激光填丝焊接铝锂合金T型接头的组织和力学性能

以铝锂合金2A97(底板)/2099(筋板)T型接头为研究对象,采用ER5356铝镁焊丝进行激光填丝焊接,分析了焊接接头的显微组织和力学性能。结果表明:接头的平均抗拉强度为425 MPa,达到2A97-T3母材抗拉强度的93%。T型接头首先在焊趾起裂,然后沿熔合线扩展,最终断在底板,呈穿晶断裂的特征。焊缝区硬度值在70~90 HV之间,比母材硬度低。焊缝区域晶粒形态主要有细等轴晶、柱状晶和等轴晶,其中细等轴晶主要在在熔合区靠近焊缝侧形成;焊缝中心等轴晶区的第二相粒子分别在晶间和晶内出现,并伴随明显的铜偏析现象。     

铝合金厚板电子束焊接头组织与特征

研究了2519铝合金厚板电子束焊接头组织分布特征,并进一步研究电子束焊过程中熔池金属的流动及凝固。结果表明:焊缝由枝晶和等轴晶组成,但其沿熔深方向焊缝中心和焊缝靠近熔合线区域存在明显组织不均匀性;沿焊缝中心的上部和中部为等轴枝晶,根部则以等轴枝晶和等轴晶混合存在;靠近熔合线边缘的焊缝自上而下分别为树枝晶、柱状晶、等轴晶,晶粒生长的方向性逐渐减弱;越接近焊缝根部,其晶粒尺寸和分布越不均匀,焊缝根部晶粒沿熔池金属凝固分层轮廓线分布,且存在微裂纹等缺陷。这些现象反映出焊接过程中熔池各区域内的焊接能量和金属流动性差异显著。     

厚板转子钢多层多道焊接头不同微区断裂韧度的分析

文中研究了厚板转子钢多层多道窄间隙焊接接头不同微区的断裂韧度,主要包括接头中热影响区粗晶区及细晶区,焊缝中的柱状晶区和等轴晶区.结果表明,焊缝内柱状晶区断裂韧度（J_m）最低,断裂韧度最小值为195 kJ/m2,而热影响区的细晶区断裂韧度最高,J_m为1 265 kJ/m2.热影响区细小的等轴晶粒（1~12 μm）和晶内均匀分布的碳化物是其断裂韧度较好的主要原因.焊缝内组织主要为回火索氏体,以柱状晶形式分布在焊缝中,裂纹容易沿柱状晶晶界扩展,导致焊缝的断裂韧度较低.整个接头柱状晶与等轴晶组织交替分布,提高抗裂纹扩展能力,保证了整个焊接接头的安全可靠.     

8090Al—Li合金焊缝金属形核机制

８０９０Ａｌ—Ｌｉ合金焊缝凝固组织中除了柱状枝晶和等轴枝晶外，还有一种极细小的等轴非枝晶，它主要分布在熔合区边缘，有时也能呈带状出现在焊缝中部．等轴枝晶与柱状枝晶的结晶核心是平衡态四方Ａｌ_３Ｚｒ粒子，而等轴非枝晶的结晶核心是亚稳态立方Ａｌ_３Ｚｒ粒子．     

预置镍基合金片对异种钢UNGW接头组织及性能的影响

研究了预置镍基合金片对15CrMo/1Cr18Ni9Ti异种钢超窄间隙焊接接头组织及性能的影响。结果表明,预置镍基合金片厚度小于0.6 mm时,15CrMo熔合线附近焊缝区部分区域以A模式凝固并形成奥氏体胞状晶,而其它区域仍以FA模式凝固,其组织为等轴晶奥氏体+枝晶状铁素体,并且焊缝中心无凝固裂纹形成。预置0.9~1.2 mm厚的镍基合金片时,焊缝中心因镍偏聚而以A模式凝固,形成粗大的奥氏体柱状枝晶,并有凝固裂纹形成。预置镍基合金片厚度在0.6~0.9 mm时,可使15CrMo熔合线附近焊缝区的Ni质量分数比填充金属(ER347L)的提高约2%,明显比未预置镍基合金片的接头具有更好的抑制碳扩散效果,但当镍基合金片厚度在0.3~1.2 mm范围内变化时接头抑制碳扩散的效果并无明显变化。焊态的异种钢接头不均匀混合区内存在马氏体层,而热时效后在熔合线附近会形成一定宽度的富碳硬化区及贫碳软化区。     

