SLM制備Ti6Al4V時缺陷同拉伸性能的關(guān)系

來源：：長三角G60激光聯(lián)盟

據(jù)悉，美國Lawrence Livemore國家實驗室的學(xué)者研究了SLM制備Ti6Al4V時缺陷同拉伸性能的關(guān)系，其成果發(fā)表在《Materials & Design》上。

成果的Graphical abstract
來自美國Lawrence Livemore國家實驗室的學(xué)者在《Materials & Design》上發(fā)表了論文《Defects -dictated tensile properties of Selective laser melted Ti6Al4V》，為我們報道了如下研究成果：

一個正常的能量圖用來識別用于高能量激光制備Ti6Al4V（簡寫為Ti64）樣品的工藝圖。

圖 Lawrence Livemore國家實驗室

屈服強(qiáng)度和均勻的拉伸伸長率通過高質(zhì)量的樣品的顯微組織和缺陷的生成來判斷。

小比例的空穴（≤ 1%）可以在拉伸時生長和聚集，從而會影響到總的應(yīng)變到斷裂的過程。

圖 沉積態(tài)的Ti64樣品的相組成和顯微組織：

a) Synchrotron XRD patterns collected for six different samples. Small peaks arising from β-Ti are pointed with arrows in V8 and H8. b) 3D reconstruction of the microstructure using optical micrographs (OMs). c) EBSD mapping on the BD/LD surface of an as-printed Ti64 sample (IPF image). d) Pole figures (0001 and 11–20 reflections) of the area highlighted by the black dashed-line in c) and stereographic projection of the parent β grain reconstructed based on these two pole figures. e) IPF of the high-temperature β microstructure based on c). f) and g), Bright field TEM pictures showing the defect structures in as-built materials, consisting of compression twins, tension twins, and 〈c + a〉 dislocations.

微塑形的早起發(fā)生是由于制造的缺陷所造成的。邊緣的掃描道是缺陷產(chǎn)生的重要來源，從而影響拉伸性能。

成果簡介：
采用SLM（又叫LPBF）來沉積制備金屬，經(jīng)常會存在缺陷，如位錯、孿生、元素偏析、雜質(zhì)和氣孔等，從而正面的或負(fù)面的影響到產(chǎn)品的機(jī)械性能。在這里來自美國Lawrence Livemore國家實驗室的研究人員系統(tǒng)的研究了室溫條件下準(zhǔn)靜態(tài)時SLM制備的Ti64合金的拉伸行為，并采用當(dāng)前的原位同步X射線衍射（SXRD）和SXCT掃描技術(shù)進(jìn)行了在拉伸屈服強(qiáng)度和均勻伸長時，主要由沉積態(tài)的顯微組織來決定，而應(yīng)變-失效則同氣孔的關(guān)系非常敏感，甚至是高密度樣品時（大于99.5%）時也是如此，原位SXRD揭示了在沉積態(tài)時微塑性的萌生起源于應(yīng)力水平，顯著的顯示為早起晶格應(yīng)變偏離行為。SXCT揭示了氣孔的生長機(jī)制是在拉伸軸垂直于制造方向的時候，而在沿著制造方向時卻觀察不到這一現(xiàn)象。這一不均勻的氣孔生長機(jī)制導(dǎo)致了SLM時的應(yīng)變-失效的巨大差異。研究人員所發(fā)現(xiàn)的熔池動力學(xué)模擬和實驗結(jié)果揭示了早先并不清楚的氣孔源的生成機(jī)制，即掃描道邊緣的氣孔。同時研究人員還為大家展示了一種正常的能量圖來識別獲得高質(zhì)量樣品的優(yōu)化的工藝區(qū)間。

圖  沉積態(tài)Ti64鈦合金的樣品的機(jī)械性能

a) Stress/strain curves obtained during tensile testing at 10−3/s. b) Yield strength versus strain-to-failure taken from the data in a) and compared to the literature. c) Normalized work hardening as a function of strain for four samples, each representative of a group presented in b). Dashed lines highlight a normalized work hardening of 1 which represents the beginning of the necking. d) Yield strength versus uniform elongation taken from the data presented in a).

