Hook is a typical defect of lap joint. It can induce stress concentration and is detrimental to joint strength. Thus, hook should be eliminated. In this work, by using the vertical squeezing force of the plastic material near the lap interface, we have successfully eliminated the hook by a double-size friction stir welding process. The results show that the double-side welding method can eliminate the hook in a wide parameter range. The rotating speeds ranging from 800 to 2000 rpm can effectively depress the bending of the lap interface, thereby eliminating the hook. The joint shows high strengths after the hook is eliminated. The maximum failure load of 15.195 kN is obtained when using the rotating speed of 1200 rpm. The joints show shear fracture mode when using rotating speed lower than 1200 rpm, and tensile fracture when the rotating speed is higher than 1600 rpm.
Based on the actual working conditions of the ∅880 × 400 horizontal twin-roll caster, the influence of the rolling speed on the flow field and solidification welding line of AZ31 magnesium alloy in the roll casting area is analyzed by finite element simulation. The results show that the molten metal’s convective heat transfer efficiency is improved by two vortexes (upper and lower) in the roll casting area. When the rolling speed increases, the solidification welding line gradually approaches the outlet. And the flow velocity difference between the middle and the edge of the roll casting area gradually increases, resulting in the continuous increase of the position difference L of the characteristic solidification welding points K and K″ in the rolling direction, which increases the probability of edge cracking of the roll casting strip of magnesium alloy. In order to improve the production efficiency of the roll casting strip of magnesium alloy, it is suggested that the rolling speed should be controlled within the range of 3 4.2 m/min.
Taking the new and old P92 pipes as the object,the rod samples of fine grain heat affected zone(FGHAZ)was prepared by using rapid heat treatment process.The microstructure and properties of the two FGHAZ samples were compared through observation by SEM,TEM,microhardness and creep tests,and the effect of the pipe original state on the microstructure and mechanical properties of FGHAZ of P92 steel was revealed.The results show that compared with that of the FGHAZ of new P92 pipe,the dislocation density of the FGHAZ of old P92 pipe is lower,the size of the precipitates is bigger,which decreases the hardness and creep resistance.During creep test,the FGHAZ of the old P92 pipe has higher precipitating tendency of Laves phase,which promotes the formation of creep void,accelerates the creep fracture and shortens the creep rupture life.
To address the problems such as abnormal wear and the friction layer falling off caused by insufficient adhesion stability of clutch friction pairs formed by traditional sintering and pressing, the laser cladding process manufacturing method was used to prepare the cladding layer of the copper-based powder metallurgy friction materials by using Laserline LDF 6000–100 semiconductor laser, and the prepared friction pairs were tested and analyzed by SEM in this paper. The numerical model of the friction torque of the wet clutch was established, and the friction performance and wear tests were carried out using MM-3000 performance test machine. The friction characteristics of the wet clutch prepared by the laser cladding process were investigated under typical operating conditions. The results show that the accuracy of the numerical model is verified by the experiments; The friction layer prepared by laser cladding technology has compact structure, without obvious crack defects, and has good wear resistance. Compared with the traditional sintering and pressing technology, the wear rate of the friction pairs is reduced by 59.76%; The maximum slipping friction work under different engagement pressures increases with the increase of rotating speed; The friction torque can rapidly reach the maximum value after the clutch is fully engaged, and the equivalent friction coefficient also reaches the extreme value correspondingly at this time.
In this work, refill friction stir spot welding (RFSSW) is innovatively applied to butt weld 6082 Al alloys. The effect of the refilling time on the microstructure and properties of the welded joints was studied. The results show that for the 6082 Al alloy with a thickness of 2 mm, a tool plunge depth of 1.6 mm can successfully avoid the incomplete bonding at the joint bottom. Cracks, formed by insufficient bonding, appear at the tool retraction paths when using the short refilling times of 3 s and 4 s. A prolonged refilling time of 5 s can eliminate the crack due to sufficient diffusion time. The pin affected zone (PAZ) has coarser grains than the sleeve affected zone (SAZ) due to the insufficient tool stirring action. Prolonged refilling time results in coarser grains, lower hardness and higher tensile strength of the joints. The maximum tensile strength of 262.2 MPa is obtained when the refilling time of 5 s is used, and the joint fails at the heat affected zone (HAZ) under such a condition.
研究了纵向交变磁场辅助CMT焊接X80管线钢焊缝的显微组织和力学性能.使用光学显微镜和扫描电子显微镜对焊缝的显微组织及晶粒取向进行分析,使用万能试验机和冲击试验机对焊缝的力学性能进行检测.结果表明,加入磁场后,焊缝组织由回火马氏体向针状铁素体和粒状贝氏体转变,焊缝组织细化,焊缝的抗拉强度、冲击韧性提高.由于磁场对熔池金属的搅拌作用,焊缝中粗大的柱状晶组织被打碎,作为异质形核点,促进了针状铁素体形核,细化了晶粒,提高了焊缝的力学性能.
