Metallographic Cross Sections Research Articles

X-Ray Tomography of Intergranular Corrosion Penetration in Sensitized AA5083-H131

The increased demand for speed, range, and fuel economy in the competitive field of commercial shipping and the expanding demands of the US Navy have emphasized the need for lightweight structural materials that possess a challenging combination of material property requirements. Aluminum-magnesium alloys (AA5XXX) are an ideal lightweight substitute for steel in marine systems due to their high strength-to-weight ratio, good weldability, and excellent uniform corrosion resistance. However, alloys containing more than 3 wt.% of Mg, when exposed to service temperatures for extended periods of time, can become susceptible to intergranular corrosion (IGC) due to sensitization or the precipitation of the more anodic β-phase (Al3Mg2) at the grain boundaries.[1] From a practical engineering perspective, the incidence, distribution, and propagation rate of IGC, as influenced by metallurgical, electrochemical, and geometrical factors, are important as IGC can develop into more severe forms of damage such as intergranular stress corrosion cracking (IGSCC). Because IGC is a sub-surface form of corrosion, it is essential to understand what occurs within the bulk material, below the visible surface. IGC spreading on the surface allows for depth penetration to occur on multiple parallel sites, subsequently causing simultaneous propagation of multiple IGC fissures. These individual IGC fissures can branch out in the perpendicular directions, eventually forming IGC networks. Previous work revealed that the incidence and rate of spreading and propagation of IGC in AA5083 is governed by multiple factors: β-phase morphology and distribution, propagation direction, degree of sensitization (DoS), applied potential and current density, and exposure time.[2-5]However, these observations of IGC were obtained through activity occurring on the material surface, in the form of IGC spreading, or in conventional, 2D post-mortem, destructive analysis of metallographic cross-sections of the specimen for IGC depth penetration. Although valuable insights were acquired from these conventional 2D visualization techniques, the true 3D complex geometries associated with IGC could be inaccurately reflected, specifically with regards to networking or connectivity. IGC fissures that appear separated in 2D may actually be connected in 3D.In recent years, X-ray computed tomography (XCT) has been an important emerging technique to visualize 3D internal structures of materials, allowing improved characterization, both qualitatively and quantitatively, of degradation and damage accumulation processes such as pitting, IGC, and cracking.[6-7] This technique is based on the variations in absorption coefficients as the X-ray beam penetrates through the specimen.[8] The absorption coefficient is associated with the density and the atomic number of the material within the specimen, enabling 3D visualization of defects and providing quantitative information.This presentation explores the IGC damage evolution and kinetics in sensitized AA5083-H131 specimens with different geometries, which produced varying extent of IGC connectivity. The specimen geometries included wire (no spreading), thin foil (limited spreading), and the 3D specimen with planar surface exposed (both spreading and penetration). Multiple potentiostatic and galvanostatic tests were conducted in 0.6 M NaCl solution for different exposure times with IGC penetration in the longitudinal (rolling) direction. XCT was utilized to non-destructively characterize IGC depth and degree of connectivity as a function of applied potential, applied current density, and exposure time. Initial experimental results (Figure 1) show the extent, both in a qualitative and quantitative manner, of IGC damage in sensitized AA5083-H131 captured via XCT.The results of this study provide further knowledge regarding the extent and dependence of IGC propagation kinetics with respect to the factors studied, which will be useful in life prediction modeling and in efforts to understand the underlying mechanisms controlling IGC and IGSCC in AA5XXX. This study is part of a larger effort to develop predictive models that estimate and forecast IGC and IGSCC damage progression in AA5XXX applied in marine environments. It also aids in the development of mitigation strategies and optimization of structure design for this class of alloy. Acknowledgements Office of Naval Research under the contract ONR: N00014-08-10315 with Dr. Airan Perez as contract monitor.

Expulsion characterization in resistance spot welding by means of a hardness mapping technique

Expulsion is an undesired event during resistance spot welding because the weld quality deteriorates. It is the ejection of molten metal from the weld nugget which usually occurs due to applying a high current for a short welding time. Expulsion has a significant impact on the final yield strength of the weld, thus the detection and characterization of expulsion events is significant for the quality assurance of resistance spot welds. In this study, hardness mapping, using a scanning hardness machine, was used as a quality assurance technique for resistance spot welding. Hardness tests were conducted on a resistance spot welded sample to prepare a hardness map. The test results showed good correlation between the hardness map and metallographic cross sections. The technique also provided further fundamental understanding of the resistance spot welding process, especially regarding the occurrence of expulsion in the nugget.

