【摘要】：激光增材制造高性能金屬零部件技術(shù)具有成形結(jié)構(gòu)復(fù)雜、成形精度高、成形性能優(yōu)良等特點(diǎn),是當(dāng)前復(fù)雜精密金屬零部件或大尺寸主承力金屬構(gòu)件一次性整體成形最具前景的應(yīng)用技術(shù)之一。它不僅是鑄造、鍛造、焊接與機(jī)械加工等傳統(tǒng)加工方法的有益補(bǔ)充,而且開(kāi)啟了一種全新的金屬零件制造模式,具有重要的戰(zhàn)略研究意義。目前,激光增材制造高性能金屬零部件技術(shù)主要有兩種典型方法,一種是基于同步送粉的激光熔覆沉積技術(shù)(Laser Cladding Deposition,簡(jiǎn)稱LCD技術(shù)),另外一種是基于預(yù)置鋪粉的激光選區(qū)熔化技術(shù)(Selective Laser Melting,簡(jiǎn)稱SLM技術(shù))。然而,上述兩種技術(shù)因其成形方式與工藝參數(shù)的差別,導(dǎo)致二者在熔池形貌、冷卻速率、凝固組織以及力學(xué)性能等若干材料成形基礎(chǔ)方面存在較大差異,這為深入理解高性能金屬零部件激光增材制造技術(shù)的成形原理及應(yīng)用背景帶來(lái)了一定的困難。為此,本文將對(duì)LCD與SLM技術(shù)在上述基礎(chǔ)問(wèn)題方面表現(xiàn)出來(lái)的差異性展開(kāi)詳細(xì)的對(duì)比研究,主要研究成果如下：采用大光斑、中光斑、小光斑LCD工藝以及微光斑SLM工藝成形316L不銹鋼單道熔覆層試樣,并由此將激光增材制造技術(shù)分為四大工藝類型展開(kāi)研究。引入熔池的形狀系數(shù),特別是穿透系數(shù),來(lái)對(duì)熔池的形貌變化進(jìn)行分析。在LCD與SLM不同工藝條件下,熔池形貌與尺寸完全不同,導(dǎo)熱模式與熱影響效應(yīng)也不同,進(jìn)而最終影響到合金的組織與性能。尤其是在LCD與SLM工藝中,熔池形貌特征隨歸一化熱焓的變化表現(xiàn)出相互獨(dú)立的趨勢(shì),預(yù)示著LCD與SLM凝固過(guò)程的差異所在。采用LCD與SLM技術(shù)大、中、小、微四種不同光斑大小的工藝方案成形316L不銹鋼樣品,測(cè)量合金在不同工藝條件下的一次枝晶臂間距大小。通過(guò)一次枝晶臂間距與冷卻速率之間的經(jīng)驗(yàn)關(guān)系式,計(jì)算出熔池冷卻速率的大小。結(jié)果顯示,不同工藝條件下熔池的冷卻速率相差1～4個(gè)數(shù)量級(jí),其中,微光斑SLM工藝與大光斑LCD工藝熔池冷卻速率的差異最高達(dá)到4個(gè)數(shù)量級(jí)。能量輸入與熔池形貌的變化是導(dǎo)致冷卻速率出現(xiàn)較大差異的主要原因。冷卻速率大小決定晶粒尺寸大小。在LCD與SLM不同工藝方案成形316L不銹鋼中,柱狀晶寬度、長(zhǎng)度及單個(gè)柱狀晶的平均體積均隨能量輸入的增加而增加,且柱狀晶的縱橫比也逐漸增大。然而,柱狀晶尺寸隨冷卻速率的變化卻表現(xiàn)出不同的趨勢(shì)。在LCD成形過(guò)程中,柱狀晶尺寸與冷卻速率滿足經(jīng)典的λ=a+b／(?)(λ代表晶粒尺寸大小,于為冷卻速率,a,b為常數(shù))線性關(guān)系,這與傳統(tǒng)的凝固理論相似。而在SLM成形過(guò)程中,柱狀晶尺寸與冷卻速率平方根的倒數(shù)卻滿足三次函數(shù)關(guān)系,與傳統(tǒng)凝固理論不符合。進(jìn)一步分析表明,在LCD工藝中,由于冷卻速率與過(guò)冷度較小,導(dǎo)致晶粒的形核率與生長(zhǎng)速率比值相對(duì)較小,再加上熔池散熱方向的單向性等原因,使得LCD工藝比SLM工藝更容易形成粗大的柱狀晶組織。SLM與LCD成形過(guò)程中的多重?zé)嵫h(huán)過(guò)程使合金組織結(jié)構(gòu)變得更加復(fù)雜。在高功率SLM成形1Cr18Ni9Ti不銹鋼中,隨著熔池穿透系數(shù)的增加,熔覆層所經(jīng)歷的熱循環(huán)次數(shù)越多,導(dǎo)致液相合金的冷卻速率降低、過(guò)冷度減小,根據(jù)柱狀晶的生長(zhǎng)機(jī)制可知,這將導(dǎo)致柱狀晶尺寸變粗大。同時(shí),多重?zé)嵫h(huán)過(guò)程還具有多次固溶處理的效果,這使得合金在胞壁區(qū)域的偏析現(xiàn)象減少,合金內(nèi)元素成分更加均勻。同時(shí),在小光斑LCD成形Inconel 718合金中,隨著能量輸入的增加,熔池中液相的溫度梯度增加,成分過(guò)冷區(qū)減小,合金的組織形貌從樹(shù)枝晶轉(zhuǎn)變?yōu)闃?shù)枝晶/胞晶共存,再轉(zhuǎn)變?yōu)榘�。而�?晶粒內(nèi)亞晶界區(qū)域的拉夫斯相逐漸增多,基體內(nèi)的二次沉淀相逐漸減少。組織結(jié)構(gòu)的差異導(dǎo)致SLM成形構(gòu)件的力學(xué)性能優(yōu)于LCD成形構(gòu)件的力學(xué)性能。在LCD與SLM成形316L不銹鋼中,SLM成形樣品的顯微硬度比LCD成形樣品高100 HV左右；SLM成形樣品的最大抗拉強(qiáng)度比LCD成形樣品高180 MPa; SLM成形樣品的屈服強(qiáng)度是鍛件標(biāo)準(zhǔn)的2.5倍,而LCD成形樣品的屈服強(qiáng)度是鍛件標(biāo)準(zhǔn)值的1.5倍。在高功率SLM成形1Cr18Ni9Ti不銹鋼中,隨著熱循環(huán)次數(shù)的增多,由于晶粒尺寸與成分均勻性的雙重變化,樣品的拉伸性能變化差異較小；而在小光斑LCD成形Inconel 718合金中,隨著柱狀晶生長(zhǎng)方向與樣品沉積高度方向之間的夾角增大,樣品的拉伸性能變差,且當(dāng)試樣沉積高度方向與拉伸方向垂直時(shí)的拉伸強(qiáng)度要比二者平行時(shí)的拉伸強(qiáng)度高約50 MPa。
[Abstract]:Laser augmentation is one of the most promising technologies for one-off integral forming of complex precision metal parts or large-sized main bearing metal components. It is not only a tradition of casting, forging, welding and machining. At present, there are two typical methods for manufacturing high-performance metal parts by laser augmentation. One is laser cladding Deposition based on synchronous powder feeding. The other is laser selective melting (SLM) technology based on pre-laid powder. However, because of the difference of forming methods and technological parameters, there are great differences between the two technologies in pool morphology, cooling rate, solidification