通过热压缩实验研究了应变量及加热温度对Nb-Ti微合金钢的再结晶行为及变形后相变产物的影响.结果表明:当加热温度为1100℃时,奥氏体晶粒尺寸不均匀,导致压缩过程中各晶粒的应变累积不均匀,当总应变量为0.380时即发生了动态再结晶;奥氏体加热温度为1050和1000℃时,动态再结晶的临界应变量分别为0.668和0.552;当变形温度为850℃,应变量超过0.552时,试样在道次间隔发生静态再结晶;奥氏体化1100℃保温后压缩样品的冷却相变显微组织为贝氏体和准多边形铁素体,而1050℃和1000℃奥氏体化保温后压缩样品冷却相变后显微组织均为多边形铁素体,1000℃奥氏体化保温后压缩样品冷却相变后多边形铁素体晶粒尺寸更细小(2～5 μm).
采用化学气相沉积法在多孔泡沫铜基体表面原位生成石墨烯,然后通过冷压烧结和二次冷轧、退火的工艺制备石墨烯增强铜基复合材料.研究表明,采用化学气相沉积的方法在铜基体表面原位生成石墨烯可以提高制备的复合材料的抗拉强度以及使其保持较高的导电率.二次轧制退火后石墨烯增强铜基复合材料的相对密度达到 98.96%,经测试复合材料的导电率高达 88.7%IACS,其抗拉强度达到了 289.5 MPa较同实验条件制备的纯铜提高 32.98%.
ERNiCrFe-7A焊丝被广泛应用于核岛主设备的制造和安装,但目前此类焊材基本依赖进口.本文介绍研制的一种符合ERNiCrFe-7A标准的镍基合金焊丝,并对其焊接工艺性、焊缝金属的力学性能及微观组织进行了分析研究.结果表明:研制的焊丝焊接工艺性良好,焊道成型平整;焊缝金属在焊态和焊后热处理态下均具有较高的强度和良好的塑性,强度、伸长率、冲击功等力学性能均满足在核电工程上的安全应用条件;熔敷金属组织多为柱状晶,在晶界和晶内存在以碳化物为主的析出相,析出相形貌为不规则的块状,尺寸集中在 1～5 μm.
为优化航天产品进气道结构件的CMT焊接工艺,使用优选的对接焊工艺对 3mm厚的 5083 铝合金板进行焊接,对比了不开坡口与开坡口情况下焊缝的尺寸、显微组织与力学性能.结果表明,使用坡口结构,可以使得焊缝更加平整,减小正面余高.在无坡口情况下,焊缝内组织呈等轴晶,局部有自底向上柱状晶的倾向;在有坡口情况下,焊缝组织呈现明显向中心生长的柱状晶.不开坡口时焊缝热影响区的粗大晶粒导致接头的力学性能降低,拉伸试验时在此处断裂.开坡口后焊缝热影响区较小,几乎不对焊缝的力学性能产生影响,拉伸试验时在基板断裂.因此,开坡口的焊缝更有利于进气道产品的后续成形加工.
为了研究双重热处理对TA15 钛合金组织演化的影响,通过对TA15 钛合金进行不同加热温度和保温时间的一步和二步热处理,采用光学显微镜和维氏硬度计分别对合金的组织和硬度进行表征,分析得到不同工艺下合金α相含量的变化规律.结果表明:随着一步热处理温度和保温时间的增加,合金的等轴α相含量也逐渐增多,合金硬度也随之增大,等轴α相含量是合金硬度值的主要影响因素.随着二步热处理温度和保温时间的增加,合金的等轴α相含量均高于 40%,形成等轴组织,但此时等轴α相含量并非硬度的主要影响因素.
针对三峡升船机主驱动齿轮表面点蚀等缺陷,采用三种填充材料Ni20、Ni35、Ni60 合金粉末进行等离子熔覆修复试验,研究了熔覆层的组织性能.结果表明:采用Ni60 粉末作为填充材料,离子气流量 3 L/min、氩气流量 9 L/min,熔覆电流 135 A、预热温度 300℃时,获得的熔覆层具有最好的综合性能,熔覆层组织细密,枝晶晶粒较小,其截面硬度比基材的提高 42.3%,磨损量比基材的小 74%.
将红外热场分布规律与直方图均衡化图像增强算法相结合提出了一种新的红外图像增强方法Heat-HE.对通过试验获得的红外图像进行随机抽样,使用Heat-HE、灰度世界算法、自动白平衡三种红外增强算法进行处理,通过平均峰值信噪比、平均结构相似度两个评价指标来判断算法处理结果的优劣.结果表明,Heat-HE算法的平均峰值信噪比和平均结构相似度优于传统的灰度世界算法、自动白平衡算法.Heat-HE算法使熔覆层红外热像图片在保留原始图像特征的基础上使缺陷轮廓更为清晰、直观,能够将熔覆层缺陷与背景噪声区分开.
