Effect of a laser pre-quenched steel substrate surface on the crack driving force in a coating–steel substrate system

A mechanical model of a coating/laser pre-quenched steel substrate specimen with a crack oriented perpendicular to the interface between the coating and the hardened layer is developed to quantify the effects of the residual stress and hardness gradient on the crack driving force in terms of the J-integral. It is assumed that the crack tip is in the middle of the hardened layer of the pre-quenched steel substrate. Using a composite double cantilever beam model, analytical solutions can be derived, and these can be used to quantify the effects of the residual stress and the hardness gradient resulting from the pre-quenched steel substrate surface on the crack driving force. A numerical example is presented to investigate how the residual compressive stress, the coefficient linking microhardness and yield strength and the Young’s modulus ratio of the hardened layer to the coating influence the crack driving force for a given crack length.     

Formation of the ζ phase at an interface between an Fe substrate and a molten 0.2 mass% Al–Zn during galvannealing

An interstitial free (IF) steel was dipped in a bath of molten Zn containing 0.2 mass% Al. The as-dipped (galvanized) steel was then annealed above the melting point of Zn (galvannealed), say at 773 K, for a period ranging from 1 to 30 s. Evolution of the microstructure of the Zn-containing coating was examined by transmission electron microscopy (TEM) and the chemical composition around the Zn(Al)/Fe interface was analyzed by energy dispersion spectroscopy (EDS) in a dedicated scanning transmission electron microscope (STEM). In the as-dipped (galvanized) steel a continuous layer of Fe₂Al₅ existed at the Zn(Al)/Fe interface, but no Fe–Zn intermetallic compounds were observed. Galvannealing at 773 K for 1 s resulted in the formation of the ζ phase at the Zn(Al)/Fe₂Al₅ interface, which grew at the expense of the Zn(Al) coating during the subsequent galvannealing. Possible mechanisms of nucleation of Fe–Zn intermetallic compounds are discussed.     

In Situ SEM Observations of Fracture Behavior of Laser Welded–Brazed Al/Steel Dissimilar Joint

Laser welding–brazing of 6061-T6 aluminum alloy to DP590 dual-phase steel with Al-Si12 flux-cored filler wire was performed. The microstructure at the brazing interface was characterized. Fracture behavior was observed and analyzed by in situ scanning electron microscope. The microstructure of the brazing interface showed that inhomogeneous intermetallic compounds formed along the thickness direction, which had a great influence on the crack initiation and propagation. In the top region, the reaction layer at the interface consisted of scattered needle-like Fe(Al,Si)₃ and serration-shaped Fe_1.8Al_7.2Si. In the middle region, the compound at the interface was only serration-shaped Fe_1.8Al_7.2Si. In the bottom region, the interface was composed of lamellar-shaped Fe_1.8Al_7.2Si. The cracks were first detected in the bottom region and propagated from bottom to top along the interface. At the bottom region, the crack initiated and propagated along the Fe_1.8Al_7.2Si/weld seam interface during the in situ tensile test. When the crack propagated into the middle region, a deflection of crack propagation appeared. The crack first propagated along the steel/Fe_1.8Al_7.2Si interface and then moved along the weld seam until the failure of the joint. The tensile strength of the joint was 146.5 MPa. Some micro-cracks were detected at Fe(Al,Si)₃ and the interface between the steel substrate and Fe(Al,Si)₃ in the top region while the interface was still connected.     

Study of the influence of phase composition and iron content on the formability characteristics of zinc-iron electroplated sheet steel

This study focuses on the relationship between coating composition and deformation and friction behavior of zinc-iron electroplated sheet steel. The influence of phase composition and microhardness of the deposits and the electrodeposition process parameters on the mechanical properties of the material were determined. The influence of coating composition on the friction and galling behavior was also investigated. Both V-bend test and cup test were used to evaluate the influence of the iron content on the powdering and flaking behavior of the deposits. Finally, the adhesion of the coating to the substrate was studied by lap shear tests. Although the soft η phase appears to be the main component in zinc-iron coatings with less than 16 wt% Fe, Γ₁ particles were observed even at low iron contents. As the iron content in the coating increases, the Γ₁ fraction increases and the coating becomes harder and more brittle. Above 16 wt% Fe the deposits start to show substantial powdering and flaking during deformation. At iron contents above 30 wt%, bending of the coated product results in total coating delamination. At low iron contents, zinc-iron electroplated sheet steel exhibits a superior deformation behavior, and both cup tests and flat die tests proved the suitability of the coating for deep drawing.     

