镁合金双向挤压-螺旋变形的数值模拟与实验研究

利用双向挤压与螺旋变形的特性,本研究提出镁合金双向挤压-螺旋复合变形的新方法。采用DEFORM-3D软件模拟分析螺旋角度和凹槽半径对坯料变形过程中累积应变的影响,得出合适的结构参数并加工制造出模具进行实验研究。研究结果表明:双向挤压-螺旋复合变形可极大地提高镁合金变形过程的等效应变,随着螺旋角度和凹槽半径的增大,等效应变值也相应的增大,但不均匀程度有所变大。模具螺旋角为40°和凹槽半径为0.8 mm时,试样可获得良好的等效应变值和均匀的等效应变分布,晶粒组织显著细化。     

Electrodeposition and corrosion resistance of Ni–P–TiN composite coating on AZ91D magnesium alloy

In order to improve the corrosion resistance and microhardness of AZ91D magnesium alloy, TiN nanoparticles were added to fabricate Ni–P–TiN composite coating by electrodeposition. The surface, cross-section morphology and composition were examined using SEM, EDS and XRD, and the corrosion resistance was checked by electrochemical technology. The results indicate that TiN nanoparticles were doped successfully in the Ni–P matrix after a series of complex pretreatments including activation, zinc immersion and pre-electroplating, which enhances the stability of magnesium alloy in electrolyte and the adhesion between magnesium alloy and composite coating. The microhardness of the Ni–P coating increases dramatically by adding TiN nanoparticles and subsequent heat treatment. The corrosion experimental results indicate that the corrosion resistance of Ni–P–TiN composite coating is much higher than that of uncoated AZ91D magnesium alloy and similar with Ni–P coating in short immersion time. However, TiN nanoparticles play a significant role in long-term corrosion resistance of composite coatings.     

DP600/AZ31无匙孔搅拌摩擦点焊塑性金属流动特性及断裂行为研究

在最优焊接参数下,对1 mm DP600镀锌钢板和3 mm AZ31镁板进行无匙孔搅拌摩擦点焊试验,焊后对接头分别进行横切、纵切及层切,采用扫描电镜（SEM）分析焊接接头显微组织和断口形貌.结果表明,镁钢间的混合主要发生在搅拌针作用区域,形成“机械互锁”的组织形貌,有利于增加两种材料的有效接触,形成复相强化;轴肩作用区镁钢间搅拌不明显,界面较平滑,镁钢界面形成了金属化合物以及氧化镁,降低了接头的塑性和韧性.对DP600/AZ31无匙孔搅拌摩擦点焊焊接接头进行拉伸试验,发现焊接接头从搭接界面上断裂,断口呈“脉状花样”,为延性断裂.     

AZ91HP镁合金化学镀镍磷及镀层性能研究

目的提高AZ91HP镁合金的耐蚀及耐磨性,扩大其应用范围。方法采用H_3PO_4+Na_3PO_4酸洗液+NH_4HF_2活化的无铬前处理工艺,再直接化学镀,获得镍磷合金镀层,随后对镀层进行了热处理。对施镀前后基体和镀层的形貌、显微硬度和耐蚀性等进行了表征分析。结果 AZ91HP镁合金经H_3PO_4+Na_3PO_4酸洗+NH_4HF_2活化的无铬处理后施镀,形成的细小胞状组织均匀致密,结合力良好,镀层结构以非晶态相为主,耐腐蚀性比基体显著增高。经热处理后,镀层的硬度明显增高且在400℃时获得的镀层硬度最高,但耐蚀性有所下降。结论 H_3PO_4+Na_3PO_4酸洗体系+NH_4HF_2活化为镁合金提供了一种环保而有效的化学镀前处理方法,获得了以非晶态相为主的Ni-P镀层,提高了基体的耐腐蚀性,镀后热处理可进一步提高镀层的硬度。     

Sr对AZ91镁合金热稳定性及疲劳性能的影响

对AZ91镁合金中加入质量分数0．5％Sr后其热稳定性与疲劳性能进行了研究．结果表明：经固溶处理后。AZ91—0．5Sr合金比AZ91合金具有更好的热稳定性；高周疲劳试验中发现。在AZ91—0．5Sr合金试样断口上有规则的海滩波纹状辉纹．而AZ91合金试样没有明显的辉纹．只有少量的撕裂棱。说明AZ91-0．5Sr合金比AZ91合金具有更好的疲劳性能．     

热挤压AZ91D镁合金线材的组织与性能研究

对铸态AZ91D合金进行400~460℃不同温度下的正挤压,制备出直径为3~4 mm的线材。利用光学显微镜分析线材的组织,测试其拉伸力学性能和热膨胀系数。结果表明,在不同挤压温度下均可制备出AZ91D镁合金线材,挤压温度越低,线材晶粒越细小。线材具有优异的力学性能,经400℃热挤压成形的线材抗拉强度和伸长率高达285.6 MPa和5.3%,明显高于同牌号铸态合金的性能。线材的平均线膨胀系数为(21.3~27.4)×10-6K-1。较低挤压温度下制备的线材具有较高的力学性能与较小的线膨胀系数。     

Periodic autoregressive conditional duration

We propose an autoregressive conditional duration (ACD) model with periodic time-varying parameters and multiplicative error form. We name this model periodic autoregressive conditional duration (PACD). First, we study the stability properties and the moment structures of it. Second, we estimate the model parameters, using (profile and two-stage) Gamma quasi-maximum likelihood estimates (QMLEs), the asymptotic properties of which are examined under general regularity conditions. Our estimation method encompasses the exponential QMLE, as a particular case. The proposed methodology is illustrated with simulated data and two empirical applications on forecasting Bitcoin trading volume and realized volatility. We found that the PACD produces better in-sample and out-of-sample forecasts than the standard ACD.     

AZ31B镁合金TIG焊裂纹形成机理及其预防措施

分析了AZ31B镁合金TIG焊接头裂纹形成机理,并提出了相应的预防措施。试验表明,焊接裂纹与填充材料的化学成分有一定关系。采用锌含量较低的AZ40M作为填充材料时,焊缝没有出现裂纹;采用AZ31B作为填充材料或直接对焊时,焊缝均有裂纹。     

P91超厚壁大径管的焊接

根据生产经验、实验数据、技术资料,从焊接性、工艺评定、焊工培训、焊前准备、焊接工艺几个方面对P91大径超厚壁管的焊接进行了分析与探讨。     

基于OMAP的嵌入式TCP/IP开发

介绍了嵌入式系统TCP/IP的网络层及运动层C代码的开发过程,结合以太网控制器LAN91C96实现其链路层,将通用的上层程序移植到OMAP平台上,实现了OMAP平台的底层TCP/IP协议。