An Investigation Into the Role of Lead as a Suppressant for Tin Whisker Growth in Electronics

The effect on Sn electrodeposits of up to 10 wt.% codeposited Pb was investigated. The influence of Pb on cathodic polarization of the electrodeposition process, the microstructural and crystallographic characteristics of as-deposited films, the Sn-Cu intermetallic compound (IMC) formation between the deposits and Cu substrate and surface oxidation after ambient storage were examined. During electrodeposition, the plumbous (Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) ions were observed to inhibit stannous (Sn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) ion reduction. The use of a metallographic cross sectioning approach enabled the observation of a grain structure evolution toward a more equiaxed morphology with increasing Pb content. This was accompanied by a duplex-phase structure consisting of isolated Pb-rich particles at grain boundary intersections together with a continuous Sn-rich phase. X-ray diffraction measurements corroborate the existence of the duplex-phase structure and also indicate an as-deposited texture transition from (112) through (200) and further toward (220) with an increase in Pb content from 0 to 8 wt.%. A marked topological transition of the interfacial Cu-Sn IMCs from wedge-shaped and discretely distributed to relatively even and uniformly distributed was found to occur concurrently. After tin whisker growth had occurred from low-Pb content (up to 5 wt.%) electrodeposits on brass, Pb was generally observed to be strongly associated with actual Sn whiskers themselves, thus the spontaneous redistribution of Pb after electrodeposition has been evidenced. X-ray photoelectron spectroscopy sputter profiling indicates the absence of Pb in the Sn oxide layer and the enrichment of metallic Pb beneath the Sn oxide from a 90Sn-10Pb deposit ambient stored for 270 days. The whisker-inhibiting effect of Pb codeposition is postulated to emanate from more than one source: not only those reflected in grain structure modification, grain texture transition, and changes in the growth mode of the interfacial Cu-Sn IMCs as prevailingly acknowledged, but also significantly, the universal distribution of the soft Pb-rich phase, providing simultaneous diffusion toward potential whisker sites as well as effective strain dissipation.

Mobile X-Ray Computed Tomography for Nuclear and Aviation Industry

Abstract Digital detectors such as phosphor imaging plates (IP) and digital detector arrays (DDA) enable radiographic inspection with higher efficiency and improved image quality in comparison to the classic film technique. The mobile mechanised tomographic system TomoCAR (tomographic computer aided radiology) was developed first for inspection of circumferential welded seams of pipes. It consists of the manipulator based position control of an X-ray tube in front of the region to inspect and a digital detector array behind it. The high contrast sensitivity technique was combined with planar tomography. The tomographic reconstruction allows the three-dimensional (3D) representation of the material structure and included defects, equivalent to a metallographic cross sectioning. The hardware design is based on a specially developed flat X-ray tube and a direct converting CMOS detector array on CdTe basis. TomoCAR is applied for sizing of volumetric and planar defects. It allows the reliable detection of planar defects with openings larger than 25μm by subpixel resolution due to the achieved high signal-to-noise ratio. TomoCAR was successfully qualified as the result of a German pilot study on the basis of the European Network of Inspection and Qualification (ENIQ) guidelines for application in the nuclear power industry. Application examples are presented. The TomoCAR design was modified for in situ inspection of large aircraft components under production conditions. A gantry gate based planar tomograph was constructed and tested for inspection of the integrity of large flat CFRP panel components of up to 3 m × 9 m size. The integrity of imbedded stringers has been successfully observed. Different trials were performed to prove the finding rate of cracks in embedded stringers, which will be presented and discussed.

Studies of technological parameters influencing the nitriding behavior of spray-formed Al alloys

Despite their typically high Si content of up to 30ma.-%, spray-formed Al alloys exhibit excellent nitridability in plasma processes. This results from the exclusive primary solidified and homogeneously dispersed small Si precipitates in the base material that occur due to the specific production process. Previous studies reported on the influence of the general technological parameters during plasma nitriding, such as temperature, time and gas composition, as well as chemical composition, of spray-formed Al alloys on their nitriding behavior. However, in connection with progress in automation of process control, such as, for example, in the case of steels, a significant influence of the adaptive control of the plasma duty cycle has been detected. In cases where the specific nitriding technology of direct current (DC) plasma nitriding is used with in situ sputter cleaning, the plasma duty cycle determines different processes that occur on the sample surface.The scope of this report covers both the influence of the duty plasma cycle and the initial surface characteristics, including the specimen shape. Despite equivalent general nitriding conditions (temperature, time, voltage), over time the plasma duty cycle and the target value of plasma duty lead to different thicknesses and coverage of the nitride layer. For a better understanding of the nitriding mechanism, analyses of interrupted nitriding tests were carried out. The results of plasma nitriding have been characterized by examining metallographic cross sections with optical microscopy, surface coverage by scanning electron microscopy (SEM), as well as measuring aluminum nitride layer thickness with different methods. Mechanical properties were studied using microindentation. Adhesion was tested by scratch tests.