Ti64由于具有高強(qiáng)度、低密度、高耐蝕性和生物相容性，而成為應(yīng)用最為廣泛的鈦合金，從而廣泛的應(yīng)用到航空航天、海洋、醫(yī)療和動力工程。然而，Ti64合金難以鑄造和機(jī)加工，這是因為該合金溶體具有較高的反應(yīng)活性和較低的熱傳導(dǎo)性。增材制造技術(shù)吸引了人們的廣泛注意。但在實際應(yīng)用過程中，一個主要的問題在于SLM制備的Ti64存在較差的拉伸韌性，這是因為在SLM制備過程中存在脆性的馬氏體相α’，這是在采用SLM制備鈦合金時的一個非常嚴(yán)重的問題，需要克服。早起的研究，大多聚焦在如何將α’相分解成更為韌性的α+β相上來，采取的策略是改變工藝參數(shù)和/或后續(xù)熱處理。改變β相使其成為強(qiáng)且韌的相的研究，目前還很少看到有報道。作為比較，很少有研究用于揭示SLM制備Ti64時的拉伸性能的影響因素，這一性能不僅受到內(nèi)在的顯微組織和缺陷的影響，同時還對氣孔等非常敏感，這是增材制備時常見的問題。

圖 在拉伸測試時原位 SXRD 在P1垂直方向（ (a, c, e)）和P1平行方向 (b, d, f)時的測試結(jié)果

Lattice strain and FWHM as a function of the macroscopic engineering stress for the six planes: . a) and b) Engineering strain/stress curves for the two samples. The crosses mark the end of the uniform elongation. Note that SXRD data collected after necking are not shown in this figure. c) and d) Evolution of the lattice strain along both tensile and compressive directions in the two samples. Note that the {0002} peak in both samples could not be accurately fitted along the compressive direction due to weak intensity. All lattice strains are normalized by the initial d-spacing measured in the as-printed state, just before loading. e) and f) FWHM for the same set of planes in the two samples along the tensile direction. Note that FWHM measurement accuracy is in the range of ±0.002 1/nm to ±0.005 1/nm, depending on the reflection.

早期的研究工作曾經(jīng)指出，氣孔的形狀、尺寸、分布和體積分?jǐn)?shù)、強(qiáng)烈的影響著拉伸強(qiáng)度和拉伸韌性。同時研究已經(jīng)指出，氣孔同時還會顯著的影響到制品的疲勞性能。對相對來說比較脆性的材料，少量的氣孔會嚴(yán)重的影響到拉伸韌性。當(dāng)然，也有人認(rèn)為，空穴也許會有積極的效果，甚至?xí)�由于位錯-空穴的相互作用而可以促進(jìn)拉伸韌性性能的提高。由于Ti64是一種低韌性的合金，相似的負(fù)面效應(yīng)也許會獲得。眾所周知，SLM時，激光參數(shù)（功率、速度）、層厚、掃描策略、支撐結(jié)構(gòu)等均會影響最終的相和晶粒尺寸的，同時也會影響到氣孔的含量。無論如何，由于氣孔在高密度樣品中的作用的不確定性，氣孔對拉伸性能的影響還是值得詳細(xì)的研究的。

圖 在原位SXRD拉伸測試時織構(gòu)的變化

a) and b) {0002} peak distribution over the entire diffracted ring as a function of the macroscopic strain for both V8 and H2 samples. Note that the y-axis does not follow a linear scale since SXRD data acquisition was controlled by the time and not the strain. Stars of the same color are 65° apart, corresponding to a set of matrix/compression twins. Circles of the same color are 85° apart, corresponding to a set of matrix/tension twins. Azimuthal angles of 0 and 180 correspond to the loading direction (marked with solid white lines). In b), the blue arrows highlight compression twin activity with one peak intensity decreasing when the other one increases. c) and d) Engineering tensile curves for the same samples V8 and H2, respectively. Red crosses correspond to the locations of the blue curves in a) and b).

在這里，研究人員采用原位同步輻射SXRD技術(shù)和掃描技術(shù)SXCT來研究缺陷，即預(yù)先存在的位錯和孿生，以及氣孔對SLM制造的Ti64拉伸性能的影響。

圖 同步X-CT技術(shù)在拉伸測試前和拉伸測試后得到的實驗結(jié)果

Synchrotron X-ray computed tomography conducted before and after tensile testing in several dog-samples with a 1 × 1 mm2 gauge section. Red regions correspond to pores. a) and b) As-printed samples cut from plates P1 and P7, respectively. c) Yield strength versus strain-to-failure of the samples concerned by this SXCT study. d) Optical images of dog-bone samples cut vertically and horizontally from the plate P1. e) Same samples as in d) analyzed by XCT after failure.