structure and mechanical properties of some materials. It is difficult to understand the forming principle and application background of laser augmentation manufacturing technology for high performance metal parts. In this paper, the differences between LCD and SLM technology in the above basic problems are studied in detail. The main research results are as follows: LCD technology with large spot, medium spot and small spot is adopted. The shape coefficient of the molten pool, especially the penetration coefficient, is introduced to analyze the morphological changes of the molten pool. In particular, in LCD and SLM processes, the morphological characteristics of the molten pool show an independent trend with the change of normalized enthalpy, indicating the difference between the solidification process of LCD and SLM. The primary dendrite arm spacing of 316L stainless steel was measured under different process conditions. The cooling rate of molten pool was calculated by the empirical relationship between primary dendrite arm spacing and cooling rate. The difference of cooling rate between spot SLM and large spot LCD is up to four orders of magnitude. The variation of energy input and pool morphology is the main reason for the large difference of cooling rate. The average volume of columnar crystals and single columnar crystals increase with the increase of energy input, and the aspect ratio of columnar crystals increases gradually. However, the change of columnar crystal size with cooling rate shows a different trend. In the process of SLM forming, the reciprocal of columnar crystal size and the square root of cooling rate satisfies a cubic function, which is not consistent with the traditional solidification theory. Due to the relatively small ratio of nucleation rate to growth rate and the unidirectional heat dissipation direction of molten pool, it is easier for LCD process to form coarse columnar structure than SLM process. With the increase of penetration coefficient, the more thermal cycles the cladding layer undergoes, the lower the cooling rate and undercooling degree of the liquid alloy. According to the growth mechanism of columnar crystals, the columnar crystals become coarser and coarser. At the same time, with the increase of energy input, the temperature gradient of the liquid phase in the molten pool increases, and the supercooled zone decreases. The microstructure of the alloy changes from dendrite to dendrite/cell coexistence and then to cell. In 316L stainless steel formed by LCD and SLM, the microhardness of SLM is about 100 HV higher than that of LCD, and the maximum tensile strength of SLM is about 100 HV higher than that of LCD. The yield strength of SLM is 2.5 times higher than that of forging standard, while that of LCD is 1.5 times higher than that of forging standard. However, the tensile strength of Inconel 718 Alloy formed by LCD with small facula decreases with the increase of the angle between the growth direction of columnar crystal and the deposition height of the sample, and the tensile strength of the sample is about 50 MPa higher when the deposition height is perpendicular to the tensile direction.
【學(xué)位授予單位】：華中科技大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：TH16

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