Corrosion behavior of stainless steel 316 in sub- and supercritical aqueous environments: Effect of LiOH additions

Corrosion control in nuclear power systems is examined via addition of LiOH to the coolant at pressurized water reactor (PWR) and supercritical water-cooled reactor (SCWR) operating conditions in stainless steel tubing. The loss of metal to the coolant is analyzed using the voltammetry method. The SEM/EDX analysis of metallographic cross-sections is performed. The results indicate that an adequate pH control is possible for water temperatures up to 500°C. Above this temperature pH control becomes progressively more difficult as dielectric constant and density of water decrease. Significant hydrogen production is detected at 650°C.

X-Ray and Optical Videography for 3D Measurement of Capillary and Melt Pool Geometry in Laser Welding

Abstract This paper describes a method to reconstruct the 3D shape of the melt pool and the capillary of a laser keyhole welding process. Three different diagnostic methods, including X-Ray and optical videography as well as metallographic cross sections are combined to gain the three dimensional data of the solidus-liquidus-surface. A detailed description of the experimental setup and a discussion of different methods to combine the 2D data sets of the three different diagnostic methods to a 3D-model will be given. The result will be a static 3D description of the welding process.

Determination of the corrosivity of Pb-containing salt mixtures

The composition of deposits from waste firing boilers is dominated by calcium-, alkali-chlorides and sulfates. Additionally, significant quantities of heavy metals such as Zn and Pb are usually present during waste combustion. In this high temperature corrosion study, two boiler materials, a low alloy ferritic steel (10CrMo9-10) and an austenitic stainless steel (AISI 347), were exposed to PbCl2–ZnCl2–KCl, PbCl2–KCl and PbCl2–K2SO4 salt mixtures as well as to the single salts of PbCl2, K2SO4 and KCl. In the tests, the salt or the salt mixture was put on a pre-oxidized and cleaned steel sample and then heat treated at 400, 500 and 600°C for 168h (1week) in a horizontal tube furnace. After the test, the metallographic cross-sections of the specimens were characterized with a SEM/EDX analyzer and the thickness of the corrosion layer was determined for each sample. The corrosion layer thickness measurements showed severe corrosion on 10CrMo9-10 already at 400°C when 5wt.% PbCl2 was mixed with KCl or K2SO4 With all the PbCl2 containing salt mixtures at temperatures around and above 500°C a significant corrosion was observed on both materials. In order to estimate the relative corrosivity of PbCl2 containing salt mixtures, the results from the laboratory experiments in this work were compared to an earlier study done with a corresponding ZnCl2 containing salt mixture.

Laser-Mig Hybrid Welding Of Aluminium To Steel — Effect Of Process Parameters On Joint Properties

Laser MIG hybrid welding was recently suggested as a feasible process for joining of aluminium to steel for both structural as well as tailored blank applications. To promote an understanding of the process and the effect of process parameters on joint properties, laser MIG hybrid welding experiments were performed to join aluminium alloy AA6016 to DC05 zinc-coated steel sheets, in the thickness range of 1 mm, in a butt joint configuration. Among the process parameters varied were laser power, MIG arc power, wire feed rate, welding speed and arc position relative to the abutting edges. By metallographic cross-sections and tensile tests, the effect of these process parameters on joint properties such as wetting length, intermetallic phase layer thickness and tensile strength could be elucidated. Based on these results, a process parameter envelope resulting in adequate and reproducible joint properties (sound weld bead, sufficient and regular wetting, thin intermetallic phase layer, tensile strength exceeding 180 MPa) was established. Within this parameter envelope, corrosion behaviour was rated not critical, and forming behaviour showed promising results.

