以往復式擠型法改善鋁合金/鎂合金機械性質及超塑性行為之研究

Study on the improved mechanical properties and high-strain-rate superplasticity of Al and Mg alloys processed by reciprocating extrusion

李士瑋(Shih-Wei Lee)

指導教授 ： 葉均蔚

國立清華大學/工學院/材料科學工程學系/博士(2006年)

摘要

本研究為改善鋁合金及鎂合金之性質以擴展其應用性，針對非析出硬化型之高強度鋁合金5056與5083及析出硬化型之高強度鋁合金7075及低密度双相鎂合金Mg-15Al-1Zn與Mg-20Al-1Zn施以往復式擠型以提升性質，首先觀察不同次數擠製後微結構的改變與演進，包括介在物、析出相、晶粒尺寸與分佈狀態。隨後測試其室溫機械性質與高溫超塑性的表現，研究中包含探討微結構細化及性質提升的原因，並提出一些模型與理論計算來驗證其現象。

5056鋁合金經往復式擠型10次後介在物可以被細化至2微米以下並均勻散佈，而晶粒尺寸則由樹枝晶細化至4.6微米，晶粒內更發現尺寸0.2微米的次晶粒，其存在證明往復式擠型過程中確實發生動態再結晶現象。往復擠型產生的細晶結構在靜態退火中穩定性可以高達500℃；而在500℃、應變速率5x10^-3 s^-1下，5056合金往復擠型10次後的最大伸長量為460%，超塑性發生區域的m值約為0.45。經由計算，此合金的超塑性行為可以利用晶界滑移公式來描述。而往復擠型10次後室溫的強度與5456-H34相當，但伸長率更高，可歸因於細晶結構與介在物尺寸縮小的效應。

5083合金經合金設計添加較多晶粒穩定元素Zr、Cr及Mn，以5083ZM為代號，經由往復式擠型後發現介在物與晶粒的細化程度與5056合金相似，尺寸分別為2微米及4.5微米。但5083ZM合金藉由細微散佈相阻礙效果可使得靜態退火的穩定化極限溫度比5056合金提高25℃。將擠型10次後的試片在500℃、應變速率5x10^-2 s^-1時可達最大伸長量1013%，m值約為0.45，可見5083ZM合金具有高速超塑性。另在固定應變速率1x10^-2 s^-1下，亦發現400℃至500℃具有約600-800%伸長率，顯示在400℃即具有高速超塑性。由於400℃以上的流變應力值都小於15MPa，更顯示超塑性成型所須應力甚低。經由起始應力與活化能的計算，證明5083ZM合金在500℃下的超塑性是屬晶界滑移機制，散佈相增加會提高超塑活化能。本研究提出富Zr契合相提升超塑性能的解釋模型，可成功地解釋細晶契合相何以優於非契合相。

Mg-Al合金經合金設計以双相結構來改善鎂合金的機械性質與高溫超塑性，鋁含量分別為9、15及20 wt%，第二相b相的體積分率由10%增加至45%，經325℃往復擠型10次後平均晶粒尺寸細化至9.2微米、2.8微米、2.5微米。隨著鋁含量的增加，Mg-Al合金的降伏強度明顯的增加，但是伸長率卻因而降低。Mg-20Al-1Zn幾乎不具伸長率，不適用於結構材料。Mg-15Al-1Zn合金降伏強度為303MPa、最大抗拉強度為361MPa，伸長率尚有3.6%；此合金若再經由T6處理，降伏強度更提高為363MPa、最大抗拉強度432MPa，伸長率為2.8％，其比強度因而超過7075-T6合金，可用於更高比強度但韌性較不要求的應用。在275℃、300℃及325℃下，Mg-15Al-1Zn與Mg-20Al-1Zn合金超塑性質遠優於AZ91D，且具有高速超塑性。此高速超塑性將使Mg-15Al-1Zn合金的超塑成行具有成本上的競爭力。利用拉伸試片表面與剖面觀察可以發現超塑性變形後，微結構中的b相具有微觀超塑性的現象，證明低熔點的b相具有調適合金高速超塑性變形的特性，並且隨其體積分率的增加可以增加Mg-Al合金系統的高速超塑性表現。本研究提出一双相Mg-Al合金的共同晶界滑移模型解釋其高速超塑性行為。

