)变(bian)形(xing)量(liang)热轧后TC4钛合(he)金的显(xian)微组织和力学(xue)性能(neng)变化规(gui)律(图14),随(sui)着(zhe)变形量的增加(jia),TC4钛(tai)合金(jin)中得超细(xi)晶粒数(shu)量明显增加(jia),位(wei)错缠(chan)结(jie)程(cheng)度(du)增加,亚(ya)晶的数量(liang)增多,TC4钛(tai)合金(jin)的屈服(fu)强度和抗(kang)拉(la)强度(du)明(ming)显(xian)增(zeng)加,伸长(zhang)率(lv)在(zai)60%变形(xing)量(liang)之(zhi)前增加,而后明显减(jian)小。轧(ya)制过(guo)程中(zhong)应变量(liang)对(dui)钛及钛(tai)合金终(zhong)态(tai)产品(pin)的组(zu)织形态具有(you)较(jiao)大影响(xiang),工程上(shang)通过调控火(huo)次之间的变(bian)形量(liang)来(lai)控制轧件(jian)的(de)组(zu)织形态(tai)。
3.3应变(bian)速(su)率
钛及钛合(he)金的轧制(zhi)速(su)率较钢(gang)材(cai)低(di),热轧工艺(yi)中应变(bian)速率(lv)对(dui)钛合(he)金(jin)轧件(jian)显微(wei)组(zu)织(zhi)有(you)着显著影响(xiang),随(sui)着应(ying)变(bian)速率的增加位(wei)错(cuo)积累速(su)率(lv)较(jiao)高(gao),而动(dong)态回(hui)复(fu)引(yin)起(qi)的位错湮(yan)灭(mie)速(su)率较(jiao)低(di),因(yin)此(ci)钛(tai)合(he)金轧件(jian)中(zhong)位错(cuo)密(mi)度迅(xun)速(su)积累(lei)到(dao)较高(gao)水平,位(wei)错之间(jian)相互(hu)作(zuo)用形成(cheng)位错(cuo)缠(chan)结(jie)等,塑(su)性(xing)变形能(neng)力受(shou)限(xian),因(yin)而轧(ya)件(jian)的变(bian)形抗(kang)力较(jiao)大,轧件的(de)流(liu)变(bian)应(ying)力明(ming)显升(sheng)高。位(wei)错(cuo)和晶界(jie)等晶体(ti)缺(que)陷会为(wei)二次α相提供(gong)较(jiao)大的(de)异质形(xing)核位点。因此(ci),作为二次(ci)α相的异质形(xing)核(he)位(wei)点的晶(jing)体(ti)缺(que)陷(xian)随着应变速率的(de)增(zeng)加而急剧增(zeng)加(jia),导致二(er)次(ci)α相(xiang)在(zai)晶(jing)体缺(que)陷处析出,分(fen)布(bu)不规则[50],这对(dui)轧件(jian)的(de)最终(zhong)质量(liang)会(hui)产(chan)生(sheng)影(ying)响(xiang)。因(yin)而合(he)适的(de)应(ying)变速(su)率(lv)对(dui)改善(shan)轧(ya)件(jian)质量起(qi)到关键作(zuo)用(yong)。
LUOYM等(deng)[45]研究发现应(ying)变速(su)率对TC4钛(tai)合(he)金(jin)组织的影(ying)响取决(jue)于(yu)变(bian)形温度(du):实(shi)验测得(de)合(he)金的β转(zhuan)变温度(du)为(wei)1263K,初(chu)生α相的(de)晶粒尺寸(cun)在1203K以上(shang)随应变(bian)速(su)率的增(zeng)加(jia)而(er)减小(xiao),但在(zai)1203K以(yi)下(xia)则呈现(xian)振(zhen)荡趋势(shi);在(zai)1223K以(yi)上(shang),随(sui)着应(ying)变(bian)率(lv)的增(zeng)加,体积(ji)分数曲线呈振荡(dang)趋(qu)势,但(dan)在1223K以(yi)下(xia),初生α相的(de)体(ti)积(ji)分(fen)数(shu)减(jian)小(xiao)。YANGLQ等[51]研究(jiu)了初(chu)始等(deng)轴(zhou)组(zu)织TC4钛合(he)金在(zai)850~930℃温(wen)度范围、0.01~1s-1应(ying)变速(su)率范(fan)围(wei)和70%应(ying)变量条(tiao)件(jian)下(xia)显微(wei)组(zu)织演(yan)变和(he)力(li)学(xue)性(xing)能(neng)变(bian)化(hua)规(gui)律。