Interface fracture behavior of zinc coatings on steel: Experiments and finite element calculations

Hot-dipped galvanized steels are widely used in the automotive industry. The formability and damage resistance of zinc coatings depend strongly on their microstructure and adhesion to the steel substrate. In order to improve the mechanical performance of zinc coatings, the influence of their thickness, grain orientation and grain size on the zinc coating/steel substrate interface cracking behavior was studied. To this end, scanning electron microscopic observations during in situ tensile testing of zinc coated IF steel sheets were performed. After partial delamination of the zinc coating, cross sections of zinc coated steel were prepared to determine the location and extent of the interface cracking and the crystallographic orientation of the delaminated zinc grains. A two-grain model using a finite element method is proposed to analyze the zinc coating/steel substrate interface cracking behavior. In addition, the coating adhesion strength can be estimated based on this model. Both calculations and experimental observations show that: (i) a preferential zinc grain orientation with the [0001] direction parallel to the interface and (ii) a small grain size mitigate zinc coating/steel substrate interface cracking.     

Dissolution mechanism of solid nickel in liquid zinc saturated with Fe

The dissolution behavior of solid nickel in static liquid zinc saturated with Fe at 723 K was studied. The results show that when immersing solid Ni in liquid Zn saturated with Fe, the intermetallic compound layers consisted of γ and δ phases are formed on nickel substrate, which is the same as that in liquid pure zinc. However, some F2 particles are formed in the liquid near the solid/liquid interface. These Г2 particles can easily heterogeneously nucleate on （particles and grow fast. The dissolution process is governed by diffusion of nickel atom across a concentration boundary layer in liquid Zn saturated with Fe, and is different from a mixed control mechanism of nickel in liquid pure zinc. The participation of Г2 particles makes the dissolution of solid Ni in the liquid accelerated.     

Experimental and finite element analysis for fracture of coating layer of galvannealed steel sheet

Mechanical properties of galvannealed (GA) steel sheet used for automotive exposed panel and predicted failure phenomenon of its coating layer were evaluated using finite element method. V-bending test was performed to understand better the fracture of coating layer of GA steel sheet during plastic deformation. Yield strength of the coating layer was calculated by using a relative difference between hardness of coating layer measured from the nano-indentation test and that of substrate. To measure shearing strength at the interface between substrate and coating layer, shearing test with two specimens attached by an adhesive was carried out. Using the mechanical properties measured, a series of finite element analyses coupled with a failure model was performed. Results reveal that the fracture of coating layer occurs in an irregular manner at the region where compressive deformation is dominant. Meanwhile, a series of vertical cracks perpendicular to material surface are observed at the tensile stressed-region. It is found that 0.26-0.28 of local equivalent plastic strain exists at the coating and substrate at the beginning of failure. The fracture of coating layer depends on ductility of the coating layer considerably as well.     

Method for the evaluation of the low-cycle fatigue strength of welded joints using the J-integral energy criterion

Summary

The purpose of this study was to investigate effects of aluminium in the Zn coating on electrode life in welding galvanized steel sheet. Three hot‐dip galvanized and one electro‐galvanized steel sheet types were prepared for this study. Aluminium content in the coatings varied within the range 0.26–0.87 mass% for the three types of hot‐dip galvanized sheet.

The approach used here included EDX, AEX analysis of the coating layers, electrode life tests and EPMA analysis of electrodes after 900 spots were welded.

The results were as follows:

1. The electrode lives of HDG materials were changed at approximately 0.3–0.4 mass% Al content in Zn coating. Materials with low‐Al coating content showed over three times longer electrode lives than materials with high‐Al coating content.

2. Although the thickness of Al oxide layers was in proportion to the Al content in Zn coatings, the obvious correlation between electrode life and thickness of Al oxide layers was not observed.

3. In the case of low‐Al coating content, it was observed that Fe‐Zn alloy grew from the steel‐coating interface to the Zn coating. It was considered that, in the initial stage of welding, the content of Fe in Zn coating increased immediately.