7075合金利用往復式擠型法擠型1、5、10及20次後發現平均晶粒尺寸在擠型5次即達到細化極限，為3.5微米；而介在物顆粒在擠型10次後得到最佳的散佈與最小的尺寸0.6微米。往復擠型20次後7075合金降伏與最大抗拉強度比起始材分別降低了9%與11%，但延展性、斷面縮減率、破裂應變與KIC卻增加78%、270%、390%與210%。強度損失的現象與柔軟的無析出帶體積分率增加有關。隨擠型次數增加，拉伸試片的頸縮變形現象越來越明顯；破斷面由沿晶破斷逐漸變為100%的穿晶破斷，此現象與晶粒及介在物尺寸的細化有關，本研究對前述之相關性皆作深入探討。由於韌性的大幅改善，擠型20次可使7075合金的設計強度高達550MPa，比傳統7075合金提高200MPa。

關鍵字

鋁合金 ； 鎂合金 ； 往復式擠型 ； 高速超塑性 ； 細晶 ； 共同晶界滑移 ； 破裂應變 ； 沿晶破斷 ； 動態再結晶

並列摘要

The aim of this study is to improve the properties of aluminum and magnesium alloys and expand their applications. The unique method, named “reciprocating extrusion”, was applied on conventional aluminum alloys, such as 5056, 5083 and 7075, and new dual-phase magnesium alloys, such as Mg-15Al-Zn and Mg-20Al-Zn. The evolution of microstructures and mechanical properties at room temperature and superplasticity at high temperatures were examined. The relationship between refined microstructures and enhanced properties were built up, and theoretical calculations and models were proposed to prove some outstanding phenomena.

The grain size in 5056 aluminum alloy was reduced to 4.6 microm and the coarse inclusions refined to 2 microm after ten passes of reciprocating extrusion. A subgrain structure was formed in the interior of the fine grains, indicating that dynamic recrystallization occurred during extrusion. Dynamic recrystallization in the billet is proposed to be repeatedly induced with the number of extrusion passes until a limiting grain size was obtained. Thereafter, dynamic recrystallization was no longer activated because grain boundary sliding, instead of dislocation gliding, accommodated the deformation strain required for extrusion. The fine-grained structure and its substructure were stable up to 500℃. The maximum elongation of the alloys after ten passes extrusion is 460% at 5x10^-3 s^-1 and 500℃ with a m value of 0.45. After calculation, the dominant of superplasticity in the alloys is grain boundary sliding. The alloys extruded with ten extrusion passes exhibited a superior combination of strength and ductility over commercial 5456-H34. The enhancement of mechanical properties is related to the fine grain structure and refined inclusions.

The 5083 aluminum alloys modified with grain refiner, 0.25% Zr and 0.46% Mn, were processed by reciprocating extrusion to yield high-strain-rate superplasticity above 400 ℃ and superior room-temperature mechanical properties. Without any prior homogenization treatment, ten extrusion passes could give the cast billets an equiaxed grain structure with a grain size of about 4.5 microm and a subgrain size about 0.2 microm, and a uniform distribution of fine inclusions and dispersoids in the matrix. The fine-grained structure was stable up to 525 ℃, giving the alloy a high-strain-rate and low-stress superplasticity over a wide operating temperature of 400-500 ℃. In the tensile test at 500 ℃, a maximum elongation of 1013% and a low flow stress of 7.7 MPa at 5x10^-2 s^-1 were achieved. The apparent and true activation energies for low temperatures (300-400 ℃) without high-strain-rate superplasticity were 220.6 and 208 kJ/mol, respectively, whereas those at high temperatures (400-500 ℃) were 88.4 and 98.7 kJ/mol, respectively. Further analysis confirms that grain boundary sliding is the dominant mechanism over the high-strain-rate region from 1x10^-2 s^-1 to 5x10^-1 s^-1 at 500 ℃, and power-law breakdown mechanism occurs over the strain rate from 5x10^−4 s^−1 to 1x10^−2 s^−1 at 300 ℃. The high-strain-rate superplasticity was more strongly enhanced by Zr addition than addition of Cr and Mn. Two enhancing mechanisms for the maximum superplastic elongation and the optimum strain rate are proposed. This study concludes that the effectiveness of Zr is caused by the fineness and the coherency of Zr-rich dispersoids in the matrix.

