電(dian)火(huo)(huo)花表(biao)面(mian)強化(hua)(hua)是(shi)利用工(gong)(gong)(gong)具(ju)電(dian)極與工(gong)(gong)(gong)件間在(zai)氣(qi)體中(zhong)產生的火(huo)(huo)花放電(dian)作用,把作為電(dian)極的導(dao)電(dian)材料熔滲進(jin)工(gong)(gong)(gong)件表(biao)層,形成合金化(hua)(hua)的表(biao)面(mian)強化(hua)(hua)層,改善工(gong)(gong)(gong)件表(biao)面(mian)的物理及(ji)化(hua)(hua)學(xue)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)(dian)(dian)火(huo)花(hua)表面(mian)(mian)強(qiang)化(hua)的原(yuan)理是在(zai)工具(ju)電(dian)(dian)(dian)極(ji)與工件(jian)之(zhi)間(jian)(jian)接(jie)上(shang)直(zhi)流(liu)電(dian)(dian)(dian)源或(huo)交流(liu)電(dian)(dian)(dian)源,由于振動(dong)器的作用使電(dian)(dian)(dian)極(ji)與工件(jian)間(jian)(jian)的放(fang)電(dian)(dian)(dian)間(jian)(jian)隙頻繁變化(hua),工具(ju)電(dian)(dian)(dian)極(ji)與工件(jian)間(jian)(jian)不斷產生火(huo)花(hua)放(fang)電(dian)(dian)(dian),從而(er)實現對金屬表面(mian)(mian)的強(qiang)化(hua)。
電(dian)(dian)(dian)(dian)(dian)(dian)(dian)火(huo)花(hua)強化(hua)過程如圖(tu)3-20所示(shi)。當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極與工(gong)(gong)件(jian)之間(jian)的(de)距離較大時(shi),電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源經電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻R對(dui)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)極在(zai)(zai)振動器(qi)的(de)帶(dai)動下(xia)向模具靠近(見(jian)圖(tu)3-20a)。當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極與模具之間(jian)的(de)間(jian)隙接近到某個距離時(shi),間(jian)隙中的(de)空氣在(zai)(zai)強電(dian)(dian)(dian)(dian)(dian)(dian)(dian)場的(de)作用下(xia)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)離,產生火(huo)花(hua)放(fang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(見(jian)圖(tu)3-20b),使(shi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極和工(gong)(gong)件(jian)在(zai)(zai)發生放(fang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)部分的(de)金屬局(ju)部熔(rong)化(hua),甚至汽化(hua)。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極繼(ji)續接近工(gong)(gong)件(jian)并與工(gong)(gong)件(jian)接觸(chu)時(shi),火(huo)花(hua)放(fang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)停止,在(zai)(zai)接觸(chu)點(dian)流過短路電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流,使(shi)該處繼(ji)續加熱,由于(yu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極以適當壓力壓向工(gong)(gong)件(jian),使(shi)熔(rong)化(hua)的(de)材料互(hu)相粘接、擴散而形成(cheng)合金或新的(de)化(hua)合物(wu)(見(jian)圖(tu)3-20c)。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極在(zai)(zai)振動器(qi)的(de)作用下(xia),離開了工(gong)(gong)件(jian),放(fang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)部分急劇冷卻(見(jian)圖(tu)3-20d)。經多(duo)次(ci)放(fang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian),并相應地(di)移動電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極的(de)位置,則在(zai)(zai)零件(jian)表面形成(cheng)強化(hua)層。
電火(huo)(huo)花表(biao)面強化(hua)(hua)過(guo)程(cheng)中發(fa)生了物理化(hua)(hua)學變化(hua)(hua),主(zhu)要包括(kuo)超(chao)高(gao)速淬火(huo)(huo)、滲(shen)碳、滲(shen)氮、電極(ji)材(cai)料的轉移等。
a. 超高速淬火(huo)
電(dian)火花(hua)放(fang)電(dian)在(zai)模具表面的極(ji)小面積上產生高溫,使該處的金屬熔化(hua)和部分汽化(hua),當火花(hua)放(fang)電(dian)在(zai)極(ji)短的時間內停止后,被加熱了(le)的金屬會以(yi)很快的速度冷卻下(xia)來。這相當于(yu)對模具表面層(ceng)進行了(le)超速淬火。
