【摘要】：鈦合金因具有密度小、耐熱性好、強(qiáng)度高和抗腐蝕性好等優(yōu)點(diǎn)而被廣泛應(yīng)用于現(xiàn)代航空制造工業(yè)中。然而,傳統(tǒng)熔焊方法會(huì)造成如氣孔、熱裂紋、高殘余應(yīng)力和大變形等缺陷,嚴(yán)重降低鈦合金接頭的性能。攪拌摩擦焊(Friction stir welding,FSW)是一種固相連接技術(shù),其焊接過(guò)程中的溫度峰值低于材料的熔點(diǎn),因此可避免諸多熔化焊缺陷。近年來(lái),隨著FSW技術(shù)在低熔點(diǎn)材料上的應(yīng)用日趨成熟,國(guó)內(nèi)外學(xué)者將更多的精力用于鈦合金等高熔點(diǎn)材料的焊接過(guò)程中,以期獲得良好的焊接接頭。眾所周知,FSW過(guò)程中攪拌頭軸肩與工件之間的產(chǎn)熱遠(yuǎn)大于攪拌針且TC4鈦合金的熱導(dǎo)率很低,因此焊接過(guò)程中沿焊縫垂直方向會(huì)存在較大的溫度梯度,進(jìn)而在焊縫底部出現(xiàn)撕裂缺陷。該撕裂缺陷的存在使得鈦合金FSW的工藝參數(shù)范圍較窄,不利于在鈦合金結(jié)構(gòu)件中的實(shí)際應(yīng)用與推廣。為解決此問(wèn)題,本文提出一種背部加熱輔助的FSW工藝。首先,研究常規(guī)工藝下TC4鈦合金接頭成形規(guī)律和攪拌頭的磨損。當(dāng)攪拌頭轉(zhuǎn)速為200rpm時(shí),焊縫表面氧化現(xiàn)象嚴(yán)重且飛邊較大,焊縫內(nèi)的撕裂缺陷幾乎貫穿整個(gè)焊縫底部。隨著攪拌頭轉(zhuǎn)速的降低,接頭表面成形趨于良好,內(nèi)部撕裂缺陷逐漸減小。當(dāng)轉(zhuǎn)速為100rpm時(shí),FSW接頭內(nèi)的缺陷消失。針對(duì)攪拌頭的鐓粗、粘結(jié)、機(jī)械磨損和氧化磨損等磨損形式,本文主要采取了低焊速、氣體保護(hù)和預(yù)置小孔等措施。其次,研究背部加熱工藝下TC4鈦合金接頭成形規(guī)律。與常規(guī)工藝相似,接頭表面同樣存在飛邊、凹陷和氧化等缺陷,但背部加熱可明顯擴(kuò)寬工藝參數(shù)范圍。當(dāng)攪拌頭轉(zhuǎn)速為200rpm時(shí),接頭內(nèi)部仍存在撕裂缺陷,但其尺寸相對(duì)于常規(guī)工藝下明顯減小。當(dāng)轉(zhuǎn)速降低至150rpm時(shí),撕裂缺陷消失。隨著攪拌頭轉(zhuǎn)速繼續(xù)降低,接頭內(nèi)部無(wú)缺陷且表面成形良好,原因是背部加熱工藝減小了沿焊縫垂直方向的溫度梯度。另外,背部加熱工藝可增強(qiáng)TC4鈦合金材料的流動(dòng)性,減小攪拌頭的磨損。由于背部加熱工藝減小了溫度梯度,接頭中的組織更加均勻,接頭中整個(gè)焊核區(qū)均為片層狀組織。最后,對(duì)常規(guī)工藝和背部加熱工藝下接頭的力學(xué)性能進(jìn)行測(cè)試。當(dāng)攪拌頭轉(zhuǎn)速為100rpm時(shí),常規(guī)工藝下的接頭抗拉強(qiáng)度為1014MPa,達(dá)到母材的98.9%;延伸率為6.8%,為母材的54.4%,接頭斷裂在焊核區(qū)邊緣。背部加熱工藝下接頭的抗拉強(qiáng)度為958.45MPa,比常規(guī)工藝略低,但是仍可達(dá)到母材的93.6%。兩種工藝下接頭焊核區(qū)硬度均高于母材。
[Abstract]:Titanium alloys are widely used in modern aviation manufacturing industry due to their advantages of low density, good heat resistance, high strength and good corrosion resistance. However, traditional welding methods can cause defects such as porosity, hot cracks, high residual stress and large deformation, which seriously reduce the properties of titanium alloy joints. Friction stir welding (Friction stir welding,FSW) is a solid-state bonding technique, in which the peak temperature in the welding process is lower than the melting point of the material, so many defects can be avoided. In recent years, with the application of FSW technology in low melting point materials, scholars at home and abroad will devote more attention to the welding process of high melting point materials, such as titanium alloys, in order to obtain good welding joints. It is well known that the heat production between the shaft shoulder of the mixing head and the workpiece is much higher than that of the stirring needle and the thermal conductivity of the TC4 titanium alloy is very low during the welding process, so there will be a large temperature gradient along the vertical direction of the weld during the welding process. A tear defect appears at the bottom of the weld. Due to the existence of the tear defect, the range of process parameters of titanium alloy FSW is narrow, which is not conducive to the practical application and popularization of titanium alloy structural parts. In order to solve this problem, a back heating assisted FSW process is proposed in this paper. Firstly, the forming law of TC4 titanium alloy joint and wear of stirring head were studied. When the speed of the stirring head is 200rpm, the surface oxidation of the weld is serious and the flange is large, and the tear defect in the weld almost runs through the bottom of the weld. With the decrease of the rotating speed of the mixing head, the surface forming of the joint tends to be good, and the internal tear defect decreases gradually. When the speed is 100rpm, the defects in the joint disappear. Aiming at the wear forms such as upsetting, bonding, mechanical wear and oxidation wear of agitator, this paper mainly adopts some measures such as low welding speed, gas protection and preset small holes. Secondly, the forming law of TC4 titanium alloy joint under back heating is studied. Similar to the conventional process, the surface of the joint also has defects such as flange, depression and oxidation, but the back heating can obviously widen the range of process parameters. When the speed of the mixing head is 200rpm, there is still tear defect inside the joint, but its size is obviously smaller than that in the conventional process. When the rotational speed is reduced to 150rpm, the tear defect disappears. As the rotating speed of the mixing head continues to decrease, there is no defect inside the joint and the surface is well formed. The reason is that the back heating process reduces the temperature gradient along the vertical direction of the weld. In addition, the back heating process can enhance the fluidity of TC4 titanium alloy material and reduce the wear of the stirring head. Due to the decrease of the temperature gradient in the back heating process, the microstructure of the joint is more uniform, and the whole nugget zone in the joint is lamellar. Finally, the mechanical properties of joints under conventional and back heating processes are tested. When the speed of the stirring head is 100rpm, the tensile strength of the joint is 1014 MPA, which reaches 98.9 of the base metal, and the elongation is 6.8, which is 54.4% of the base metal, and the joint breaks at the edge of the nugget. The tensile strength of the joint under the back heating process is 958.45 MPA, which is slightly lower than that of the conventional process, but it can still reach 93.6% of the base metal. The hardness of joint nugget zone in both processes is higher than that of base metal.
【學(xué)位授予單位】：沈陽(yáng)航空航天大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TG407

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