图15a~图(tu)15c白色(se)箭(jian)头所指区(qu)域是(shi)拉长的α晶(jing)粒(li)之(zhi)间动态再结(jie)晶形(xing)核(he)区域(yu),在(zai)高(gao)应(ying)变速(su)率下合金(jin)没(mei)有足够的(de)时间释(shi)放(fang)畸(ji)变能,动态再(zai)结(jie)晶形核(he)数(shu)量(liang)明(ming)显增多。从(cong)图15d的真应(ying)力-真(zhen)应变曲(qu)线可以看出(chu)高(gao)应(ying)变(bian)速率(lv)的(de)应力值较(jiao)低(di)应变(bian)速率应力值(zhi)更(geng)高,造成(cheng)该(gai)现(xian)象(xiang)主要(yao)是(shi)由(you)于随(sui)应(ying)变速率升(sheng)高(gao),一方面(mian)激发位(wei)错(cuo)增(zeng)值速(su)率(lv),加(jia)工(gong)硬化效应(ying)明(ming)显(xian),另(ling)一(yi)方面合金(jin)动态再结(jie)晶软化没(mei)有足够时间发(fa)生,塑性(xing)变(bian)形(xing)无(wu)法充分(fen)完(wan)成,动(dong)态软化(hua)效(xiao)应(ying)减(jian)弱(ruo)。钛及钛合(he)金具(ju)有相(xiang)对较低(di)的(de)热(re)导(dao)率,无法(fa)有(you)效地传(chuan)递(di)和(he)分(fen)散(san)轧制(zhi)过程中(zhong)产生(sheng)的热量(liang),高(gao)速轧(ya)制(zhi)过程中(zhong)热量聚集,温度(du)梯(ti)度上升(sheng),增加塑(su)性(xing)流动失稳的(de)可(ke)能(neng)性。
3.4冷(leng)却速(su)率
TC4钛(tai)合(he)金的(de)相变(bian)行(xing)为随着(zhe)冷却(que)速度(du)的(de)增加依次(ci)经(jing)历扩散相变(bian)、块(kuai)状(zhuang)相变和马氏体相变[52]。当冷却速度(du)较快时(shi),固(gu)溶原(yuan)子(zi)没有(you)足(zu)够(gou)的时(shi)间扩(kuo)散,会形(xing)成(cheng)过(guo)饱(bao)和的固(gu)溶(rong)α′马氏(shi)体(ti)[53]。冷却(que)过程(cheng)中,根(gen)据(ju)冷(leng)却速度(du)不同(tong),可能发生(sheng)的(de)相变(bian)有α′、a″马氏(shi)体(ti)相(xiang)变、无热(re)ω相变和等(deng)温(wen)α相(xiang)变。常见(jian)的(de)淬(cui)火(huo)过程(cheng)的(de)主要相(xiang)变有(you)α′、a″马(ma)氏(shi)体(ti)相(xiang)变(bian)和(he)无(wu)热(re)ω相(xiang)变(bian),选(xuan)择(ze)合(he)适的(de)冷(leng)却速(su)度(du)和(he)冷却方式(shi)对(dui)提升钛材产品质(zhi)量(liang)起到(dao)明显的(de)促(cu)进作用(yong)。
梁(liang)爽等[54]在(zai)TC4钛(tai)合金(jin)完成热(re)轧(ya)工(gong)艺(yi)之后,采(cai)用3种冷却(que)方式:空(kong)冷(leng)、层流冷却和(he)水(shui)冷(leng),研究(jiu)结果(guo)表(biao)明,空(kong)冷(leng)条(tiao)件下,部(bu)分α相被拉(la)长,轧(ya)件的(de)抗(kang)拉(la)强度(du)呈现各(ge)向异(yi)性(xing),塑(su)韧性均不高(gao),综(zong)合(he)性(xing)能较层(ceng)流冷(leng)却和水冷要差;层(ceng)流冷(leng)却(que)条(tiao)件下,被拉长的(de)α相(xiang)数量(liang)明显增加(jia),轧件的各(ge)向抗(kang)拉(la)强(qiang)度较(jiao)高;水(shui)冷(leng)条(tiao)件(jian)下,轧件(jian)的抗(kang)拉(la)强度各(ge)向(xiang)异性(xing)得到(dao)明显(xian)的改善(shan),断面(mian)收缩(suo)率(lv)及伸长(zhang)率(lv)均(jun)较高(gao),综合(he)性在(zai)3种冷却方式(shi)中最(zui)佳(jia)。