铸坯心部可压缩孔洞尺寸极限分析

为分析初始坯料中允许存在(后继可以通过轧制压合)的最大孔隙性缺陷尺寸,对一定轧制工艺下所能压合的棒材心部孔隙性缺陷的极限尺寸进行了模拟研究。     

搅拌摩擦焊搭接hook缺陷研究现状

搅拌摩擦焊搭接hook缺陷是材料焊接过程中由于材料受热及物理畸变变形形成的固有物理缺陷,在焊接构件使用过程中对构件静态及动态性能影响大。本文从hook缺陷表现形貌及形成原因出发,综述了新型连接技术搅拌摩擦焊在搭接接头中缺陷成型机理,分析了搅拌摩擦焊焊接材料搭接焊接时材料的配置位置、焊接刀具形貌及长度以及焊接工艺参数等对hook缺陷形貌及尺寸的影响,结合国内外研究现状简述了hook缺陷对焊接接头静态力学性能及动态裂纹敏感性的影响程度,并就目前影响因素情况对后期该缺陷改进提出了相关建议。     

粘焊变质层对硬质合金刀具前刀面温度分布的影响

通过分析硬质合金刀具前刀面粘焊变质层几何参数和物理参数,采用拉普拉斯变换、逆变换以及热力学第一定律,并结合切削的实际工况,建立了刀具前刀面有无粘焊变质层的传热理论模型和有限元模型,并进行理论模型解算与有限元模型模拟仿真。结果表明,粘焊变质层的存在会影响刀具基体温度梯度变化,且仿真温度梯度变化通过理论模型得到了验证,解决了实验无法验证刀具前刀面温度梯度变化的问题。选用不同粘焊变质层厚度以及导热系数变化进行刀具内部温度分布数值模拟,获得了粘焊变质层对刀具前刀面温度分布的影响规律,并通过实验测量确定有粘焊变质层的切削温度与数值模拟值误差在10%之内,验证了有限元模型的准确性。     

热等静压对铸件致密化及组织演变机理的影响研究

研究了铸件制品的热等静压致密化机理,分析了铸件制品热等静压的致密化模型,综合应用合金的蠕变理论和粉末冶金的烧结理论,利用现有的物理和数学模型对铸件在热等静压过程中的内部孔洞类缺陷闭合过程机理进行了阐述。确认了热等静压过程中起主要致密化作用的孔洞形变机制,为选择制定合适的热等静压工艺参数提供了理论参考。     

铝合金建筑型材氧化焊合线缺陷分析

氧化型材焊合线缺陷问题一直是生产过程中最令人困扰的问题.本文主要针对氧化型材焊合线问题进行的研究,通过调整模具结构,优化挤压工艺参数,最终达到避免焊合线缺陷的效果.     

玄武岩三维细观孔隙模型重构与直接拉伸数值试验

由于岩石材料的不透明性和多孔隙特性, 通过传统的物理试验或数值模拟很难真实体现其内部三维细观结构. 本文基于CT扫描技术、边缘检测算法、滤波算法、三维点阵映射与重构算法, 构建了可以表征玄武岩试样内部孔隙结构的三维细观非均匀数值模型. 结合并行计算进行直接拉伸数值试验, 研究了内部孔隙结构特征对试样破坏机制及抗拉强度的影响. 研究结果表明: 加载初期在试样孔隙处产生初始裂纹, 随着荷载的增加初始裂纹逐渐沿横向扩展最终形成宏观拉伸破坏裂纹, 并且孔隙含量和分布位置对试样拉伸断裂的位置具有重要影响. 随着孔隙率增高, 试样破坏过程中的声发射数目和能量逐渐减小. 拉伸破坏模式呈现脆性破坏特征, 同时孔隙的存在削弱了试样的抗拉强度.      