4. In the case of high‐Al coating content, Fe‐Al alloy was observed at coating‐steel interfaces instead of Fe‐Zn alloy. It was known that Fe‐Al alloy suppresses the Fe‐Zn alloying reaction. Zn coating was not alloyed with Fe in initial stages of welding.

5. From these results, it was concluded that aluminium in coatings affected electrode life by changing the melting point of coating layers between the electrode and the steel. The melting point of low‐Al content coating layers rose because of the diffusion of Fe into the Zn coating. This phenomenon decreased electrode wear and electrode life was long. In contrast, the melting point of high‐Al content coating layers remained low. This phenomenon caused electrode alloying easily and also increased electrode wear. As a result, electrode life became shorter.

     

加热温度对Al-Si涂层热成形钢组织及性能的影响

利用粗糙度仪、扫描电镜、硬度计、辉光放电原子发射光谱仪等检测方法,研究分析了热冲压成形工艺过程中的加热温度对Al-Si涂层22MnB5热成形钢组织及性能的影响。结果表明,随着加热温度的升高,Fe沿垂直于表面方向由热成形钢基体向Al-Si涂层表面的迁移量逐渐增大,O沿垂直于表面方向由Al-Si涂层表面向热成形钢基体的迁移量逐渐增大,且迁移的最大深度约为2.80 μm。Fe沿垂直于表面方向由热成形钢基体向Al-Si涂层表面的迁移量直接决定了Fe-Al-Si相的形态、生成位置及界面结合层厚度。随着加热温度的升高,Al-Si涂层表面粗糙度R_a、峰值计数R_pc值先增大后减小;当加热温度为930 ℃时,涂层表面粗糙度R_a达到最大值1.89 μm,峰值计数R_pc值达到最大值218。随着加热温度的升高,Al-Si涂层总厚度从27.78 μm增加至40.46 μm,界面结合层厚度从1.08 μm增加至15.11 μm。当加热温度为930 ℃时,热成形钢基体的硬度达到最大值505 HV0.2。     

MoSi2/Mo涂层热震过程中界面应力分布和裂纹扩展的有限元模拟

在氢气保护下将MoSi₂/Mo涂层加热至1000 ℃,再迅速冷却至室温进行热震循环,表征了材料在热震循环过程中裂纹的演变过程并评估了MoSi₂/Mo涂层的热冲击行为。采用Abaqus软件计算了MoSi₂/Mo涂层在热冲击过程中的应力分布,讨论了热震循环中裂纹的发展过程。结果表明：Mo基体与MoSi₂涂层之间存在较高的热冲击应力,这将导致裂纹的萌生和扩展。计算结果显示：在最初的10次热震循环中,涂层产生了垂直于界面的裂纹,在界面上没有出现裂纹,涂层与基体仍结合良好;在随后的热震循环中开始出现界面裂纹,界面裂纹开始于垂直裂纹的末端区域,当垂直裂纹与界面裂纹汇聚,会导致涂层剥离和涂层失效。     

Study of fracture behaviour of bond coats on nickel superalloy by three-point bending of microbeams

A microbeam testing geometry is designed to study the variation in fracture toughness across a compositionally graded NiAl coating on a superalloy substrate. A bi-material analytical model of fracture is used to evaluate toughness by deconvoluting load–displacement data generated in a three-point bending test. It is shown that the surface layers of a diffusion bond coat can be much more brittle than the interior despite the fact that elastic modulus and hardness do not display significant variations. Such a gradient in toughness allows stable crack propagation in a test that would normally lead to unstable fracture in a homogeneous, brittle material. As the crack approaches the interface, plasticity due to the presence of Ni₃Al leads to gross bending and crack bifurcation.     