Examination of the Mg-Zn Phase Formation in Hot-Dip Galvanized Steel Sheet

Abstract Zinc-magnesium layers have been produced by a combination of conventional hot-dip galvanizing and PVD magnesium coating. A subsequent short heat treatment causes the diffusion of magnesium into the zinc layer. The influence of the maximum temperature of the heat treatment on the diffusion of magnesium and on the type and distribution of the formed intermetallic Mg-Zn phases has been examined in detail in a temperature range from 300 to 400°C by means of X-ray phase analyses and metallographic cross sections. With increasing temperature a sequential intermetallic Mg-Zn phase formation has been detected. At 300°C, an initial magnesium rich, intermetallic MgZn2 surface layer is formed, which, at higher temperatures from 335 to 379°C, is progressively transformed into a zinc rich Mg2Zn11 layer by interdiffusion. In this context, the nucleation of the Mg2Zn11 phase takes place on the interface between the MgZn2 and the Zn phase. Above the peritectic temperature of 381°C, the microstructure is composed of a zinc matrix in which precipitated MgZn2 is dispersed.

Progress in NDT of resistance spot welding of aluminium using ultrasonic C-scan

An ultrasonic C-scan technique was evaluated for non-destructive testing (NDT) of resistance spot welding of aluminium. It was established through fundamental trials that removal of surface indentation from the welding electrode is necessary in order to obtain credible non-destructive assessments of aluminium spot weld size and shape. The non-destructive test results showed good correlation with peeled samples and metallographic cross sections. The technique also provided further fundamental understanding of aluminium resistance spot welding (RSW) process; especially with respect to the presence of an outer fused ring. The fundamental aspect of this study was underpinned using the NDT technique for a real assembly; challenging the technique with access issues, different material combinations, multiple joint stacks and independent measurements. The non-destructive test analyses obtained correlated well with actual tear down results and offered further encouragement that the C-scan technique is viable as a production tool if a suitably flattened surface can be provided.

A micro-level strain analysis of a high-strength dual-phase steel

Abstract The deformation behaviour of a dual-phase steel with a tensile strength of about 1 000 MPa is investigated at the micro level. In-plane strains are determined by means of in-situ tensile tests in the scanning electron microscope and automatic local deformation analysis (based on digital image correlation). Maps of strain at high and low magnification reveal the network-like formation of deformation bands. In detailed high resolution analyses the sites of highest strain (“hot spots”) are identified. Observations in metallographic cross-sections indicate that these hot spots are frequently the origin of damage initiation.

Lead sheathing of ship hulls in the Roman period: Archaeometallurgical characterisation

Abstract An archaeometallurgical analysis of samples of lead sheathing from five ships of the Roman period was carried out in order to determine their composition and microstructure, and to obtain a better understanding of their manufacturing processes. The examinations included optical microscopy of metallographic cross-sections, microhardness tests, scanning electron microscopy, including energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. The results show that the samples were all composed of lead covered with an oxide layer. The sheet thicknesses, microhardness values and microhardness distribution, as well as the grain size distribution, led to the conclusion that all of the sheets were produced by the same technology, using hammering, and were probably used for the same purpose. The presence of antimony was observed in the sample from the Roman ship from Caesarea, which may hint at an Italian (Sardinian) origin of the material, and perhaps of the ship.

Low-hazard metallography of moisture-sensitive electrochemical cells

A low-hazard approach is presented to prepare metallographic cross-sections of moisture-sensitive battery components. The approach is tailored for evaluation of thermal (molten salt) batteries composed of thin pressed-powder pellets, but has general applicability to other battery electrochemistries. Solution-cast polystyrene is used to encapsulate cells before embedding in epoxy. Nonaqueous grinding and polishing are performed in an industrial dry room to increase throughput. Lapping oil is used as a lubricant throughout grinding. Hexane is used as the solvent throughout processing; occupational exposure levels are well below the limits. Light optical and scanning electron microscopy on cross-sections are used to analyse a thermal battery cell. Spatially resolved X-ray diffraction on oblique angle cut cells complement the metallographic analysis.

Laser surface alloying of 1045 AISI steel using Ni–CrB2 powder

Laser surface alloying is a process whose purpose is to improve the surface properties by incorporating alloying elements into the surface. The advantages of using laser for surface treatment are: formation of a non-equilibrium or amorphous phase as well as homogenisation and refinement of the microstructure, all without affecting the substrate properties. Powder (50 wt-%Ni–50 wt-%CrB2) was injected into a melt pool created by a CW–CO2 laser on AISI1045 steel plates. In order to alloy the entire surface, the sample was scanned at scan speeds in the range of 600–6000 mm min–1 and the laser power was in the range of 1750–2500 W. The powder feed rate was 1·6 g min–1, the laser beam was 2 mm in diameter, with 60% overlap between successive laser paths. Metallographic cross-sections were made of the samples. For each sample the following properties were characterised: layer depth, microhardness (HV), layer microstructure and composition. It has been found that the scan speed and the laser power affect the depth of the melt pool, the microstructure, the hardness and the treated layer composition. The laser boronised surface exhibits better wear resistance than D2 tool steel hardened to 59 ± 1 HRC. This will be discussed based on numerical analysis of the laser/material interaction.