欧(ou)梅桂等(deng)[55]对TC4钛(tai)合(he)金(jin)进(jin)行(xing)0.1~80℃·s-1不同(tong)冷(leng)却(que)速(su)度(du)处(chu)理(li),分(fen)析(xi)冷(leng)却速度对(dui)合(he)金(jin)显微(wei)组(zu)织和(he)力学性(xing)能(neng)的影(ying)响。当(dang)冷却速度为0.1℃·s-1时(shi),TC4钛合(he)金(jin)的显(xian)微(wei)组(zu)织(zhi)呈(cheng)现为(wei)片层(ceng)状β转变(bian)组(zu)织(zhi)(图(tu)16a),冷(leng)却(que)速度(du)达(da)到0.5℃·s-1时(shi),层(ceng)状(zhuang)β转变(bian)组织明(ming)显(xian)细化(图(tu)16b)。冷却速度的提(ti)升使得β片(pian)层(ceng)厚(hou)度减(jian)小,提高了(le)TC4钛合金的强度(du)(表1),这(zhe)主要(yao)是由(you)于增(zeng)多的相界面阻碍了位错(cuo)运(yun)动(dong)。冷(leng)却速度(du)过大(da)时(shi)TC4钛(tai)合(he)金(jin)组(zu)织转(zhuan)变(bian)的(de)六方(fang)晶(jing)格(ge)的(de)过饱和固溶(rong)马(ma)氏体,使(shi)得(de)合(he)金(jin)的伸长(zhang)率(lv)和断面收缩(suo)率降(jiang)低(di)(表1)。通(tong)常(chang)情(qing)况下,钛(tai)合金(jin)冷(leng)却时具(ju)有很强(qiang)的尺寸效应,在相关研究(jiu)中,应(ying)考虑到(dao)轧件(jian)尺(chi)寸,设(she)计(ji)更宽泛的冷(leng)速(su)范(fan)围或(huo)充(chong)足(zu)的冷速(su)形式(shi)。
4、热(re)轧(ya)过程中(zhong)缺陷形(xing)成(cheng)机理
与(yu)钢(gang)、铝(lv)等合(he)金(jin)相(xiang)比(bi),钛合(he)金(jin)加工(gong)温度窗口窄(zhai)、工(gong)艺(yi)参(can)数(shu)敏感性强(qiang),从而容易出现一(yi)系(xi)列宏(hong)微(wei)观问题(ti),如裂(lie)纹(wen)、组(zu)织(zhi)不均匀等缺(que)陷(xian)。本(ben)节针对(dui)TC4钛(tai)合金(jin)热(re)轧时出现的(de)表面裂纹(wen)、塑性(xing)流动(dong)及(ji)绝热(re)剪切带(dai)进(jin)行分析,以(yi)期研究(jiu)上(shang)述缺陷的(de)形成机理,为(wei)优(you)化(hua)TC4钛合(he)金(jin)轧制(zhi)工(gong)艺(yi)提供参(can)考(kao)。
4.1热轧(ya)裂纹(wen)
TC4钛(tai)合金(jin)的导热(re)系(xi)数通(tong)常(chang)比(bi)普通(tong)钢要(yao)低,开(kai)轧温度过(guo)高(gao)轧(ya)件表面(mian)和(he)中心(xin)区域(yu)的(de)温差(cha)将(jiang)会(hui)急剧升(sheng)高(gao),伴(ban)随着(zhe)相(xiang)变(bian)将会导致(zhi)轧件变(bian)形(xing)不均(jun),轧件出(chu)现组织缺陷,如(ru)板带(dai)边裂(lie)(图(tu)17a)、表(biao)面裂纹(图17b)等问(wen)题[56]。钛(tai)合(he)金(jin)在(zai)热轧热(re)轧(ya)过程(cheng)中(zhong),氧元(yuan)素与轧(ya)件(jian)表(biao)面的(de)钛(tai)亲和(he)力(li)强,不断往(wang)基(ji)体(ti)里(li)面扩散(san),在轧件(jian)表面形(xing)成富(fu)氧(yang)层,容易出现(xian)裂纹,对轧(ya)件的塑性和(he)韧性(xing)产(chan)生伤害[57],相关(guan)研(yan)究表(biao)明TC4钛合(he)金(jin)轧件裂纹处氧、氮(dan)元(yuan)素含量(liang)异常(chang)偏(pian)高(gao),加剧轧(ya)件加(jia)工(gong)性能恶(e)化(hua)[57]。崔岩等[58]研究(jiu)了(le)TC4钛合(he)金和纯钛(tai)在(zai)热轧工(gong)艺中(zhong)轧件吸氧形成富氧(yang)层的现(xian)象(xiang),结果发(fa)现TC4钛(tai)合(he)金(jin)相较(jiao)于纯(chun)钛在结构上(shang)更(geng)为疏松(song)多(duo)孔,表(biao)明TC4轧件(jian)吸氧(yang)速率和总(zong)量(liang)明显多于(yu)纯钛。