连铸管坯的缺陷

<正> 肉眼观察时,连铸坯缺陷可分为内部缺陷、外表面缺陷和形状缺陷。一、内部缺陷一部分内部缺陷与连铸时的凝固条件有关,因此首先说明连铸坯的组织。一般情况下,连铸坯的凝固组织由三个区域组成:外层等轴晶区、柱状晶区和芯部等轴晶区。外层细晶粒区是由于钢液与水冷的结晶器接触时有强烈的散热而形成的。当钢液存在局部较大的过冷度时,形成无数的晶核,由此而形成细小的多边形晶粒。继续凝固时,沿温度梯度大的方向上的晶粒比其     

焊缝弧坑二维等轴树枝状晶的研究

本文报道了铁镍基合金鱼骨型裂纹试样焊缝弧坑裂纹断口扫描电镜观察发现的二维等轴树枝状晶的真实形貌,并摄下了该树枝状晶主干端部的四个对称椭圆形凸台及有关层状结晶生长线等树枝状晶生长的一些有规律的现象的照片。在讨论中提出了半熔化区自然晶核生长初期产生对称异号螺形位错的假设,并提出了二维等轴树枝状晶的对称异号螺形位错型层状结晶生长机理的假说,对研究和改善焊缝的凝固结晶,以及研究和控制焊缝的凝固裂纹将有一定的裨益.     

C24S新型铝锂合金焊接工艺研究

对2 mm C24S铝锂合金薄板进行了GTAW和LBW机器人焊接工艺试验,采用光学显微镜(OM)和扫描电镜(SEM)分析了两种方法下接头显微组织,并测试了接头的力学性能。结果表明:GTAW所得接头出现气孔缺陷,焊缝和热影响区组织粗大。焊缝由粗大的胞状晶组成,析出物呈条带状沿晶界分布,抗拉强度仅为母材的60.3%。LBW焊接头成形良好,焊缝由尺寸一致的细小等轴晶组成,且焊缝中弥散分布着大量细小的颗粒状析出物。LBW焊焊缝在各区域沿熔深方向组织分布存在差异,靠近热影响区形成了细晶区,细晶区的宽度以及焊缝中心的显微组织与焊接时熔池金属流动行为密切相关。LBW焊接头呈现出高强度、高伸长率和高硬度的综合力学性能,抗拉强度达到347.8 MPa,为母材的79.1%,焊缝硬度高达105.4 HV,且拉伸断口中分布着细小、尺寸均匀的韧窝。     

AZ91D镁合金的快速凝固特征

采用单辊实验技术研究了AZ91D镁合金的急冷快速凝固特征.合金条带的凝固组织沿厚度方向分为3个晶区:近辊面细晶区、内部柱状晶区和自由面等轴晶区.XRD分析表明,在快速凝固条件下,L→α-Mg β-Mg17Al12的共晶反应受到抑制,条带凝固组织由过饱和的单相α-Mg固溶体组成.TEM分析发现,α-Mg晶内和晶界上离散分布着的少量Mg17Al12质点,系α-Mg固溶体在连续冷却过程中脱溶析出的产物;α-Mg中存在着大量的位错线和位错胞.随着辊速增大,晶粒更加细化,柱状晶区减小,位错密度增大,条带强度增加,塑性降低,电阻率增大.     