热喷涂涂层界面断裂韧性的反向三点弯曲法试验

以热喷涂NiCrBSi涂层/钢基体为材料模型,利用将涂层置于受压侧的反向三点弯曲法试验,对热喷涂涂层界面断裂现象进行分析并建立相应的界面断裂韧性计算模型。结果表明,界面裂纹起始于三点弯曲试样中部,对应于加载压头与涂层接触区域正下方的界面上,并向两侧扩展,伴随涂层屈曲,形成分层屈曲的破坏形貌。根据分层屈曲形貌建立计算模型,通过分层屈曲几何参数与屈曲临界应力、涂层内真实应力以及界面断裂韧性的关系,获得界面断裂韧性值。     

双相不锈钢表面激光熔覆钴基合金组织和性能研究

目的提高2205双相不锈钢的耐磨性和耐腐蚀性能。方法采用激光熔覆技术,在2205双相不锈钢基体表面制备钴基合金熔覆层。用X射线衍射仪、光学显微镜检测钴基合金熔覆层的相组成和显微组织,用能谱仪测定熔覆层和基体界面区域的Fe和Cr元素分布,确定熔覆层界面过渡区域的宽度。用显微硬度计和湿砂磨粒磨损试验机,测试熔覆层硬度和耐磨性能。采用扫描电镜观察摩擦表面的磨损特性,分析钴基合金熔覆层的磨损机理。用电化学工作站测试熔覆层的电化学腐蚀特性,并用2205双相不锈钢作为对比试样做相应的性能试验。结果熔覆层由γ-Co固溶体和少量的Cr7C3、Cr2Ni3化合物相组成,界面处的熔覆层相组织是少量的平面晶和胞状晶,其他区域是发达的树枝晶。由于熔覆层由多道搭接和多层熔覆形成,树枝晶生长有方向性,但不是成固定的方向,并出现明显的分层现象。熔覆层过渡区范围为50μm左右,熔覆层平均显微硬度达477HV(0.1),远高于2205双相不锈钢基体(265HV(0.1))。当磨程达到3354m时,熔覆层的质量损失仅为10.3 mg,约为基体质量损失的1/3。在3.5%NaCl溶液中,熔覆层具有较高的极化电阻与电荷转移电阻和较小的自腐蚀电流。结论熔覆层组织致密,无气孔、裂纹等缺陷,与基体呈良好的冶金结合,钴基合金熔覆层具有良好的耐磨粒磨损性能和耐腐蚀性能。     

Corrosion protection of mild steel by applying TiO<Subscript>2</Subscript> nanoparticle coating via sol-gel method

TiO₂ nanoparticle coatings possess good thermal and electrical properties and they are resistant to oxidation, corrosion, erosion and wear in high temperature environments. This property is very important factor in the applications such as pipelines, castings and automotive industry. In this investigation a uniform TiO₂ nanoparticle coating has been applied on mild steel, using sol-gel method. The coating was deposited on mild steel substrate by dip coating technique. The morphology and structure of the coating were analyzed using SEM, AFM and X-ray diffraction. The anticorrosion performances of the coating have been evaluated by using electrochemical techniques. It is worthy to note that the film uniformity was retained in high temperatures and no crack and flaking off from the substrate was observed. The Tafel polarization measurements provide an explanation to the increased resistance of TiO₂ nanoparticle coated mild steel against corrosion and i_corr was decreased from 18.621 to 0.174 μA/cm². The article is published in the original.     

Interface fracture property of PEO ceramic coatings on aluminum alloy

This paper combines the four-point bending test, SEM and finite element method to study the interface fracture property of PEO coatings on aluminum alloy. The interface failure mode of the coating on the compression side is revealed. The ceramic coating crack firstly along the 45° to the interface, then the micro crack in the coating deduces the interface crack. The plastic deformation observed by SEM shows excellent adhesion property between the coating and substrate. The plastic deformation in the substrate is due to the interfacial crack extension, so the interface crack mode of PEO coatings is ductile crack. The results of FEM show that the compression strength is about 600 MPa.     