圖 氣孔的分布和體積分?jǐn)?shù)

a) and b), respectively in the samples P1 (Horizontal and Vertical) before (as-printed) and after (postmortem) tensile test, measured by SXCT. UR and NR are the uniform and necking regions of the dog-bone sample after test, respectively.

圖 上圖中（a）中的2D橫截面和3D重構(gòu)的結(jié)果：

a) Locations of the two cross-sections displayed in b) and c). b) Cross-section corresponding to the green plane in a). This plane cut through the laser turn-around induced pores. c) Cross-section corresponding to the blue plane in a). This plane follows the pores aligned along the laser track. The dimensions correspond to the average distance between pores calculated from the projected intensity profiles in red and blue.

圖 本實驗后得到的正常的能量圖用于工藝判斷 （見紅色的部分）

a previous study conducted with Ti64 pillars printed in the same conditions [37], and the literature. a) Normalized heat input diagram (log10 scale) where the dash lines are the results of the product of 1/h⁎ and E⁎ (see text). b) Sample density as a function of the normalized heat input (semi-log10 scale).

圖：采用 ALE3D代碼在不同的激光參數(shù)下進(jìn)行掃描時得到的氣孔形成的模擬圖

The first column (a, b, c) corresponds to the set P1 (laser power of 100 W, and speed of 600 mm/s) while the second column (d, e, f) corresponds to the set P7 (laser power of 250 W, and speed of 1300 mm/s). The interface between the liquefied or solidified Ti64 and the ambient is represented in yellow color. The red envelope shows the solid/liquid interface. a) and d) show the melt pool from below the surface. b) and e) the side views at a given time during printing, while c) and f) are side views after the laser was turned off.



本研究的主要結(jié)論如下：

L-PBF制備 Ti64樣品得到的顯微組織為為馬氏體，其強(qiáng)度可以達(dá)到 (>1110 MPa),該強(qiáng)度的提供得益于復(fù)雜的顯微組織的組成，主要是細(xì)小的α′板條、元素偏析和高密度的缺陷，缺陷主要是位錯、和孿生。

1.構(gòu)建了一個正常的能量圖，該圖可以使我們直接比較其他SLM制備和EBM制備Ti64鈦合金的樣品時的工藝窗口。該工藝圖可以進(jìn)一步的識別本工作中的高密度樣品。

2.我們對高密度樣品進(jìn)行的拉伸測試，沿著不同的制備方向和在不同的測量幾何形狀的條件下，顯示出增材制造的Ti 64樣品的屈服強(qiáng)度和均勻的伸長率主要由材料內(nèi)在的顯微組織所確定。而應(yīng)變-失效則強(qiáng)烈的依賴于氣孔的存在，即使是其體積率 <1%。均勻的應(yīng)變-失效可以推薦為在增材制造材料時拉伸韌性的有效評估手段。

3.TEM和XRD分析結(jié)果進(jìn)一步的證明了高密度位錯和孿生在沉積態(tài)顯微組織中的存在，其主要來源于馬氏體的轉(zhuǎn)變和在打印過程中的塑性變形。材料的強(qiáng)度的增加得益于位錯和孿生的自由移動或相互作用。小比例的β相在SLM制備的樣品中也存在。

4.原位同步X射線衍射顯示，在沉積態(tài)的Ti64合金中，微塑性的萌生在應(yīng)力水平比較低的時候，只有宏觀屈服強(qiáng)度的一半。這表明變形行為的比較小的差異，甚至是在不同的參數(shù)下加工的樣品或在不同的方向上加載也是如此。

5.X-CT結(jié)果揭示了氣孔的聚集和生長長大，從而造成的失效機(jī)制，這是當(dāng)拉伸軸垂直于制造方向的時候，導(dǎo)致了更加有限的應(yīng)變-失效，而這一機(jī)制并沒有在平行于制造方向的時候觀察到。

6.多物理場模擬結(jié)果揭示了我們并沒有知道的在SLM制備鈦合金時的氣孔源的生成機(jī)制：掃描道邊緣的生成。他們造成了空穴在粉末顆粒之間的存在，從而看起來對拉伸性能的影響非常大。

文章來源：Defects-dictated tensile properties of selective laser melted Ti-6Al-4V,Materials & Design,Volume 158, 15 November 2018, Pages 113-126,https://doi.org/10.1016/j.matdes.2018.08.004