電火花表面(mian)強化(hua)是利(li)用工具電極(ji)與工件(jian)(jian)間在氣(qi)體(ti)中(zhong)產(chan)生(sheng)的火花放電作用,把作為電極(ji)的導(dao)電材料熔滲進(jin)工件(jian)(jian)表層(ceng),形成合金化(hua)的表面(mian)強化(hua)層(ceng),改(gai)善工件(jian)(jian)表面(mian)的物理及化(hua)學性能。
電火花表面強化層的性能主要決定于模具本身和電極材料,通常所用的電極材料有TiC、WC、ZrC、NbC、Cr3C2、硬質合金等。電火花強化表面因電極材料的沉積發生有規律的、較小的長大,除此之外,模具沒有其他變形。其心部的組織與性能也不發生變化,因此十分適用于工件表面強化處理。
金屬電(dian)火花(hua)表(biao)面強化(hua)的(de)原理(li)是在工(gong)具電(dian)極與(yu)工(gong)件之間(jian)接上直(zhi)流電(dian)源或交流電(dian)源,由于振動器的(de)作用使電(dian)極與(yu)工(gong)件間(jian)的(de)放(fang)電(dian)間(jian)隙頻繁變化(hua),工(gong)具電(dian)極與(yu)工(gong)件間(jian)不(bu)斷產(chan)生火花(hua)放(fang)電(dian),從(cong)而實現對金屬表(biao)面的(de)強化(hua)。
電(dian)(dian)(dian)(dian)(dian)(dian)(dian)火(huo)花強(qiang)化(hua)過程如圖(tu)3-20所(suo)示。當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)與(yu)(yu)工(gong)件(jian)(jian)之(zhi)間(jian)(jian)的(de)(de)距離(li)(li)較大時(shi),電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源經(jing)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻R對電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容充電(dian)(dian)(dian)(dian)(dian)(dian)(dian),電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)在(zai)振動(dong)(dong)器(qi)的(de)(de)帶(dai)動(dong)(dong)下(xia)向模(mo)具靠近(見圖(tu)3-20a)。當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)與(yu)(yu)模(mo)具之(zhi)間(jian)(jian)的(de)(de)間(jian)(jian)隙(xi)接近到某個距離(li)(li)時(shi),間(jian)(jian)隙(xi)中的(de)(de)空氣在(zai)強(qiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)場的(de)(de)作用(yong)下(xia)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)離(li)(li),產生(sheng)火(huo)花放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(見圖(tu)3-20b),使(shi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)和工(gong)件(jian)(jian)在(zai)發生(sheng)放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)部(bu)分(fen)的(de)(de)金屬(shu)局部(bu)熔化(hua),甚至汽化(hua)。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)繼續接近工(gong)件(jian)(jian)并與(yu)(yu)工(gong)件(jian)(jian)接觸(chu)時(shi),火(huo)花放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)停止,在(zai)接觸(chu)點流過短路(lu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流,使(shi)該處繼續加熱,由于電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)以適當壓力壓向工(gong)件(jian)(jian),使(shi)熔化(hua)的(de)(de)材料(liao)互相粘接、擴散(san)而形成(cheng)(cheng)合金或新的(de)(de)化(hua)合物(見圖(tu)3-20c)。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)在(zai)振動(dong)(dong)器(qi)的(de)(de)作用(yong)下(xia),離(li)(li)開了工(gong)件(jian)(jian),放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)部(bu)分(fen)急劇冷卻(見圖(tu)3-20d)。經(jing)多(duo)次放電(dian)(dian)(dian)(dian)(dian)(dian)(dian),并相應地移動(dong)(dong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)的(de)(de)位置,則在(zai)零件(jian)(jian)表面形成(cheng)(cheng)強(qiang)化(hua)層。
電(dian)(dian)火(huo)花表面強化(hua)(hua)過程中發生了物理化(hua)(hua)學變(bian)化(hua)(hua),主要包(bao)括超高(gao)速淬火(huo)、滲碳、滲氮、電(dian)(dian)極材料的轉移等。
a. 超高速淬火
電火花(hua)放(fang)電在(zai)模(mo)具表面的極(ji)小面積上產(chan)生高溫(wen),使該處的金(jin)屬熔(rong)化(hua)和部(bu)分(fen)汽化(hua),當火花(hua)放(fang)電在(zai)極(ji)短(duan)的時(shi)間內停止后,被加熱了的金(jin)屬會以(yi)很(hen)快的速度冷(leng)卻下來。這相當于(yu)對模(mo)具表面層進(jin)行(xing)了超速淬火。