进一步从图(tu)18可以看(kan)出(chu),TC4钛(tai)合(he)金(jin)的富氧层(ceng)明显大(da)于纯钛,其表(biao)面裂(lie)纹(wen)较(jiao)纯钛严(yan)重(zhong)。虽(sui)然TC4钛(tai)合(he)金现(xian)存热轧(ya)工艺中(zhong)设计(ji)了(le)防(fang)氧(yang)化(hua)措施(shi),如涂敷(fu)防氧化材(cai)料,但这些(xie)材(cai)料(liao)较(jiao)注(zhu)重(zhong)防氧(yang)化,而未(wei)过多(duo)重视其热塑性。TC4钛合金(jin)在轧制(zhi)后仍存在(zai)大(da)量(liang)裂纹(wen),经分(fen)析后发(fa)现(xian)这(zhe)些(xie)裂(lie)纹周边较(jiao)多(duo)的(de)富氧层(ceng)。因(yin)此,亟(ji)需(xu)开发(fa)出(chu)钛合(he)金轧(ya)制专(zhuan)用的、具有一(yi)定(ding)热(re)塑(su)性(xing)的(de)防护(hu)剂(ji)。同(tong)时(shi),在热轧过(guo)程中(zhong)需要(yao)设(she)定(ding)合适(shi)的(de)热轧(ya)参数,尤其(qi)在道次(ci)之间重视抢温(wen)工(gong)作。
4.2局部(bu)流(liu)动(dong)失稳
钛合(he)金(jin)在(zai)热轧工艺中(zhong),组(zu)织中的加(jia)工(gong)硬(ying)化和动态软化同时发(fa)生(sheng),在加工(gong)硬化(hua)的塑性做(zuo)功和诱发(fa)动态软(ruan)化(hua)的高温热(re)量共同(tong)作用(yong)下(xia),钛(tai)材局(ju)部区域(yu)将会(hui)出(chu)现(xian)温升(sheng)过快(kuai)的(de)现(xian)象,导致(zhi)局部(bu)组织软化(hua),而塑(su)性(xing)变形会率先集(ji)中(zhong)在(zai)此区域。局(ju)部(bu)和(he)整体(ti)的变形不(bu)均匀(yun)导致(zhi)组织(zhi)演变(bian)差异,即产(chan)生(sheng)局(ju)部流(liu)动失(shi)稳现(xian)象(xiang)[59]。TC4钛(tai)合(he)金的比热容(rong)较(jiao)钢(gang)材(cai)等(deng)金属要(yao)低得(de)多,热轧过程(cheng)中(zhong)的(de)产(chan)生的(de)热量(liang)散(san)布不(bu)均(jun),局部区域(yu)的(de)温升和软化较为(wei)突(tu)出(chu),因此(ci),常(chang)常可(ke)以(yi)观察到局(ju)部塑(su)性流(liu)动失(shi)稳(wen)现(xian)象的(de)发(fa)生[60]。
刘诚等[61]在(zai)TC4-DT钛(tai)合(he)金(jin)热(re)变形实验(yan)中,研(yan)究(jiu)了(le)轧(ya)件局(ju)部(bu)流(liu)动失稳区域(yu)组织(zhi)形貌形成机理(li)(图19),晶(jing)粒变形(xing)的均(jun)匀(yun)性(xing)明显(xian)较低(di)。分(fen)析(xi)发现(xian)这是(shi)由(you)于在(zai)1s-1和10s-1的高应(ying)变速(su)率(lv)下,极(ji)短时(shi)间(jian)内完成(cheng)的塑性做(zuo)工(gong)的能(neng)量(liang)在(zai)轧件中(zhong)无法(fa)迅速散(san)去(qu),造(zao)成局部组(zu)织区(qu)域的(de)温度(du)上(shang)升(sheng)过快,动态(tai)软(ruan)化作用明(ming)显,发(fa)生局部流(liu)动(dong)失稳(wen)现象(xiang)。研究(jiu)结(jie)果表明高(gao)应变(bian)速率(lv)下TC4钛合(he)金更(geng)容(rong)易发生(sheng)局部流动失稳,流变(bian)应力(li)不易(yi)达(da)到稳(wen)态(tai)。局(ju)部(bu)流动塑(su)性(xing)失稳(wen)导(dao)致(zhi)TC4钛合金(jin)的(de)裂(lie)纹(wen)形成(cheng)、力学性(xing)能(neng)下(xia)降(jiang)和(he)变(bian)形(xing)能力(li)减(jian)小(xiao),降低了(le)材料(liao)的负(fu)荷(he)能力和(he)寿命,通过(guo)调整(zheng)轧制(zhi)工艺和方式能(neng)够(gou)有效(xiao)减(jian)弱流(liu)动(dong)塑性失稳(wen)的发(fa)生。