A superior combination of specific strength and low-temperature high-strain-rate superplasticity of two-phase Mg-15Al-1Zn and Mg-20Al-1Zn alloys could be achieved with reciprocating extrusion directly from as-cast billets. The volume fraction of b phase increases from 10% to 45% with increasing Al content from 9 wt% to 20 wt%. The average grain size of Mg-15Al-1Zn and Mg-20Al-1Zn alloys could be refined to 2.8 and 2.5 microm, respectively, after ten passes at 325℃. The yield strength increases but elongation reduces with increasing aluminum content in Mg-Al alloy. In the as-extruded state, the Mg-20Al-1Zn alloy has less ductility. However, the elongation, yield strength and ultimate tensile strength of Mg-15Al-1Zn were 3.6%, 306 MPa and 376 MPa, respectively. After T6 heat treatment, Mg-15Al-1Zn alloy has an excellent improvement on yield and ultimate tensile strength, which are 363 MPa and 432 MPa, respectively. It is noticed that the specific strength of Mg-15Al-1Zn is larger than 7075-T6. The pronounced strengthening mechanism was attributed to the high volume fraction of fine-grained hard Mg17Al12 phase in the refined a-Mg matrix. At 275, 300 and 325℃, both the Mg-15Al-1Zn and Mg-20Al-1Zn have better high-strain-rate superplasticity than AZ91D. The optimum elongation of Mg-15Al-1Zn at least 1610% in company with a high m value of 0.7 was obtained at the strain rate of 1x10^-2 s^-1 when tested at 325 ℃. The excellent superplasticity at high strain rates was also mainly contributed by the high volume fraction of Mg17Al12 phase, which displays easier grain boundary sliding than alpha-Mg phase. The apparent activation energy for superplastic flow is thus smaller than that of the boundary diffusion in Mg. In addition, the mechanisms for the filament formation and cooperative grain boundary sliding are discussed. The increase on volume fraction of beta phase could improve the performance of high-strain-rate superplasticity in Mg-Al alloys. A cooperative grain boundary sliding mechanism is proposed to explain how beta phase could enhance the superplasticity in two-phase Mg-Al alloys.

The reciprocating extrusion method was also applied on 7075 Al alloy to refine the grains and inclusions with the aim of improving mechanical properties. Various extrusion passes, that is 1, 5, 10, 20, were chosen to investigate the microstructure evolution and property variation. Experimental results indicate that grain refining largely ceases after five extrusion passes whereas significant inclusion refining continues up to the 20th pass. Comparing the properties of the 20-pass extrudates with those of the starting material with zero pass, yield strength and tensile strength decrease by 9% and 11%, respectively, while elongation, reduction of area, fracture strain and KIC increase by 78%, 270%, 390%, and 210%, respectively. However, compared to the typical properties of 7075 alloys, yield strength, tensile strength, elongation and KIC are all improved and increase by 11%, 3 %, 68%, and 200%, respectively. The ductility and toughness improvement is attributable to the refinement of both grains and inclusions, while the strength loss results from the increased volume fraction of soft PFZ associated with grain refinement. Investigation of tensile fracture surfaces demonstrates the transition of the fracture mode from predominantly intergranular to completely transgranular. This phenomenon in company with the extensive necking behavior is consistent with the significant increasing of ductility and toughness. The reciprocatingly extruded 7075-T6 alloys exhibit superior strength-ductility and strength-toughness combinations to conventional ones. This characteristic could largely increase the designed stress standard for the airframe structure based on the fail-safe design concept.

並列關鍵字

aluminum alloys ； magnesium alloys ； reciprocating extrusion ； high-strain-rate superplasticity ； fine grain structure ； cooperative grain boundary sliding ； fracture strain ； transgranular ； dynamic recrystallization

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