7075铝合金搅拌摩擦加工的组织结构及性能表征
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摘要
7075铝合金作为可热处理强化高强铝合金,被广泛应用于航天、航空、军工以及汽车等工业领域。但在7075铝合金熔融焊接过程中,容易产生较大的变形、热应力,以及气孔、裂纹等缺陷,严重影响7075铝合金的应用。搅拌摩擦焊接作为一种新型的固相焊接技术,为7075铝合金的焊接与加工提供了有效的加工方法。然而,由于应用的现状,目前对7075合金的搅拌摩擦焊接/搅拌摩擦加工研究主要集中于对时效态7075铝合金的搅拌摩擦加工方面,对固溶态7075铝合金的搅拌摩擦加工研究报导相对较少。本文通过对不同热处理状态(固溶态及时效态)7075铝合金进行搅拌摩擦加工后的显微硬度及光学组织研究,并结合对硬度特征数值处的背散射电子通道衬度相、透射电镜和背散射电子衍射研究,讨论了加工过程中的温度梯度场与合金形变机制、第二相及织构演变的联系规律。
对比了两种搅拌摩擦加工方式对不同热处理状态合金的的微观组织结构的影响。详细研究了两种状态合金平行于轧向搅拌摩擦加工后的金相组织。结果表明:固溶态合金在加工区横截面的轴肩影响区、搅拌区和前进侧的交界处,以及在焊核区漩涡区、搅拌区与前进侧的交界处,存在较为明显的孔洞型缺陷,而时效态合金并没有在这两处发现缺陷;固溶态合金在轴肩影响区与前进侧的界面变化幅度较为平缓,时效态合金界面过渡则较陡。
对两种状态合金分两次进行了系统的显微硬度实验,建立了硬度分布图。各层面显微硬度分布总体上呈现出W形状。焊核中心的硬度从底面至表面呈现出逐渐增大的趋势。时效态合金焊核区硬度的回升比固溶态合金的硬度回升明显。同时,在3mm层面附近,时效态合金出现明显的低硬度区,而固溶态合金则没有发现软化带。此外,两次硬度试验表明,自然时效对两种状态合金的硬度分布存在影响。
根据两种状态合金硬度分布图的特征,通过扫描电镜、背散射电子通道衬度相和透射电镜对特定硬度数值位置处的组织结构进行了分析。研究表明,焊核区晶粒由细小等轴晶粒组成,且从底部至顶部晶粒越来越大。焊核区底部至顶部晶界相逐渐减少,但晶内析出相却逐渐增多。时效态合金在焊核区晶内析出的第二相明显要多于固溶态。
对两种状态合金的背散射电子衍射实验表明,两种状态合金在热机影响区和焊核区的晶粒的取向变化差别不大。越靠近焊核区,回退侧的热机影响区呈现出小角度晶界逐渐减少,大角度晶界逐渐增加的渐变式的变化。随着逐渐靠近焊核区,晶粒织构越来越弱。两种状态合金焊核区晶粒主要由大角度界面构成,内部有少量的小角度界面,大角度界面的百分数占到了80%以上。经搅拌摩擦加工处理后,由于晶粒发生偏转,两种状态合金在焊核区均形成了弱的剪切织构。
7075 aluminum alloy,as a heat treatmentable high-strength aluminum alloy,has been widely used in aerospace,aviation,military and automotive industries. However,it is generally considered difficult to be the fusion welding process due to the poor solidification microstructure and porosity in the fusion zone. As a new type of solid state welding method, Friction Stir Welding provides an effective processing methods for welding and processing of 7075 aluminum alloy. But,possibly due to practical application status of 7075 aluminum alloy, the studies have been mainly focused on the processing microstructures and mechanical properties of the age-treated 7075 aluminum alloy. There is much less data on the microstructure evolution and mechanical properties of solution treated 7075 alloy. In this paper,through a large number of micro-hardness and OM experiments and combined with ECC,TEM and EBSD study for specific location of hardness profiles, the solution and age treated 7075 aluminum alloy were investigated by friction stir processing respectively. The relationship among temperature gradient,deformation mechanism,the precipitates distribution and texture evolution are discussed.
The effect of processing modes on the microstructures of the two alloys was compared. The optical micro-structures that were processed paralleling to the roll direction were studied in more details. The results showed that the solution treated alloy have defects found at the intersection between Shoulder Flow Zone,Stir Zone and Advancing Side, and the intersection of Swirl Zone、Stir Zone and TMAZ on transvere cross-section of processing zone, while no defects were found in the age treated alloy. Interface between the SFZ and AD changes smoothly in the solution treated alloy while the interface transition in the age treated alloy changes steeply.
Micro- hardness on the cross section of both alloy were test systemetically and the maps of hardness distribution were established. Micro-hardness profiles at the certain planes of solution-treated and age-treated 7075 Al exhibits Letter W morphology, typically as that in the most friction stir welds. Hardness in the center of the WZ gradually increased from bottom to surface. Meanwhile,apparent low hardness zone were found at the 3mm plane of the age treated alloy while no softened bands weren’t found in the solution treated alloy. In addition,two hardness tests showed that the natural aging has great influence on the hardness distribution in the two alloys.
Microstructures at specific location of hardness profiles were studied by SEM-ECC and TEM. The results showed that the sizes of fine equiaxed grains in the WZ increase from bottom to top of sheet. The amounts of intergruanlar precipitates gradually decreased from bottom to top while that of intragranular precipitates increased from bottom to top. The amount of intragranular precipitates in the age treated alloy were more than that of the solution treated alloy. However,in the 3mm level of hardness distribution,distribution of the second phase is more uniform in the alloy under solution heat treatment more than aging.