唐钢FTSC工艺生产SS400热板卷的轧制孔洞缺陷控制

 针对唐钢FTSC薄板坯连铸连轧线生产SS400热板卷的轧制孔洞缺陷,对连铸坯的低倍组织、铸坯温度分布和动态软压下工艺进行了研究。结果表明：铸坯存在内部横裂纹和疏松缺陷,内部裂纹产生在0扇形段,裂纹处硫元素呈正偏析;铸坯内部裂纹和疏松缺陷在后续轧制中不能够焊合,是导致轧制孔洞缺陷的直接原因。通过优化动态软压下参数,将压下终点向前提和减小0扇形段的压下量;增加二次冷却强度,提高冷却均匀性;提高钢水质量、加强设备精度管理和推进恒拉速操作,可以完全避免中碳钢SS400热轧板卷轧制孔洞缺陷的产生。     

铝合金点焊过程中铜铝合金化的有限元分析

为了研究铜铝合金化反应的机理,本文建立了一种基于铜和铝扩散反应进行的有限元模型。由于热传导方程和扩散方程在表达形式上的相似性,所以用模拟热传导的方式来模拟铝在铜电极中的单向扩散过程,以温度分布表示扩散浓度的分布情况。研究表明：若只考虑铝在铜中的单向扩散,将试验中的数据按比例进行换算,这样计算出来的试验结果与模拟结果比较相近。     

热轧工艺对铸坯冶金缺陷焊合能力的影响

针对中轧线,紫金线圆钢热顶锻完好率存在较大差异的现象,通过生产试验,着重分析了不同轧制工艺对连铸坯内部冶金缺陷焊合能力的影响。     

氢致钢内部疲劳裂纹萌生和扩展的有限元分析

对氢致钢内部疲劳裂纹的萌生和扩展进行了数值模拟.首先用有限元法分析了氢在疲劳载荷作用下向钢中缺陷处扩散富集的过程,然后计算得到氢含量分布结果.根据夹杂理论将氢富集区视为在缺陷附近分布的弹性夹杂,用有限元法计算得到的氢含量场求出夹杂处的应力强度因子,进而建立疲劳裂纹萌生和扩展的判据.比较了在不同加载条件下氢致疲劳裂纹萌生和扩展的规律.用梯形法修正了Sofronis和McMeeking的瞬态扩散有限元公式,发现用梯形法可以缓解加载初期较高的浓度梯度和应力梯度引起的计算结果震荡的情况,这对于计算开裂判据是十分重要的.最后讨论了提高模拟精度和改进模型的方法.      

空气相对湿度对焊缝质量的影响分析

通过对法兰盘焊接时存在的气孔缺陷的研究与分析,找出了问题存在的各种原因,由于焊接环境的特殊性,空气相对湿度是引起气孔缺陷的主要原因,针对空气相对湿度较高这一原因,提出了焊前预热、改进工艺和选择高水平操作者的改进措施,使得产品质量有了很大的提高。     

纯镍压力管道的焊接及检验

纯镍材焊接时易引起焊缝金属和热影响区晶粒粗大,易产生热裂纹、气孔等缺陷,通过优化焊接工艺和焊后的检测手段可以保证其焊接质量,从而保证设备的安全运行。     

铝合金焊接性能及焊接接头性能的研究

在高铁、地铁列车的制造中,铝合金材料更是列车车体的主要材料之一,然而由于铝合金材料在焊接性能、焊接接头性能方面仍存在一定的不足,经常会出现气孔、裂纹等缺陷,因此高铁、地铁列车铝合金车体的焊接施工质量仍然很难保证。本文对铝合金的焊接性能以及焊接接头性能进行了分析。     

Global and Local Mechanical Properties of Autogenously Laser Welded Ti-6Al-4V

Ti-6Al-4V sheets, 3.2-mm in thickness, were butt welded using a continuous wave 4 kW Nd:YAG laser welding system. The effect of two main process parameters, laser power and welding speed, on the joint integrity was characterized in terms of the joint geometry, defects, microstructure, hardness, and tensile properties. In particular, a digital image correlation technique was used to determine the local tensile properties of the welds. It was determined that a wide range of heat inputs can be used to fully penetrate the Ti-6Al-4V butt joints during laser welding. At high laser power levels, however, significant defects such as underfill and porosity, can occur and cause marked degradation in the joint integrity and performance. At low welding speeds, however, significant porosity occurs due to its growth and the potential collapse of instable keyholes. Intermediate to relatively high levels of heat input allow maximization of the joint integrity and performance by limiting the underfill and porosity defects. In considering the effect of the two main defects on the joint integrity, the underfill defect was found to be more damaging to the mechanical performance of the weldment than the porosity. Specifically, it was determined that the maximum tolerable underfill depth for Ti-6Al-4V is approximately 6 pct of the workpiece thickness, which is slightly stricter than the value of 7 pct specified in AWS D17.1 for fusion welding in aerospace applications. Hence, employing optimized laser process parameters allows the underfill depth to be maintained within the tolerable limit (6 pct), which in turn prevents degradation in both the weld strength and ductility. To this end, the ability to maintain weld ductility in Ti-6Al-4V by means of applying a high energy density laser welding process presents a significant advantage over conventional arc welding for the assembly of aerospace components.     