激光选区熔化成形过程是涉及多物理现象的耦合过程.采用数值模拟能够有效地研究成形过程中的多物理现象,探究缺陷形成机理,优化工艺参数设置.本文对目前国内外激光选区熔化粉末尺度的数值模拟进行了综述,介绍了数值模拟的关键模型,分析了粉末尺度下缺陷研究的数值模拟,总结展望了激光选区熔化粉末尺度数值模拟的发展方向与趋势.
SiC作为潜在的下一代核反应堆包壳材料,实现其稳定可靠的连接是SiC基包壳管投入实际工程应用的重要一环.使用镍含量为 55at%的NiTi粉和Ni粉混合膏状钎料(NiTi+5Ni钎料)在 1350℃保温 10 min的工艺下实现了SiC陶瓷的连接.结果表明:焊缝界面区域组织为TiC连续反应层;中心区域基体组织为δ-Ni2Si+Ni3Si2 共析组织,其中弥散分布着大量的TiC颗粒.接头平均室温剪切强度可达 125 MPa,断裂位于靠近界面的SiC陶瓷内部.界面处TiC与SiC之间良好的晶格匹配度以及焊缝中心区域的TiC颗粒对接头热残余应力的缓解,提升了接头的力学性能.
采用静电自组装法将氧化石墨烯(GO)分散在铝粉中,通过放电等离子烧结(SPS)制备了石墨烯铝基复合材料.采用改进的Hummers方法制备带负电荷的GO片,并用十六烷基三甲基溴化铵(CTAB)处理铝粉以获得表面正电荷.由静电自组装获得的氧化石墨烯-铝粉(GO-Al粉),实现氧化石墨烯在铝粉上的均匀吸附.SPS制备石墨烯铝基复合材料(Gr-Al)的快速烧结过程能够还原氧化石墨烯,同时减少有害碳化物的形成.使用拉曼光谱(Raman)、红外光谱(FT-IR)、透射电镜(TEM)和扫描电镜(SEM)对复合材料进行了相关测试,同时研究了Gr-Al复合材料的硬度和导热性能.结果表明,GO片均匀吸附在CTAB包覆的Al粉表面,实现了石墨烯在Al基体中的均匀分散.与铝基体材料相比较,0.2wt%石墨烯铝基复合材料的显微硬度和导热系数均明显提高,显微硬度(45.7 HV)提高 41%,导热系数(203 W/(m·K))提高 65%.
利用数值模拟对铝合金变速箱箱体挤压铸造过程进行分析,预测了原始工艺方案中箱体缩松缩孔缺陷的分布情况,通过分析其形成原因添加了合理的冷却系统.在此基础上运用正交试验对工艺参数进行了多目标优化.结果表明:优化后的箱体无缩松缩孔缺陷,二次枝晶间距减小到 36.37 μm.通过金相分析,验证了箱体挤压铸造工艺的正确性.
采用Gleeble-3500 热模拟实验机、金相法和室温力学性能测试等,研究了轧制温度对建筑结构用Q420钢组织和力学性能的影响.结果表明,随着轧制温度的降低,铁素体和珠光体显微组织晶粒尺寸减小,铁素体体积分数下降而珠光体体积分数增加.此外,对不同轧制温度试样进行碳复型TEM分析可知,随着轧制温度降低,试样中出现了更多更细小的析出物.最后对不同轧制温度试样进行力学性能分析,表明由于组织细化和更多细小碳化物的析出,轧制温度较低的试样具有更高的屈服强度和抗拉强度.
高熵合金普遍具有高强度、高硬度、强抗腐蚀性以及优异的磁学和电化学性能等,是近些年来研究的热点.对近 3 年内FeCoMnNi系高熵合金的制备方法、处理工艺、影响因素等性能研究进展进行了总结,对新兴高熵合金材料在未来可能的应用和发展方向进行预测.
分别采用两道次轧制过程中中间退火和未中间退火两种工艺制备了Cu/Al复合板,研究了中间退火处理对复合板界面结合性能的影响.研究结果表明,两种工艺条件下制备的复合板界面均牢固结合,但采用中间退火处理工艺更有利于提升轧后Cu/Al复合板的结合强度.其中,采用未中间退火的两道次轧后复合板的剥离强度为 28.29 MPa,而采用中间退火工艺的复合板剥离强度则达到了34.05 MPa.这是因为经过中间退火处理后进行二道次轧制制备的复合板界面由冶金结合区和物理结合区共同组成,协同作用强化了界面的结合性能.