激光增材制造GH3625高温合金激光焊接头组织及力学性能

对激光增材制造GH3625高温合金进行激光对接焊实验,分析了焊接接头的显微组织演变规律和力学性能。结果表明,在热影响区,仅在上层晶界处析出大量Laves相,导致晶界发生明显粗化。沿焊缝上层至下层,熔合区和中间区由上层的胞状晶、柱状晶和等轴细晶逐渐转变为下层的柱状晶,且紧贴熔合线生长的等轴细晶的数量逐渐增多,中心区均为树枝晶。细小颗粒状γ′相弥散分布于焊缝;大量Laves相在中间区枝晶间析出,且形态由上层的条状逐渐转变为下层的颗粒状。接头抗拉强度为872MPa,达到母材抗拉强度的98.2%,伸长率达到母材的90.7%。接头断面近似呈45°斜面,断裂形式为脆性和韧性混合断裂方式。     

Microstructure–properties relationships in martensitic stainless steel resistance spot welds

The paper addresses the phase transformations and mechanical response of martensitic stainless steel resistance spot welds. The fusion zone microstructure consists of carbon-rich martensite together with a relatively high amount of retained delta ferrite along the grain boundaries with a transition in solidification mode from equiaxed to columnar dendritic grains across the fusion zone. The heat affected zone microstructure is featured by martensitic matrix together with carbide precipitation. The very high hardness of the fusion zone and the heat affected zone, the sharpness of the notch at sheet/sheet interface, which is located in the hard microstructural zone, and the presence of delta ferrite in the weld nugget play important roles in failure characteristics and mechanical performance of the joint.     

Evolution of grain structure in AA2195 A1-Li alloy plate during recrystallization

The evolution of the grain structures in AA2195 Al-Li alloy plate warm-rolled by 80% reduction during recrystallization annealing at 500℃ was investigated by electron backscatter diffraction, scanning electron microscopy and transmission electron microscopy. It is found that the elongated grain structures are caused by the lamellar distribution of recrystaUization nucleation sites, being lack of large second phase particles （〉 1μm）, and dispersive coherent particles （such as δ′ and β′concentrated in planar bands. The recrystallization process may be separated into three stages： firstly, recrystallization nucleation occurs heterogeneously, and the nuclei are concentrated in some planar zones parallel to rolling plane. Secondly, the grain boundaries interacted with small particles concentrate in planar bands, which is able to result in the elongated grain structures. The rate of the grain growth is controlled by the dissolution of these small particles. Thirdly, after most of small particles are dissolved, their hindrance to migration of the grain boundaries fades away, and the unrecrystallized zones are consumed by adjacent recrystallized grains. The migration of high angle grain boundaries along normal direction leads a gradual transformation from the elongated grains to the nearly equiaxed, which is driven by the tension of the grain boundaries.     

金属铸态组织细化的机制辨析

金属铸态组织细化的关键是获得大量细小等轴晶。本文通过实验,对细小等轴晶形成的几种假说进行了鉴别,指出了铸锭中等轴晶主要来源于型壁,并初步探讨了浇注温度、对流等因素对铸态组织形成的影响。     

脉冲电流频率对2219铝合金焊缝组织性能的影响

采用超快速变换复合高频脉冲方波变极性TIG电弧焊接方法进行2219-T87高强铝合金焊接,研究脉冲方波电流频率对焊缝组织性能的影响.结果表明,加入脉冲方波电流后,焊缝组织明显细化,焊缝区由粗大柱状晶向细小等轴晶转变.当脉冲电流频率达超音频段时,在焊缝中部还存在一种呈带状分布的细小等轴非枝晶组织,并与等轴树枝晶组织交替分布.与未加入脉冲电流相比,当脉冲电流频率为60kHz时,接头抗拉强度和断后伸长率分别增加约17.6%和66%.在一定范围内提高脉冲电流频率,可显著减小焊缝熔合区宽度,改善接头硬度分布,明显提高接头塑性,但对接头强度的影响作用并不明显.     