Interfacial features of TiAl alloy/316L stainless steel joint brazed with Zr−Cu−Ni−Al amorphous filler metal

TiAl alloy and 316L stainless steel were vacuum-brazed with Zr?50.0Cu?7.1Ni?7.1Al (at.%) amorphous filler metal. The influence of brazing time and temperature on the interfacial microstructure and shear strength of the resultant joints was investigated. The brazed seam consisted of three layers, including two diffusion layers and one residual filler metal layer. The typical microstructure of brazed TiAl alloy/316L stainless steel joint was TiAl alloy substrate/α₂-(Ti₃Al)/AlCuTi/residual filler metal/Cu₉Zr₁₁+Fe₂₃Zr₆/Laves-Fe₂Zr/α-(Fe,Cr)/316L stainless steel substrate. Discontinuous brittle Fe₂Zr layer formed near the interface between the residual filler metal layer and α-(Fe,Cr) layer. The maximum shear strength of brazed joints reached 129 MPa when brazed at 1020 °C for 10 min. The diffusion activation energies of α₂-(Ti₃Al) and α-(Fe,Cr) phases were ?195.769 and ?112.420 kJ/mol, respectively, the diffusion constants for these two phases were 3.639×10^?6 and 7.502×10^?10 μm²/s, respectively. Cracks initiated at Fe₂Zr layer and propagated into the residual filler metal layer during the shear test. The Laves-Fe₂Zr phase existing on the fracture surface suggested the brittle fracture mode of the brazed joints.     

Estimation of interfacial fracture toughness based on progressive edge delamination of a thin transparent coating on a polymer substrate

Evaluation of interfacial toughness of sub-micron-thickness layers deposited on a ductile substrate is a challenging task which has motivated different experimental approaches. Fragmentation testing was used in the present study as a means of interface characterization of a silicon–nitride-coated polyimide substrate. During the test, after an initial rapid segmentation–cracking phase, the coating fragments developed edge delaminations which propagated in a stable manner with further increase in the applied strain. The debonding process was modelled by the finite element method incorporating a cohesive zone at the front of the interfacial crack. The edge cracks were found to be dominated by mode II loading. By fitting the predicted delamination evolution to the experimental data for coating fragments of differing geometry, the mode II critical energy release rate was estimated at 30 J m^?2.     

金属基陶瓷涂层/钢基体界面裂纹的应力强度因子

将三点弯曲断裂力学试验与有限元分析(FEA)结合起来计算LX88A涂层与Q345钢界面裂纹的复应力强度因子.结果表明,对于三点弯曲试样,当发生界面断裂的临界载荷较小时,涂层试样的界面裂纹尖端附近存在K控制区,但K因子随临界载荷的增大,K因子控制区消失,发生失效的现象.对于三点弯曲试样,当临界载荷超过一定值时,线弹性断裂力学已经不能描述界面裂纹尖端场.因此,在后续研究中有必要使用弹塑性断裂力学和概率断裂力学对此类界面裂纹进行分析.     

Hot dip fine Zn and Zn-Al alloy double coating for corrosion resistance at coastal area

A new hot dip Zn-7Al alloy coating was performed on a structural steel by double coating of fine Zn and Zn-7 wt.% Al alloy, to prevent severe corrosion in coastal area. The alloy-coated steels were exposed to seaside, quasi-industrial, and rural districts to compare with conventional Zn coating. Double coating was significantly effective in preventing corrosion, particularly in a seaside. It was estimated from the exposure test for 10 years that the life of the Zn-7Al alloy-coated steel would be almost four times that of the Zn-coated steel in the seaside. A bending test showed that no exfoliation occurred at the interface between the coated alloy and substrate steel. TEM observation revealed that the excellent adhesiveness of the doubly coated fine Zn and Zn-7Al alloy to the steel substrate was due to formation of the interface region consisting of heterogeneous fine phase mixture of zinc, aluminium and iron.     

Ti/Al异种合金电弧熔钎焊接头界面断裂行为分析

采用TIG电弧的方法实现了钛合金与铝合金熔钎焊连接,分析了不同焊丝形成的熔钎焊接头的界面组织和断裂特征.结果表明,纯铝接头界面为单一的TiAl3相,裂纹主要沿着TiAl3反应层与焊缝之间的界面扩展.拉伸时首先从坡口拐角启裂,当裂纹扩展至接头反面时,断裂扩展形式转变为从焊缝金属撕裂,接头抗拉强度为139MPa.添加Al-Cu-La焊丝的接头界面结构为TiAl3+ Ti2Al20La双化合物层,拉伸时沿TiAl3反应层与钛合金界面开裂,以界面内的微裂纹为裂纹源并向反应层内扩展,属于准解理断裂,接头抗拉强度达270 MPa.稀土La元素作用下形成的双化合物层是提高接头强度的关键.