Effects of alloying elements on the microstructure and inclusion formation in HSLA multipass welds

The effects of Ti, Ni, Mo and Cr on microstructural development, and the chemical composition of the non-metallic inclusions, in high strength low alloy multipass (HSLA) weld metal have been considered. Increasing titanium content, in the range of 50 to 400 ppm, has not caused any major effects on microstructural development. With a further increase in the hardenability, by Ni, Mo and Cr additions, the microstructure has changed from a mixture of allotriomorphic ferrite, Widmanstätten ferrite, acicular ferrite and microphases to a mixture of acicular ferrite, bainite, low carbon martensite and microphases. In weld metals with low titanium content, manganese and silicon were the main chemical elements present in inclusions. Increasing the titanium content in the weld metal leads to an increase in the titanium content of the inclusions. For a very high titanium content, ≈ 700 ppm, the amount of titanium in the inclusions varies in the range of 60 to 70 wt.%.     

基于微观结构的高强铝合金应力集中系数研究

为了更好地理解铝合金材料的微观力学性能,基于MATLAB编写了Voronoi算法的微观结构模拟程序,并将程序导入ABAQUS有限元软件建立铝合金晶粒模型.推导出六结点内聚线单元模型的界面单元刚度矩阵,利用内聚力模型的内聚力-位移关系描述铝合金晶粒界面间的粘着力(法向力)和摩擦力(切向力),建立了微观晶粒结构的有限元模型.研究结果表明:单个夹杂粒子随着弹性模量的增加应力集中系数先减小再增加;相对于单个夹杂粒子,两个夹杂粒子的应力集中会增加,当d/r接近2时应力集中系数明显增加,当d/r值处在6左右时应力集中系数基本恢复到单夹杂粒子时的大小.夹杂粒子的形状、数量及分布状态对结构微观应力集中均有影响.     

高比强多孔铝合金的压缩变形性能

采用渗流法制备了新型多组元、高比强多孔铝合金，研究了合金的单向压缩变形特征和能量吸收性能，讨论了孔壁厚度均匀性、孔隙率和强化热处理对性能的影响．结果表明：提高多孔铝合金的孔壁厚度的均匀度和强化热处理都显著提高多孔铝合金的压缩吸能性能；随着孔隙率的降低多孔铝合金的压缩强度增加。     

Cold cracking in DC-cast high strength aluminum alloy ingots: An intrinsic problem intensified by casting process parameters

For almost half a century the catastrophic failure of direct chill (DC) cast high strength aluminum alloys has been challenging the production of sound ingots. To overcome this problem, a criterion is required that can assist the researchers in predicting the critical conditions which facilitate the catastrophic failure of the ingots. This could be achieved at first glance by application of computer simulations to assess the level and distribution of residual thermal stresses. However, the simulation results are only able to show the critical locations and conditions where and when high stresses may appear in the ingots. The prediction of critical void/crack size requires simultaneous application of fracture mechanics. In this paper, we present the thermo-mechanical simulation results that indicate the critical crack size distribution in several DC-cast billets cast at various casting conditions. The simulation results were validated upon experimental DC-casting trials and revealed that the existence of voids/cracks with a considerable size is required for cold cracking to occur.     

随焊激冷防止高强铝合金焊接热裂纹的数值模拟

采用热弹塑性有限元方法对随焊激冷防止焊接热裂纹的工艺过程进行了数值模拟 ,动态定量地描述了温度场、位移场和应变场的演变过程 ,清晰地揭示了随焊激冷防止焊接热裂纹的机理及主要参数的影响规律 .研究表明 ,随焊激冷能够通过冷源作用区的冷却收缩造成对其前方处于BTR金属的横向挤压 ,推迟塑性拉伸应变的开始形成温度并减缓其发展速率 ,因而降低热裂纹倾向 ;采用较小的激冷距离有利于降低热裂纹倾向 ,在激冷距离较大时提高激冷功率仍有防止热裂纹的作用 ;加大外部拘束会在一定程度上削弱随焊激冷防止热裂纹的效果 .     

Influence of alloying on hydrogen-assisted cracking and diffusible hydrogen content in Cr-Mo steel welds

Study of hydrogen-assisted cracking and measurement of diffusible hydrogen content in different Cr-Mo steel welds shows that under identical conditions, susceptibility to cracking increased and diffusible hydrogen content decrease with increase in alloy content. Hydrogen permeation studies show that hydrogen diffusivity decreases and solubility increases with increase in alloy content. Thus decrease in diffusible hydrogen content with increase in alloying is attributed to increase in apparent solubility and decrease in apparent diffusivity of hydrogen. Analysis of the results indicates that variation of diffusible hydrogen content and apparent diffusivity of hydrogen with alloy content can be represented as a function of carbon equivalent CE₁ originally proposed to predict the hardness in the heat-affected zone of alloy steel welds.     