4.3绝热剪(jian)切(qie)带
高(gao)速冲(chong)击(ji)、装(zhuang)甲侵(qin)彻和(he)轧制(zhi)成形等高(gao)应变速(su)率(lv)下[62],材料容(rong)易发生(sheng)严(yan)重(zhong)的(de)塑性变形行为,即(ji)绝(jue)热(re)剪切(qie)现象[63]。“绝热(re)”[64]是指(zhi)仅(jin)仅(jin)几十(shi)微秒(miao)的(de)变形(xing)过程中,塑(su)性功转(zhuan)化(hua)的热量无法(fa)有效(xiao)散(san)去,轧件中(zhong)出(chu)现绝(jue)热区(qu)域(yu),该(gai)区(qu)域(yu)被称(cheng)为(wei)绝热剪(jian)切(qie)带(dai)(Adia-baticShearBand,ΑSΒ)。绝热(re)剪(jian)切(qie)与局(ju)部(bu)流动失(shi)稳的形(xing)成(cheng)机(ji)理相似,但是(shi)相较(jiao)于(yu)局部(bu)流(liu)动失(shi)稳,ASB经历(li)快(kuai)速(su)升(sheng)温-急剧冷却过(guo)程(cheng),狭长带(dai)状的、剪切变形(xing)高度(du)局(ju)部化的(de)变(bian)形(xing)区域(yu)甚至能(neng)够(gou)穿越(yue)整个(ge)轧(ya)件(jian),对(dui)轧(ya)件的(de)质(zhi)量(liang)会(hui)造成严(yan)重(zhong)破坏(huai)。TC4钛(tai)合(he)金具(ju)有(you)较低的(de)比(bi)热(re)容和(he)热(re)传(chuan)导系(xi)数[65],且(qie)α相的滑(hua)移系较(jiao)少(shao),在(zai)剪(jian)切(qie)变形(xing)中呈现(xian)出(chu)对温(wen)度和(he)应(ying)变(bian)率(lv)的(de)高(gao)敏感性[66]。
LUOYM等(deng)[67]研究(jiu)了(le)应(ying)力状(zhuang)态(包括动态剪切和(he)单轴(zhou)动(dong)态(tai)压(ya)缩)对热(re)轧(ya)TC4钛(tai)合金绝(jue)热剪(jian)切带(dai)的影响(xiang)。结(jie)果(guo)表明(ming):单轴(zhou)压缩(suo)时(shi)TC4钛(tai)合(he)金热轧板(ban)横(heng)向试(shi)样(yang)的绝(jue)热剪切(qie)敏(min)感性(xing)最(zui)高(gao),轧(ya)制方(fang)向(xiang)的试(shi)样(yang)绝(jue)热剪(jian)切(qie)敏(min)感(gan)性(xing)最(zui)低(di),而(er)在动态(tai)剪切条(tiao)件下(xia),轧(ya)件的绝热剪(jian)切(qie)敏感性(xing)的(de)各向异性则表现(xian)出与(yu)之(zhi)相(xiang)反(fan)的(de)趋(qu)势。EL-MΑGDE等(deng)[68]通过对(dui)比(bi)TC4钛合(he)金、Al合金和(he)Mg合金(jin)在(zai)不同应变速(su)率(lv)下的(de)流(liu)动行为(wei)和(he)延展(zhan)性(xing),发(fa)现TC4钛(tai)合(he)金板(ban)材(cai)绝热剪(jian)切(qie)带的生(sheng)成,对应变(bian)速(su)率具(ju)有较高(gao)的敏感性(xing)。杨柳(liu)青等(deng)[65]通(tong)过(guo)研(yan)究(jiu)TC4钛(tai)合(he)金(jin)绝热剪(jian)切(qie)带中显微(wei)组(zu)织演(yan)变,发现轧(ya)件(jian)ASB区域(yu)主要(yao)由(you)基体、过渡区(qu)和(he)ASB组(zu)成(cheng)(图(tu)20),从(cong)基(ji)体到ASB区域(yu)显微(wei)组织的演变(bian)规(gui)律(lv)为:等轴α晶(jing)粒(li)和(he)间隙(xi)β晶(jing)粒(li)逐渐(jian)被(bei)拉(la)长,小角(jiao)度晶(jing)界逐(zhu)渐成长为(wei)大(da)角度晶界(jie),并(bing)伴随着亚(ya)晶(jing)和(he)再结(jie)晶晶粒数量的增长(zhang),ASB区域(yu)由(you)于(yu)发(fa)生(sheng)动(dong)态再(zai)结晶,晶粒尺寸较小(xiao),且(qie)没(mei)有明显的(de)织(zhi)构(gou)取(qu)向(xiang)。