b. 滲氮
在電(dian)火花放(fang)電(dian)通道區(qu)域(yu)內(nei),溫(wen)度很高(gao)(gao),空氣中的(de)氮(dan)分子呈(cheng)原子狀(zhuang)態,它(ta)和受(shou)高(gao)(gao)溫(wen)而熔化的(de)金(jin)屬(shu)有關的(de)元素(su)合成高(gao)(gao)硬度的(de)金(jin)屬(shu)氮(dan)化物,如氮(dan)化鐵、氮(dan)化鉻(ge)等。
c. 滲碳(tan)
來(lai)自石(shi)墨電極或(huo)周圍介質的(de)碳元素(su),熔(rong)解在受(shou)熱而熔(rong)化(hua)的(de)鐵(tie)中,形成金屬的(de)碳化(hua)物(wu),如碳化(hua)鐵(tie)、碳化(hua)鉻(ge)等。
d. 電極材料的轉(zhuan)移(yi)
在操作(zuo)壓力和火花放(fang)電的條件下(xia),電極(ji)材料(liao)轉移到模(mo)具金(jin)屬(shu)熔融表面(mian),有關金(jin)屬(shu)合金(jin)元素(W、Ti、Cr等)迅(xun)速擴散(san)在金(jin)屬(shu)的表面(mian)層。
電火花表面強化層(ceng)具有如(ru)下特征(zheng):
a. 當采(cai)用硬質(zhi)合金作電極材料時,硬度可達1100~1400HV(約(yue)70HRC以(yi)上)或更高,耐熱性(xing)(xing)、耐蝕性(xing)(xing)和疲(pi)勞強度都(dou)大(da)(da)大(da)(da)提高。
b. 當使(shi)用(yong)鉻(ge)錳、鎢(wu)鉻(ge)鈷合金、硬質合金作工具電極(ji)強化45鋼時,其耐磨性比原表層提高(gao)2~2.5倍。
c. 用(yong)石墨作電極材料強(qiang)(qiang)化45鋼(gang),用(yong)食鹽水作腐蝕(shi)性(xing)試驗,其(qi)耐(nai)(nai)蝕(shi)性(xing)提高90%;用(yong)WC、CrMn作電極強(qiang)(qiang)化不(bu)銹鋼(gang)時,其(qi)耐(nai)(nai)蝕(shi)性(xing)提高3~5倍。
d. 硬化(hua)層厚度為0.01~0.08mm。
鋼制(zhi)模(mo)具工(gong)作(zuo)表面(mian)的(de)電(dian)(dian)(dian)火(huo)(huo)花強化通常采用(yong)硬質合(he)金電(dian)(dian)(dian)極。為(wei)了使(shi)被強化的(de)表面(mian)光(guang)潔(jie),事先必須(xu)將模(mo)具和電(dian)(dian)(dian)極表面(mian)清洗(xi)干(gan)凈,然后手持振(zhen)動(dong)(dong)器(qi),將電(dian)(dian)(dian)極沿模(mo)具工(gong)作(zuo)表面(mian)移動(dong)(dong),并保持適當壓力,使(shi)火(huo)(huo)花放電(dian)(dian)(dian)均勻連續。
電(dian)火花熔(rong)滲(shen)合金(jin)化層的形成是(shi)一(yi)個漸近過(guo)(guo)程(cheng)(cheng),在每一(yi)電(dian)規(gui)范(fan)下(xia),合金(jin)化層厚度出(chu)(chu)現最大值(zhi),在通常使用的電(dian)容范(fan)圍內,最佳單(dan)位面(mian)積涂(tu)覆時(shi)間為6~12min/c㎡。過(guo)(guo)分延長涂(tu)覆時(shi)間將出(chu)(chu)現層厚減薄的趨勢,并使性能(neng)惡(e)化。可用直(zhi)線方程(cheng)(cheng)式(shi)表示:
如電極YG8、電壓60V,頻率250Hz,電容(rong)60μF,最佳涂覆時間(jian)為(wei)6.75min/c㎡,合金(jin)(jin)化(hua)層厚(hou)度為(wei)13μm;電容(rong)322μF,涂覆時間(jian)為(wei)11.99min/c㎡,合金(jin)(jin)化(hua)層厚(hou)度為(wei)27μm。
為了(le)降(jiang)低合(he)(he)金(jin)(jin)化(hua)層(ceng)的熱疲勞應力和電火花合(he)(he)金(jin)(jin)化(hua)處(chu)理的應力,可穿插1~2次500℃×4h去(qu)應力退火,這(zhe)樣可獲得(de)性能優良、層(ceng)深(shen)較厚(hou)的表面合(he)(he)金(jin)(jin)化(hua)層(ceng)。改換電極(ji)材料,可使合(he)(he)金(jin)(jin)化(hua)層(ceng)繼續增厚(hou),電極(ji)斷(duan)面尺寸不影響合(he)(he)金(jin)(jin)化(hua)層(ceng)的厚(hou)度。鋼(gang)中w(C)小于0.8%時(shi),隨鋼(gang)中碳(tan)含量的增加合(he)(he)金(jin)(jin)層(ceng)增厚(hou);w(C)大于0.8%時(shi),隨鋼(gang)中碳(tan)含量的增加合(he)(he)金(jin)(jin)層(ceng)變薄。
用YG8、Nb、Ti、Ta合金化,工件表面將獲得極(ji)高的顯(xian)微硬度值。
電火花(hua)合金(jin)層比未經電火花(hua)合金(jin)化處(chu)理(li)的(de)模具的(de)熱(re)疲(pi)勞性能提(ti)高3倍,抗(kang)氧(yang)化性能提(ti)高兩倍,在各(ge)種試驗介質中的(de)耐(nai)蝕性提(ti)高3~15倍;表(biao)3-44所(suo)示為3Cr2W8V鋼的(de)處(chu)理(li)效果。
電火花表(biao)(biao)面(mian)強(qiang)(qiang)化(hua)應用(yong)效果顯著(zhu)。例如用(yong)YG8作電極(ji),對(dui)3Cr2W8V鋼模(mo)具(ju)(ju)進行電火花強(qiang)(qiang)化(hua)處理以(yi)后,模(mo)具(ju)(ju)在各類酸堿中(zhong)的耐蝕性提高4~15倍;而(er)Cr12鋼模(mo)具(ju)(ju)刃口部位(wei)經電火花表(biao)(biao)面(mian)強(qiang)(qiang)化(hua)后,模(mo)具(ju)(ju)的平均(jun)使用(yong)壽命由5萬次提高到20萬次。