形核剂成分对高碳钢凝固过程的影响

高碳钢连铸坯存在的最大质量问题是中心缩松、中心偏析严重.解决这一问题的关键是扩大铸坯等轴晶区比例,细化晶粒.本文以Fe-C合金作为形核剂,研究了形核剂含碳量对高碳钢凝固过程的影响机理.研究结果表明,向钢液中加入形核剂可有效地扩大等轴晶区.对高碳钢,中碳铁合金形核剂既可扩大等轴晶区,又可细化晶粒;而低碳形核剂可以扩大等轴晶区,但细化晶粒效果差.为有效地发挥形核剂的作用,对不同钢种要合理地选择形核剂含碳量.     

Coupled macro-micro modeling for prediction of grain structure of Al alloy

A 3D stochastic modeling was presented to simulate the dendritic grains during solidification process of aluminum alloy. Shape functions were proposed in 2D and 3D to describe equiaxed dendritic shape. A growth model was presented to describe the growth of a nucleated grain and the capturing of the neighboring cells. On growing,each grain continues to capture the nearest neighboring cells to form the final grain shape. If a neighboring cell has been captured by other grains, the growth along this direction stops, which can reflect the grains impingement phenomenon occurring in solidification process. 2D and 3D calculations were performed to simulate the evolution of equiaxed dendritic grains. In order to verify the modeling results, step-shaped sample castings were cast in sand mold. The microstructure in various positions of the sample was observed. In addition the quantitative metallographic analysis also has been done to evaluate the grain size. Experimental and numerical results agree well.     

Effects of annealing on microstructure and properties of linear friction welded dissimilar titanium joints

Linear friction welds of Ti–6Al–4V (TC4, according to Chinese classification) to Ti–6·5Al–3·5Mo–1·5Zr–0·3Si (TC11, according to Chinese classification) were subject to post-weld heat treatment (PWHT) at 650°C×4 h (PWHT1) and at 950°C×1 h+530°C×4 h (PWHT2) under air cooling. In the as welded joint, TC4 had recrystallised sufficiently compared to TC11. After PWHT1, the α grains in the TC4 weld centre zone grew to some extent and many superfine equiaxed recrystallised α grains precipitated along β boundaries on the TC11 side. The grain growth along the weld line in the weld centre zone under PWHT2 was evident. PWHT2 reduced the joint tensile strength due to the coarsening of the microstructure. The joint microhardness decreased after PWHT.     

Propagation and termination of an internal crack in continuously cast austenitic stainless steel analyzed by ∑ values and the Schmid factor

The formation mechanism of an internal crack was clarified from the viewpoint of the crystallography and thermal expansion. An inverse pole figure map obtained by EBSD pattern showed that the crack propagated along the grain boundaries having high ∑ values within the columnar zone. After the crack initiation, these positions were considered to undergo cracking followed by propagation toward the equiaxed side. Near the termination position, the grains ahead of crack propagation had a Schmid factor higher than 0.45 consuming elastic strain energy. Thermal expansion measurements showed that the grain with (0 0 1) orientation had the largest expansion while that with (0 1 1) the smallest. The grain boundaries neighboring the combination of (0 0 1) and (0 1 1) grains had the largest thermal stress. Therefore, thermal stress contributed to the initiation of cracking. It was thus proposed to enlarge the equiaxed zone to prevent cracking by discontinuing the crack propagation.     

Effect of ball milling time on microstructures and mechanical properties of mechanically-alloyed iron-based materials

The microstructures and mechanical properties of an iron-based alloy (Fe-13Cr-3W-0.4Ti-0.25Y-0.30O) prepared by mechanical alloying were investigated with scanning electron microscope, optical microscope, X-ray diffractometer and hardness tester. The results show that the particle size does not decrease with milling time because serious welding occurs at 144 h. The density of the alloy sintered at 1 523 K is affected by the particle size of the powder. Finer particles lead to a high sintered density, while the bulk density by using particles milled for 144 h is as low as 70%. In the microstructures of the annealed alloy, large elongated particles and fine equiaxed grains can be detected. The elongated particle zone has a higher microhardness than the equiaxed grain area in the annealed alloys due to the larger residual strain and higher density of the precipitated phase.