5、热轧(ya)过程(cheng)数(shu)值模(mo)拟(ni)
科(ke)学实验(yan)、理论推(tui)导(dao)和数(shu)值(zhi)模(mo)拟已经(jing)成为(wei)当今(jin)材料研(yan)究领域3种主要研(yan)究(jiu)手段(duan),热轧(ya)过程中轧(ya)件的(de)应(ying)力(li)场、温度(du)场(chang)以及(ji)微观组(zu)织(zhi)演(yan)变(bian)的准确(que)预测和精(jing)确控(kong)制,对(dui)于通过(guo)热加工(gong)和(he)热(re)处理获(huo)得(de)钛(tai)合金所(suo)需的机(ji)械性能非(fei)常(chang)重(zhong)要。随着计(ji)算能(neng)力的快(kuai)速发(fa)展(zhan)以(yi)及(ji)对(dui)数值(zhi)模拟(ni)机理(li)的(de)深入理解(jie),有(you)限(xian)元模拟(ni)及介(jie)观组织模拟(ni)等(deng)模(mo)拟(ni)方法(fa)已经(jing)被开(kai)发(fa)出(chu)来并成功(gong)地应用(yong)于模(mo)拟钛(tai)合(he)金热轧工(gong)艺流(liu)程。
5.1有(you)限元模(mo)拟
有(you)限元(yuan)法是基(ji)于(yu)连续介(jie)质(zhi)假设,对连续(xu)体进(jin)行离(li)散计算的(de)一(yi)种(zhong)方法[69]。目前,有(you)限(xian)元(yuan)法多用于宏(hong)观(guan)尺度(du)的(de)模(mo)拟(ni),众多(duo)商业(ye)有限元软(ruan)件(jian)如ANSYS、Abaqus、MSC.Marc和Procast等(deng)在模拟的精(jing)准(zhun)性(xing)上(shang)取得了巨(ju)大(da)的成(cheng)功(gong)[70]。在钛(tai)合(he)金热(re)轧工(gong)艺研(yan)究过程(cheng)中(zhong)发(fa)现,轧件质量缺(que)陷(xian)问(wen)题(ti),如(ru)表(biao)面(mian)裂(lie)纹(wen)、板(ban)凸(tu)度(du)差(cha)及边部(bu)裂纹等,多是由(you)于(yu)热轧(ya)工(gong)艺参数(shu)选(xuan)择不(bu)当导致的(de)。钛(tai)合(he)金(jin)具(ju)有(you)相对较(jiao)低的导热系数,热轧(ya)中(zhong)的(de)板坯中(zhong)心(xin)与(yu)表面、中(zhong)部(bu)与(yu)边(bian)缘(yuan)存在(zai)明显(xian)温(wen)度(du)差(cha),温(wen)度(du)不(bu)均(jun)进一(yi)步引发(fa)不(bu)连续的(de)塑性变形,从而诱发(fa)轧制质(zhi)量(liang)缺(que)陷(xian)。徐森等(deng)[47]利用有限元(yuan)软(ruan)件MSC.Marc建(jian)立(li)TC4钛(tai)合金(jin)热(re)轧(ya)模(mo)型(xing),研究热(re)轧工(gong)艺过(guo)程中板坯温度(du)变(bian)化规(gui)律,研(yan)究发(fa)现(xian)轧(ya)件表面(mian)与(yu)中心达(da)到最(zui)大温度(du)差120℃时,由于(yu)温差过(guo)大(da),轧件表面(mian)出(chu)现(xian)开(kai)裂(lie)缺(que)陷,模拟结(jie)果与(yu)现场实测值(zhi)符(fu)合(图21)。SHIJ等[71]采用(yong)有(you)限元热力(li)耦(ou)合法(fa)和(he)Abaqus软(ruan)件对(dui)TC4钛(tai)合金半成(cheng)品(pin)管(guan)进(jin)行(xing)研(yan)究,分(fen)析(xi)开(kai)轧(ya)温(wen)度和(he)轧制速(su)度对(dui)TC4钛(tai)合(he)金温(wen)度、轧(ya)制力(li)和功(gong)率(lv)的(de)影(ying)响(xiang),研究(jiu)了900℃和(he)2.93m·s-1输(shu)送(song)速度下的(de)TC4轧(ya)件温(wen)度场、应力(li)场(chang)和(he)应(ying)变场情况(kuang)(图(tu)22)。