EBSD experiments show that the two alloys have similar grain orientation in the TMAZ and WZ. Low angle boundaries ( LABs) and high angle boundaries ( HABs) in the TMAZ of retreating side increase and decrease, respectively, and grain texture becomes weak when the detected site is approaching to the WZ. The grains are constituted by the HABs that accounted for more than 80% in the WZ. Due to shearing of the pin,the weak shear textures are formed in the WZ by rotation of the grains after the FSP.
对比了两种搅拌摩擦加工方式对不同热处理状态合金的的微观组织结构的影响。详细研究了两种状态合金平行于轧向搅拌摩擦加工后的金相组织。结果表明:固溶态合金在加工区横截面的轴肩影响区、搅拌区和前进侧的交界处,以及在焊核区漩涡区、搅拌区与前进侧的交界处,存在较为明显的孔洞型缺陷,而时效态合金并没有在这两处发现缺陷;固溶态合金在轴肩影响区与前进侧的界面变化幅度较为平缓,时效态合金界面过渡则较陡。
对两种状态合金分两次进行了系统的显微硬度实验,建立了硬度分布图。各层面显微硬度分布总体上呈现出W形状。焊核中心的硬度从底面至表面呈现出逐渐增大的趋势。时效态合金焊核区硬度的回升比固溶态合金的硬度回升明显。同时,在3mm层面附近,时效态合金出现明显的低硬度区,而固溶态合金则没有发现软化带。此外,两次硬度试验表明,自然时效对两种状态合金的硬度分布存在影响。
根据两种状态合金硬度分布图的特征,通过扫描电镜、背散射电子通道衬度相和透射电镜对特定硬度数值位置处的组织结构进行了分析。研究表明,焊核区晶粒由细小等轴晶粒组成,且从底部至顶部晶粒越来越大。焊核区底部至顶部晶界相逐渐减少,但晶内析出相却逐渐增多。时效态合金在焊核区晶内析出的第二相明显要多于固溶态。
对两种状态合金的背散射电子衍射实验表明,两种状态合金在热机影响区和焊核区的晶粒的取向变化差别不大。越靠近焊核区,回退侧的热机影响区呈现出小角度晶界逐渐减少,大角度晶界逐渐增加的渐变式的变化。随着逐渐靠近焊核区,晶粒织构越来越弱。两种状态合金焊核区晶粒主要由大角度界面构成,内部有少量的小角度界面,大角度界面的百分数占到了80%以上。经搅拌摩擦加工处理后,由于晶粒发生偏转,两种状态合金在焊核区均形成了弱的剪切织构。
7075 aluminum alloy,as a heat treatmentable high-strength aluminum alloy,has been widely used in aerospace,aviation,military and automotive industries. However,it is generally considered difficult to be the fusion welding process due to the poor solidification microstructure and porosity in the fusion zone. As a new type of solid state welding method, Friction Stir Welding provides an effective processing methods for welding and processing of 7075 aluminum alloy. But,possibly due to practical application status of 7075 aluminum alloy, the studies have been mainly focused on the processing microstructures and mechanical properties of the age-treated 7075 aluminum alloy. There is much less data on the microstructure evolution and mechanical properties of solution treated 7075 alloy. In this paper,through a large number of micro-hardness and OM experiments and combined with ECC,TEM and EBSD study for specific location of hardness profiles, the solution and age treated 7075 aluminum alloy were investigated by friction stir processing respectively. The relationship among temperature gradient,deformation mechanism,the precipitates distribution and texture evolution are discussed.
The effect of processing modes on the microstructures of the two alloys was compared. The optical micro-structures that were processed paralleling to the roll direction were studied in more details. The results showed that the solution treated alloy have defects found at the intersection between Shoulder Flow Zone,Stir Zone and Advancing Side, and the intersection of Swirl Zone、Stir Zone and TMAZ on transvere cross-section of processing zone, while no defects were found in the age treated alloy. Interface between the SFZ and AD changes smoothly in the solution treated alloy while the interface transition in the age treated alloy changes steeply.
Micro- hardness on the cross section of both alloy were test systemetically and the maps of hardness distribution were established. Micro-hardness profiles at the certain planes of solution-treated and age-treated 7075 Al exhibits Letter W morphology, typically as that in the most friction stir welds. Hardness in the center of the WZ gradually increased from bottom to surface. Meanwhile,apparent low hardness zone were found at the 3mm plane of the age treated alloy while no softened bands weren’t found in the solution treated alloy. In addition,two hardness tests showed that the natural aging has great influence on the hardness distribution in the two alloys.
Microstructures at specific location of hardness profiles were studied by SEM-ECC and TEM. The results showed that the sizes of fine equiaxed grains in the WZ increase from bottom to top of sheet. The amounts of intergruanlar precipitates gradually decreased from bottom to top while that of intragranular precipitates increased from bottom to top. The amount of intragranular precipitates in the age treated alloy were more than that of the solution treated alloy. However,in the 3mm level of hardness distribution,distribution of the second phase is more uniform in the alloy under solution heat treatment more than aging.
EBSD experiments show that the two alloys have similar grain orientation in the TMAZ and WZ. Low angle boundaries ( LABs) and high angle boundaries ( HABs) in the TMAZ of retreating side increase and decrease, respectively, and grain texture becomes weak when the detected site is approaching to the WZ. The grains are constituted by the HABs that accounted for more than 80% in the WZ. Due to shearing of the pin,the weak shear textures are formed in the WZ by rotation of the grains after the FSP.
引文
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