A study on weldability of zinc-coated steel sheets in lap joint configuration

The weldability of Zn-coated steel sheets 0.7 mm thick was investigated using resistance spot welding process. The effect of welding current, welding time and holding time on weld nugget characteristics, microstructure, and mechanical properties was discussed. Then, the possibility of replacing this welding process with laser beam welding was outlined. In this respect, quality of weld joints as a function of zinc removal by grinding prior to welding was evaluated. It is found that resistance spot welding current and time are the most significant parameters in affecting both expulsion and Zn-induced porosity. Expulsion was avoided and Zn-induced porosity was reduced with the decrease in welding current and/or welding time. Zn-induced porosity was completely eliminated by zinc-removal by grinding prior to welding. The best weld joint concerning nugget characteristics, soundness and tensile shear strength was obtained using welding current of 10 kA, weld cycle of 20, holding cycle of 18. Unlike resistance spot welds, high quality of CO₂ laser welds free from Zn-induced porosity could be made without zinc removal by grinding before welding.     

高频焊管的常见缺陷及预防措施实践

高频焊接制管是最快最有效的制管方法,但高频焊接制管过程中会产生各种各样的缺陷。分析了高频焊接制管生产中夹杂物、熔合不足、粘焊、气孔、跳焊等常见焊接缺陷产生的原因及相应预防措施。     

铁水罐焊接缺陷的产生及防止措施

介绍了100t铁水罐焊接过程中产生的各种内部缺陷，并对产生缺陷的原因进行了具体分析，采取了相应的防止措施，最终取得了理想的焊接效果，满足了用户的使用要求。     

Weldability Evaluation and Microstructure Analysis of Resistance‐Spot‐Welded High‐Mn Steel in Automotive Application

In this paper, resistance spot weldability of high‐Mn steels were investigated in order to get high reliability in welded joints of automotive components. Microstructural characterizations, cross‐tensile test (CTT), microhardness tests of spot welded parts were conducted. The effects of weld current on the microstructural characteristics, mechanical properties, and fracture modes were investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The hardness in the weld nugget was observed to be lower than that in the base metal (BM). In CTT, the failure initiation was observed to occur at the boundary of the weld nugget. Also welding imperfections of welded parts were investigated. Liquation cracking in heat affected zone (HAZ), porosity, and shrinkage cavity were found most common welding defects in welded parts. Furthermore, the effects of welding imperfections on weld quality and failure criteria were identified and discussed.     

A Quantitative Model of Keyhole Instability Induced Porosity in Laser Welding of Titanium Alloy

Quantitative prediction of the porosity defects in deep penetration laser welding has generally been considered as a very challenging task. In this study, a quantitative model of porosity defects induced by keyhole instability in partial penetration CO₂ laser welding of a titanium alloy is proposed. The three-dimensional keyhole instability, weld pool dynamics, and pore formation are determined by direct numerical simulation, and the results are compared to prior experimental results. It is shown that the simulated keyhole depth fluctuations could represent the variation trends in the number and average size of pores for the studied process conditions. Moreover, it is found that it is possible to use the predicted keyhole depth fluctuations as a quantitative measure of the average size of porosity. The results also suggest that due to the shadowing effect of keyhole wall humps, the rapid cooling of the surface of the keyhole tip before keyhole collapse could lead to a substantial decrease in vapor pressure inside the keyhole tip, which is suggested to be the mechanism by which shielding gas enters into the porosity.     

生产优质大截面模块的锻造工艺

钢锭内部缺陷对大截面模块内部质量的影响较大,莱钢锻压厂在生产改进加工艺,利用高温扩散部分消除钢锭内部微观偏析,同时利用平丰加长及大压下量的开坯,有效地锻合了钢锭中负Ｖ型偏析引起的疏松组织,提出了模块的内部质量。