最(zui)终(zhong)通(tong)过(guo)实(shi)际热(re)轧(ya)测(ce)试(shi)结(jie)果(guo)验(yan)证了(le)TC4钛(tai)合金无缝(feng)管(guan)轧制过程的模(mo)拟精(jing)准性,管(guan)道形状与仿(fang)真(zhen)结(jie)果吻(wen)合良好(hao)。有限(xian)元模拟(ni)结(jie)果分(fen)析为实际(ji)TC4钛合金(jin)轧(ya)制过(guo)程工艺(yi)参数的(de)制(zhi)定(ding)提(ti)供(gong)了有(you)效指导(dao),有效改善(shan)了(le)轧(ya)件的轧制(zhi)质(zhi)量(liang)缺陷(xian)。
5.2介观(guan)组织(zhi)模拟(ni)
钛(tai)合(he)金在热轧工艺过程中(zhong),显(xian)微(wei)组织(zhi)在加(jia)工(gong)硬化(hua)和(he)动态软化(hua)的(de)共(gong)同(tong)作(zuo)用下会(hui)发生显著的变(bian)化,对介(jie)观显(xian)微(wei)组织(zhi)变(bian)化(hua)的模(mo)拟,常(chang)见的有(you)几(ji)何(he)法[72]、蒙(meng)特卡(ka)罗(Monte
Carlo,MC)法(fa)[73]和(he)元胞(bao)自动机(ji)(CellularAutomaton,CA)法[74]。目(mu)前CA法在(zai)模拟TC4钛(tai)合(he)金(jin)的微(wei)观组(zu)织演(yan)变(bian)方(fang)面(mian)应(ying)用(yong)广泛(fan),尤其是在(zai)回复、再(zai)结(jie)晶、晶(jing)粒(li)长大、相变
和(he)凝固(gu)等组(zu)织演(yan)变方(fang)面的(de)研(yan)究已经(jing)取得重要(yao)的研(yan)究成(cheng)果(guo)。
在(zai)钛(tai)合金(jin)的(de)热(re)轧过(guo)程(cheng)中,晶粒变形(xing)、再结晶(jing)形(xing)核及(ji)长大的动态再(zai)结晶(jing)行为普遍(bian)发(fa)生,刘诚等[75]建(jian)立(li)了TC4-DT钛(tai)合金(jin)热(re)加工过(guo)程中(zhong)动态(tai)再结晶的(de)元(yuan)胞(bao)自(zi)动机(ji)模型,模拟(ni)结果表明TC4-DT在热(re)变(bian)形(xing)过(guo)程中显微(wei)组(zu)织(zhi)的变(bian)化规律(lv),并(bing)用(yong)动态再(zai)结(jie)晶(jing)理论进行(xing)了(le)解释,根据模(mo)拟(ni)条(tiao)件(jian)进行(xing)了(le)相(xiang)应(ying)的(de)热加工实验,结(jie)果(guo)表(biao)明CA模(mo)拟与(yu)实(shi)验(yan)结吻(wen)合度良(liang)好(图23)。
HANF等[76]基(ji)于(yu)位错(cuo)驱动形核(he)和(he)晶(jing)粒(li)生长动(dong)力学(xue)理论,建(jian)立了模拟(ni)动(dong)态(tai)再(zai)结(jie)晶的元胞(bao)自动机模型(xing),以(yi)分析TC4钛(tai)合(he)金在(zai)热压(ya)缩变形过(guo)程中的显微组(zu)织(zhi)演(yan)变,模拟(ni)结(jie)果(guo)表明:随(sui)着(zhe)应变的(de)增(zeng)加、应(ying)变(bian)速率的降低和(he)变形温(wen)度的升(sheng)高,再(zai)结晶(jing)体积(ji)分数(shu)和再结(jie)晶平均晶(jing)粒尺(chi)寸(cun)稳(wen)步增加(jia)(图(tu)24a)。将模拟(ni)的(de)流动应力-应变(bian)曲线与(yu)实(shi)验(yan)得到的应力-应变(bian)曲(qu)线进(jin)行对(dui)比(bi),验证(zheng)了(le)元胞(bao)自(zi)动(dong)机(ji)模型(xing)的(de)合(he)理性(xing)和(he)准(zhun)确性(图(tu)24b)。
6、展望(wang)
TC4钛合金综合(he)性能优异,已(yi)成功应(ying)用于航(hang)空航(hang)天、海(hai)洋(yang)工程(cheng)、石(shi)油化(hua)工及生物医(yi)疗等(deng)领域(yu),其(qi)占所有钛(tai)及钛合金应(ying)用总(zong)量(liang)的(de)50%以(yi)上。本文(wen)通(tong)过阐述(shu)国内外(wai)关(guan)于TC4钛合(he)金热轧过程中(zhong)组织演(yan)变和性能(neng)控(kong)制(zhi)机理的(de)研究,发(fa)现(xian)该合金(jin)在热(re)轧(ya)时(shi)仍存(cun)在较多问(wen)题,文(wen)中虽提出一些(xie)改进、优化该合金(jin)热轧工(gong)艺(yi)的方法(fa)及措施(shi),但(dan)关(guan)于TC4钛(tai)合金(jin)轧(ya)制(zhi)技(ji)术(shu)的(de)进一(yi)步发(fa)展仍(reng)需(xu)要持(chi)续(xu)完善(shan)。对(dui)TC4钛合金轧(ya)制(zhi)提(ti)出以下(xia)3点展(zhan)望。