b. 滲氮
在(zai)電(dian)火花放(fang)電(dian)通(tong)道(dao)區域(yu)內,溫度(du)很高(gao),空氣中的氮分子呈原子狀(zhuang)態,它和(he)受高(gao)溫而熔化(hua)的金(jin)屬有(you)關(guan)的元(yuan)素合成高(gao)硬(ying)度(du)的金(jin)屬氮化(hua)物,如氮化(hua)鐵、氮化(hua)鉻(ge)等。
c. 滲(shen)碳
來自石墨電極(ji)或周圍介質(zhi)的(de)(de)碳(tan)元(yuan)素,熔解(jie)在(zai)受熱(re)而(er)熔化(hua)的(de)(de)鐵(tie)中,形成金屬的(de)(de)碳(tan)化(hua)物,如碳(tan)化(hua)鐵(tie)、碳(tan)化(hua)鉻等。
d. 電極材料(liao)的轉(zhuan)移
在(zai)操作壓力和火花放電(dian)的條件(jian)下,電(dian)極材(cai)料轉移到模具金(jin)屬(shu)熔融表面,有關(guan)金(jin)屬(shu)合金(jin)元素(su)(W、Ti、Cr等(deng))迅速擴散在(zai)金(jin)屬(shu)的表面層。
電火花表面強化層具有如(ru)下特征:
a. 當采用硬質(zhi)合金作電極材料時(shi),硬度可達1100~1400HV(約70HRC以上)或更(geng)高,耐熱性、耐蝕性和疲(pi)勞強(qiang)度都(dou)大大提高。
b. 當使用鉻錳、鎢鉻鈷合金、硬質合金作(zuo)工(gong)具電極強化45鋼時(shi),其耐磨性比原(yuan)表層提(ti)高2~2.5倍(bei)。
c. 用(yong)石墨作電(dian)(dian)極材料強(qiang)化45鋼,用(yong)食鹽(yan)水(shui)作腐蝕(shi)(shi)性試驗,其(qi)耐蝕(shi)(shi)性提高(gao)90%;用(yong)WC、CrMn作電(dian)(dian)極強(qiang)化不銹鋼時,其(qi)耐蝕(shi)(shi)性提高(gao)3~5倍。
d. 硬化層(ceng)厚度為0.01~0.08mm。
鋼(gang)制(zhi)模(mo)(mo)具工作表(biao)(biao)面(mian)的電(dian)(dian)火(huo)花強(qiang)化(hua)通常采用硬質合金(jin)電(dian)(dian)極。為了(le)使被強(qiang)化(hua)的表(biao)(biao)面(mian)光潔(jie),事先必(bi)須將模(mo)(mo)具和電(dian)(dian)極表(biao)(biao)面(mian)清洗干凈,然后(hou)手持振動器,將電(dian)(dian)極沿(yan)模(mo)(mo)具工作表(biao)(biao)面(mian)移(yi)動,并保持適當壓力,使火(huo)花放(fang)電(dian)(dian)均勻連(lian)續。
電火花熔(rong)滲合金化層(ceng)的形(xing)成(cheng)是一個漸(jian)近過程,在(zai)(zai)每一電規范(fan)下,合金化層(ceng)厚度(du)出(chu)現(xian)最大值,在(zai)(zai)通常使用的電容范(fan)圍內,最佳單位面(mian)積(ji)涂覆時間(jian)為6~12min/c㎡。過分延長涂覆時間(jian)將(jiang)出(chu)現(xian)層(ceng)厚減薄的趨(qu)勢,并(bing)使性能惡化。可用直線方程式表示:
如電極YG8、電壓60V,頻率250Hz,電容60μF,最佳涂(tu)覆(fu)(fu)時(shi)間為(wei)(wei)6.75min/c㎡,合金化(hua)層厚(hou)度為(wei)(wei)13μm;電容322μF,涂(tu)覆(fu)(fu)時(shi)間為(wei)(wei)11.99min/c㎡,合金化(hua)層厚(hou)度為(wei)(wei)27μm。
為(wei)了降低合(he)金(jin)化(hua)層(ceng)(ceng)的熱疲勞應(ying)力(li)和電(dian)火花(hua)合(he)金(jin)化(hua)處(chu)理的應(ying)力(li),可穿(chuan)插(cha)1~2次(ci)500℃×4h去應(ying)力(li)退(tui)火,這樣(yang)可獲得性能優(you)良、層(ceng)(ceng)深較厚的表面(mian)合(he)金(jin)化(hua)層(ceng)(ceng)。改(gai)換電(dian)極材(cai)料,可使(shi)合(he)金(jin)化(hua)層(ceng)(ceng)繼(ji)續增(zeng)厚,電(dian)極斷(duan)面(mian)尺(chi)寸不影響合(he)金(jin)化(hua)層(ceng)(ceng)的厚度。鋼(gang)中w(C)小(xiao)于(yu)0.8%時(shi),隨(sui)鋼(gang)中碳含量的增(zeng)加(jia)合(he)金(jin)層(ceng)(ceng)增(zeng)厚;w(C)大于(yu)0.8%時(shi),隨(sui)鋼(gang)中碳含量的增(zeng)加(jia)合(he)金(jin)層(ceng)(ceng)變薄。
用YG8、Nb、Ti、Ta合金化,工件表(biao)面將(jiang)獲得極高(gao)的顯(xian)微(wei)硬度值。
電(dian)火花(hua)合(he)金層(ceng)比(bi)未經電(dian)火花(hua)合(he)金化處(chu)理的(de)模(mo)具的(de)熱疲勞(lao)性能(neng)提(ti)高3倍(bei),抗(kang)氧化性能(neng)提(ti)高兩倍(bei),在各種試驗介(jie)質(zhi)中的(de)耐蝕性提(ti)高3~15倍(bei);表3-44所(suo)示為3Cr2W8V鋼(gang)的(de)處(chu)理效果。
電(dian)火(huo)花(hua)表面強(qiang)(qiang)化(hua)應用效果顯(xian)著。例(li)如用YG8作電(dian)極,對3Cr2W8V鋼模(mo)具進行電(dian)火(huo)花(hua)強(qiang)(qiang)化(hua)處(chu)理以后(hou),模(mo)具在各(ge)類酸堿中的耐蝕(shi)性提高4~15倍(bei);而(er)Cr12鋼模(mo)具刃口部(bu)位經電(dian)火(huo)花(hua)表面強(qiang)(qiang)化(hua)后(hou),模(mo)具的平均使用壽命由(you)5萬(wan)次提高到20萬(wan)次。