显微疏松对7050铝合金厚板疲劳性能影响

铝合金材料中的显微疏松会导致其疲劳性能的降低,很大程度上限制了合金的应用。本研究针对7050-T7451铝合金厚板进行了分析,对比了不同孔隙率板材之间疲劳性能差异。本研究实验运用了X射线计算机断层成像法(XCT)对材料孔隙率进行了分析,包括孔洞尺寸、数密度以及其尺寸分布,同时运用扫描电镜(SEM)对疲劳断口特征进行了分析。结果表明,对于孔洞较少且尺寸小的样品而言,疲劳裂纹更倾向于从第二相粒子产生,而对于孔洞较大且密度较高的样品而言,裂纹更容易起始于样品表面处的大尺寸孔洞。相比于几乎没有孔洞的样品而言,含有高孔隙率的样品疲劳寿命缩短将近一个数量级,样品疲劳寿命与孔洞大小和深度呈反比关系,且不同形状与朝向的孔洞对疲劳寿命也会造成影响。同时,结果分析可知,样品整体的孔隙率对疲劳长裂纹扩展也会造成一定影响。

Quasi <i>In-Situ</i>Study on Microstructure Evolution of Al 2014 Alloy during Thermal Deformation and Following Solution Treatment

Microstructure evolution of 2014 aluminum alloy was studied by hot compression deformation at 410?C to 470?C and strain rates of 0.07 s?1 to 0.53 s?1, to provide manufacturing references for aluminum plate. The deformation temperature and especially strain rate ranges were chosen very close to the actual processing condition. The results show that the stress-strain curves display a stable flow at the given deformation conditions, corresponding a dominant microstructure evolution behavior of dynamic recovery (DRV) and few dynamic recrystallization (DRX). After solution treated at 502?C for 3 hours, quasi in-situ observation shows that static recrystallization (SRX) develop typical fine grains with several microns at grain boundaries, while static recovery (SRV) dominants the microstructure evolution during the soaking time, leading to a similar microstructure to that of the as-deformed. The average low angle grain boundaries (LAGBs) and high angle grain boundaries (HAGBs) display weak differences between as-deformed and solution treated specimens, which reveals a good thermal stability of microstructure for 2014 alloy. However, the deformation at the lower temperature has an obvious trend to induce SRX during solution soaking.

Constitutive behavior and microstructural evolution in hot deformed 2297 Al-Li alloy

The microstructural evolution mechanism and constitutive behavior of 2297 Al-Li alloy were studied via thermal compression test with the constant strain rates of 0.001–1 s-1 and the deformation temperatures ranging from 623 to 773 K.To verify the predictable ability of diverse constitutive models under different stress states,the hot compression experiments with stress triaxiality varying from-0.33 to 0.46 were conducted.The microstructures of the deformed specimens under diverse deformation conditions are probed to reveal the mechanism of hot deformation behavior.The experimental results indicate that the work-hardening and dynamic softening are competitive during the hot compression process,and the dynamic softening is more obvious under low deformation temperature and high strain rate.The microstructural analysis manifests that the dynamic recovery gets predominant at high deformation temperature to produce fine grains.Meanwhile,the dynamic recrystallization becomes more dominant as the strain rate decreases,which is sensitive to the stress triaxiality.In addition,both the modified Johnson-Cook model and straincompensated Arrhenius-type function are suitable for describing the flow behavior of 2297 alloy,while the latter reveals a more accurate prediction.However,the predictability of the two kinds of models is worsened with the transformation of stress triaxiality,and the validity of the Arrhenius-type model is restricted by high stress triaxiality.

Experimental and numerical investigation on surface quality for two-point incremental sheet forming with interpolator

The unsatisfied surface quality seriously impedes the wide application of incremental sheet forming(ISF)in industrial field.As a novel approach,the interpolator method is a promising strategy to enhance the surface quality in ISF.However,the mechanism for the improvement of surface quality and the influence of interpolator properties on surface roughness are not well understood.In this paper,the influences of process variables(i.e.tool diameter,step size and thickness of interpolators)on the forming process(e.g.surface roughness,forming force and geometric error)are investigated through a systematic experimental approach of central composite design(CCD)in two-point incremental sheet forming(TPIF).It is obtained that the increase in thickness of interpolators decreases the surface roughness in direction vertical to the tool path while increases the surface roughness in direction horizontal to the tool path.Nevertheless,the combined influence between thickness of interpolators and process parameters(tool diameter and step size)is limited.Meanwhile,the placement of interpolator has little influence on the effective forming force of blank.In addition,the geometric error enlarges with the increase of step size and thickness of interpolator while decreases firstly and then increase with an increase in tool diameter.Finally,the influencing mechanism of the interpolator method on surface quality can be attributed to the decrease of thecontact pressure due to the increase of contact area with the unchanged contact force.Meanwhile,the interpolator method eliminates the sliding friction on the surface of blank due to the stable relative position between the blank and the interpolator.

Formation of intermetallic phases in unrefined and refined AA6082 Al alloys investigated by using SEM-based ultramicrotomy tomography

The spatial arrangement,distribution and morphology of Fe-bearing intermetallics in AA6082 alloys depends on the manufacturing process of the alloy and thus influences the macroscopic properties.Here,the microstructure of a near industrial scale casting AA6082 Al alloy fabricated by:(a)direct chill casting,(b)Al-5 Ti-1 B grain refiner addition and(c)intensive melt shearing has been investigated by threedimensional visualization using SEM-based serial ultra microtomy tomography.The formation sequence of phases in AA6082 alloys is generally categorized into four stages:formation ofα-Al grains,Fe-bearing intermetallics,Mg_(2)Si phase,and eutectic rosettes.Results of three-dimensional visualization of the microstructure indicated that TiBparticles not only could nucleate Fe-bearingβ-intermetallics,but also could provide substrate for the formation of Fe-bearingα-intermetallics and Mg_(2)Si.A further deep analysis reveals that the essential condition for the formation of secondary phases such as Fe-bearing intermetallics and Mg_(2)Si phase is the build-up of a supersaturated solute front at theα-Al solid-liquid interface irrespective of the specific nucleation site.In addition,the results indicate that grain refinement processing causes the severe interconnectivity of Fe-bearingα-intermetallics.However,the intensive melt shearing is a better manufacturing process because the intermetallics are more evenly distributed and refined than with the addition of the grain refiner,thereby improving the properties of the alloy.

Formation mechanism and modeling of surface waviness in incremental sheet forming

mproving and controlling surface quality has always been a challenge for incremental sheet forming (ISF), whereas the generation mechanism of waviness surface is still unknown, which impedes the widely application of ISF in the industrial field. In this paper, the formation mechanism and the prediction of waviness are both investigated through experiments, numerical simulation, and theoretical analysis. Based on a verified finite element model, the waviness topography is predicted numerically for the first time, and its generation is attributed to the residual bending deformation through deformation history analysis. For more efficient engineering application, a theoretical model for waviness height is proposed based on the generation mechanism, using a modified strain function considering deformation modes. This work is favorable for the perfection of formation mechanism and control of surface quality in ISF.