Influence of alloying elements and density on aqueous corrosion behaviour of some sintered low alloy steels

Low alloy steels produced through powder metallurgy route of sintering followed by forging are promising candidate materials for high strength small components. Porosity in such steels poses a real challenge during acid pickling treatment, which is one of the processing steps during manufacturing. The present research work attempts to investigate the mechanism underlying the acid corrosion behaviour of some sintered low alloy steels under induced acid pickling conditions. Sintered-forged low alloy steel samples containing molybdenum (Mo), copper (Cu) and titanium (Ti) were subjected to aqueous corrosion attack by immersing the samples in 18% HCl (Hydrochloric acid) solution for 25 h. Sample weight loss and Fe (Iron) loss were estimated for the corroded samples. The morphology of the corroded surfaces was studied through metallography and scanning electron microscopy. Higher porosity alloys underwent enhanced corrosion rates. Both corrosion rate and iron loss are found to decrease linearly with reduction in porosity in all cases of the alloys. The alloying elements Mo, Ti and Cu, when added in combination, have played a complementary role in the reduction of corrosion rate by almost one order of magnitude compared to unalloyed steel. Presence of carbides of the carbide forming elements Mo and Ti played a positive role on the corrosion behaviour of the low alloy steels.     

Influence of alloying elements on thermal conductivity and high temperature strength of copper based alloys

Abstract

The effects of alloying elements and annealing temperature on thermal conductivity and softening behaviour of Cu – 0·1Ag – xP–yMg and Cu – xSn – yTe alloys (all compositions are in wt-%) have been investigated. The Cu – 0·1Ag – xP–yMg alloys, in spite of greater amounts of P and Mg, had a higher electrical conductivity and a higher softening temperature than those of a Cu – 0·1Ag – 0·031P alloy. A Cu – 0·032Sn – 0·023Te alloy had the same levels of electrical conductivity and softening temperature as those of Cu – 0·040Sn. The conductivity and softening temperature of the Cu – 0·032Sn – 0·023Te alloy are comparable with those of the Cu – 0·1Ag – 0·013P alloy currently used for continuous casting mould materials.     

Effects of microstructural heterogeneity on very high cycle fatigue properties of 7050-T7451 aluminum alloy friction stir butt welds

In this study, the very high cycle fatigue (VHCF) properties of 7050-T7451 aluminum alloy and its friction stir welding (FSW) butt welds have been investigated. The results show that the failure of FSW joints still occurs at 7.0 × 10⁸ cycles. The fatigue properties of the FSW joints are superior to those of the base material, especially in the super long life regime. Most fatigue cracks initiate at the thermo-mechanically affected zone and heat affected zone on the advancing side of the FSW joints, and the susceptibility of these zones to fatigue is attributed to the metallurgical heterogeneity.     

Influence of aluminum and vanadium alloying elements on the resistivity of titanium

The experimental data on the resistivity of titanium-aluminum and titanium-vanadium alloys in the temperature ranges of 77–1700 and 77–350 K, respectively, at alloying component concentrations below 10 at % are generalized. At these concentrations, the isotherms of the dependences ρ(c) are linear in the temperature range of the α phase (77–500 K) and in the β phase. The numerical values of the concentration angular coefficients (dρ/dc) _T = b were obtained for the Ti-Al and Ti-V systems at temperatures of 77 and 300 K and Ti-Al alloys in the β phase at T = 1400 K. The temperature derivatives (dρ/dT) _T = b _T for the α phase near the Debye temperature and the β phase, where the dependences ρ(T) are close to linear, have been calculated for Ti-Al alloys. In both cases the concentration dependences of the temperature angular coefficients b _T (c) decrease with an increase in the Al concentration; however, their magnitudes differ by an order of magnitude.     

Sulfide stress cracking resistance of API-X100 high strength low alloy steel

Sulfide Stress Cracking (SSC) of API-X100 high strength low alloy steel was evaluated in NACE solution “A” at room temperature. The corrosion rate, utilizing electrochemical polarization techniques, in the solution was 97 mpy. Proof ring testing, per NACE TM-0177, generated an SSC threshold stress value of 46% of yield strength. SSC susceptibility was caused by the high corrosion rate which formed corrosion pits that acted as crack initiation sites on the metal surface and provided more hydrogen to migrate into the steel. In addition, the X100 inhomogeneous microstructure provided a high density of hydrogen traps which promoted hydrogen embrittlement.     

Effects of laser peening on the fatigue strength and defect tolerance of aluminum alloy

The effects of laser peening (LP) on the bending fatigue strength of the 7075‐T651 aluminum alloy were investigated. Accordingly, the defect tolerance of the aluminum alloy subjected to LP is discussed on the basis of fracture mechanics. The results indicate that a deeper compressive residual stress was induced by LP compared with the case of shot peening (SP). The fatigue strengths increased when both peening types were used. Semicircular slits with depths less than 0.4 and 0.1 mm were rendered harmless on the basis of the applications of LP and SP, respectively. The apparent threshold stress intensity factor range ΔK_th,ap increased by approximately five and two times owing to LP and SP, respectively. The increase of the ΔK_th,ap was caused by the compressive residual stress induced by the peening. The Kitagawa‐Takahashi diagram of the laser‐peened specimens shows that the defect tolerance of the aluminum alloy was improved by LP.     