(1)微观(guan)组(zu)织(zhi)的(de)精细(xi)控制(zhi):TC4钛(tai)合(he)金的热轧过程(cheng)中,显微(wei)组织与(yu)力学性(xing)能(neng)的(de)复杂变(bian)化密切关联。微(wei)观(guan)结(jie)构的(de)演(yan)变不(bu)仅(jin)直(zhi)接(jie)影(ying)响材料的流(liu)变行(xing)为(wei),还(hai)在(zai)很(hen)大(da)程(cheng)度上(shang)决(jue)定(ding)了其(qi)力学性能(neng)。因此,借(jie)助(zhu)深入(ru)的变(bian)形机理研(yan)究,有必(bi)要(yao)深刻理(li)解(jie)钛(tai)合金组(zu)织(zhi)演变(bian)的规(gui)律(lv)。通(tong)过深(shen)入(ru)的(de)组(zu)织(zhi)控(kong)制(zhi)研究(jiu),优化(hua)加(jia)工(gong)参(can)数,可以精细地调(diao)控钛(tai)合(he)金的(de)微观(guan)结(jie)构(gou),从而显著提升材料的(de)质(zhi)量与性能。
(2)工(gong)艺(yi)参(can)数(shu)的智能(neng)调(diao)控:TC4钛(tai)合金(jin)对于(yu)热(re)轧(ya)工艺(yi)参(can)数,如(ru)轧制温度、应(ying)变(bian)量以(yi)及(ji)应变速率,表(biao)现出(chu)高(gao)度(du)的(de)敏(min)感性(xing)。只有(you)在(zai)适(shi)宜(yi)的(de)热(re)轧温(wen)度区(qu)间内(nei),才能获(huo)得均匀的组(zu)织、细小(xiao)的(de)晶(jing)粒以(yi)及(ji)卓越的(de)力学性(xing)能(neng)。同(tong)时,智能(neng)调(diao)控工(gong)艺参数(shu)有助(zhu)于实现协调变(bian)形,从(cong)而避(bi)免(mian)在(zai)轧(ya)制(zhi)过(guo)程(cheng)中(zhong)引发(fa)缺(que)陷的隐(yin)忧(you)。
(3)融合(he)实(shi)验(yan)与数(shu)值模拟(ni)的(de)深度(du)研(yan)究:当前(qian)TC4热(re)轧工艺研(yan)究,正(zheng)处于(yu)实验(yan)数据与(yu)数(shu)值模(mo)拟技术融合(he)的重(zhong)要(yao)阶段。结合(he)实验(yan)数(shu)据,可以(yi)深(shen)入了解(jie)轧件的(de)宏(hong)观(guan)变(bian)形(xing)以(yi)及(ji)微(wei)观组(zu)织(zhi)的演变(bian)规(gui)律(lv)。通(tong)过精准(zhun)的(de)数(shu)值(zhi)模拟,能够(gou)更好(hao)地解析材料(liao)在(zai)复(fu)杂(za)工艺条件(jian)下的(de)行(xing)为(wei)。这一深度研(yan)究路径,不(bu)仅(jin)有助于(yu)提(ti)升国内(nei)TC4钛(tai)合(he)金轧(ya)件(jian)的(de)质量(liang),还(hai)将有(you)力地推(tui)动其在高(gao)端应(ying)用领(ling)域的拓(tuo)展(zhan)。
综上(shang)所(suo)述(shu),TC4钛合金在(zai)热轧工(gong)艺(yi)中的研(yan)究具(ju)有(you)重要(yao)的(de)理(li)论(lun)和(he)实(shi)践意义(yi)。通过(guo)精细(xi)组织控制、智(zhi)能(neng)工艺调控以及实验(yan)与(yu)数(shu)值模(mo)拟(ni)的(de)深度(du)融合,可(ke)以为(wei)提升(sheng)合金(jin)质(zhi)量(liang)、拓展高端(duan)应用领(ling)域(yu)打下(xia)坚实(shi)基础(chu),进一步促进(jin)我国在(zai)钛(tai)合金领(ling)域(yu)的(de)创(chuang)新(xin)与(yu)发展。
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