Mg元素对铝合金微弧氧化陶瓷膜的影响

选择磷酸盐为主要成分的复合电解液体系,用微弧氧化方法对3种常用的商业铝合金物陶瓷膜分别用扫描电镜、能谱仪、X射线衍射等探讨了铝合金中元素Mg对陶瓷层的组织形貌、元素分布和相组成的调制作用,并借助硬度、摩擦磨损测试评价了膜层的性能。结果表明膜的相结构主要为γ-Al2O3,α-Al2O3和AlPO4,并含有少量Mg、Cu、Zn、W元素;随着Mg含量的增加表面组织更加细密,膜层中大尺寸缺陷减少;膜层中α-Al2O3含量随Mg含量的增加逐渐减少,当Mg含量接近2.5%时,膜层中只剩下γ-Al2O3;硬度与摩擦磨损测试显示Mg含量的增加导致硬度和耐磨性能下降。     

高Nb-TiAl合金的凝固组织与力学性能研究

研究了Al含量变化对高Nb-TiAl合金的凝固组织与力学性能的影响.结果表明:随着Al含量的增加,TiAl合金晶粒尺寸呈增加趋势;当Al含量为45.7%时,凝固过程中局部区域发生包晶转变,使晶粒尺寸显著增大;室温及700℃高温拉伸强度随着Al含量的增加而呈增加的趋势,但发生包晶转变致使室温及700℃高温拉伸强度下降约200MPa;Al含量对延伸率不敏感,持久性能随Al含量的增加呈增加趋势.为控制铸锭凝固后的组织与力学性能,尽量避开包晶转变区,合金中Al含量应低于45.7%.     

800 MPa级超高强度铝合金的时效析出行为

采用硬度、电导率、室温拉伸测试方法,研究110~140℃范围内时效不同时间后新型铝合金性能的变化。利用透射显微镜(TEM)观察合金的组织形貌特征。结果表明:该新型铝合金最佳的时效工艺为110℃保温24 h,此条件下合金的抗拉强度,屈服强度和伸长率分别为808,785 MPa与6.9%。时效温度是影响合金析出相种类、密度和尺寸的主要因素。在110℃时效时,合金主要的析出相是GPⅠ区、GPⅡ区和亚稳η′相。110℃时长时间(直至96 h)时效后,GPⅠ区和GPⅡ区仍能稳定存在。与110℃时效相比,在140℃时效时,析出过程加速。当140℃时效4 h后,未观察到GP区的存在,主要的析出相为η′相;140℃时效24 h后,主要的析出相为η′相和η相。     

Effect of power ultrasound on solidification of aluminum A356 alloy

The present investigation attempted to evaluate the effect of ultrasonic vibration on the nucleation and growth of aluminum alloy A356 melt. A356 melt was treated at various solid fractions isothermally with ultrasonic vibrations by dipping the acoustic radiator into the melt. Experimental result confirmed that globular grains could be effectively obtained when the melt was ultrasonically treated at the temperature close to its liquidus and subsequently cooled quickly. It further illustrated the difficulty to form globular grains when the specimens were treated at isothermal temperatures in the mushy zone. It may imply that in the given experiments cavitations-induced heterogeneous nucleation plays a more important role than dendrite fragmentation in the formation of globular grains.     

Influence of alloying elements on as-cast microstructure and strength of gray iron

Casting experiments were carried out to produce gray cast irons with compositions in the range (wt.%): Fe–3.2C–wCu–xMo–yMn–zSi, where w = 0.78–1.79, x = 0.11–1.17, y = 0.68–2.34 and z = 1.41–2.32. These key elements were varied systematically during sand casting into 30-mm diameter bars to assess their influence on the development of microstructure and mechanical properties. It was found that microstructures ranging from fully pearlitic to an intimate mixture of retained austenite and bainitic ferrite, termed ausferrite, were produced and a reasonable linear correlation was observed between ausferrite volume fraction and strength. The optimum combination of mechanical properties was achieved in an alloy of approximate composition Fe–3.2C–1.0Cu–0.7Mo–0.55Mn–2.0Si, which generated 100% ausferrite without alloy carbides. This alloy has a microstructure and mechanical properties comparable to austempered gray iron without many of the problems associated with austempering.     

Effects of sulfur content in tube steel welds on sulfide cracking

Summary Results are presented on the tendency to stress corrosion cracking in NACE solution for welded joints in tube steel with sulfur contents from 0.002 to 0.028%. Threshold stresses have been determined and test results are given on the tendency to sulfide cracking at slow strain rates. There is a relationship between the tendency to stress corrosion cracking and the microstructural features.Translated from Fiziko-Khimicheskaya Mekhanika Meterialov, No. 6, pp. 51–56, November–December, 1992.     

Effects of sulfur content in tube steel welds on sulfide cracking

Materials Science - Results are presented on the tendency to stress corrosion cracking in NACE solution for welded joints in tube steel with sulfur contents from 0.002 to 0.028%. Threshold stresses...