The iron anchors from the Tantura F shipwreck: typological and metallurgical analyses

The Tantura F shipwreck was discovered in 1995 in Dor (Tantura) lagoon, about 70 m offshore. It was a coaster that plied the Levant coast during the local early Islamic period. Among the finds exposed in the wreck site were two iron anchors of the T-shaped type. This type of anchor, dated to between the second half of the fourth and the thirteenth centuries AD, is found throughout the Mediterranean. The anchors were analyzed by typological and archaeometallurgical methods, including radiography, metallographic cross-sections, microhardness tests, SEM/EDS analysis and Optical Emisson Spectroscopy (OES) analysis. Light microscopy revealed heterogeneous microstructure consisting of ferrite, Widmanstätten ferrite-pearlite or pearlite, which is typical of wrought iron made by bloomery. The metallographic and microhardness results revealed that decarburization had occurred, probably during the final hot-working process. The OES analysis, supported by SEM/EDS data, showed that the anchors are similar in composition. Soda-blast cleaning followed by chemical etching revealed the forge-welding lines, clarifying the manufacturing process, which is similar in the two anchors. Thus, it is likely that both anchors belonged to the same ship and, hence, were in situ. This information extends the limited knowledge of technologies and materials used, specifically for the development of metallurgy in the Eastern Mediterranean during the early Islamic period, and enlarges the database of the typology of anchors of that period.

Electroless ternary Ni–W–P alloys containing micron size Al 2O 3 particles

In the present investigation electroless ternary NiWP–Al 2O 3 composite coatings were prepared using an electroless nickel bath. Second phase alumina particles (1 µm) were used to codeposit in the NiWP matrix. Nanocrystalline ternary NiWP alloys and composite coatings were obtained using an alkaline citrate based bath which was operated at pH 9 and temperature at 88 ± 2 °C. Mild steel was used as a substrate material and deposition was carried out for about 4 h to get a coating thickness of 25 ± 3 µm. Metallographic cross-sections were prepared to find out the coating thickness and also the uniform distribution of the aluminum oxide particles in NiWP matrix. Surface analysis carried out on both the coatings using scanning electron microscope (SEM) showed that particle incorporation in ternary NiWP matrix has increased the nodularity of composite coatings compared to fine nodular NiWP deposits. Elemental analysis of energy dispersive X-ray (EDX) results showed that codeposited P and W elements in plain NiWP deposit were 13 and 1.2 wt.%, respectively. There was a decrease in P content from 13 to 10 wt.% with a marginal variation in the incorporated W (1.01 wt.%) due to the codeposition of aluminum oxide particles in NiWP matrix. X-ray diffraction (XRD) studies carried out on as-plated deposits showed that both the deposits are X-ray amorphous with a grain size of around 3 nm. Phase transformation studies carried out on both the coatings showed that composite coatings exhibited better thermal stability compared to plain NiWP deposits. From the XRD studies it was found that metastable phases such as NiP and Ni 5P 2 present in the composite coatings heat treated at major exothermic peak temperature. Annealed composite coatings at various temperatures revealed higher microhardness values compared to plain NiWP deposits.

3D two scale X-FEM crack model with interfacial frictional contact: Application to fretting fatigue

A global approach coupling experiments and numerical modeling is proposed to enhance crack prediction. Fretting tests have been conducted under conditions leading to crack formation. Three dimensional crack shapes have been reconstructed from metallographic cross-sections and described with level set functions. They are then used as input data in 3D two-scale X-FEM model detailed in this paper. Frictional contact conditions holding between the crack faces are accounting for. The methodology developed to describe those contact conditions at the pertinent scale is detailed. A 3D fretting fatigue test is modeled to illustrate the coupling between experiments and numerical modeling.

Enhanced melt pool stirring in welding with dynamic polarised laser beam

The stirring of the molten material during CO2 laser welding with a dynamic polarised laser beam (DP) was studied. The key element of the set-up is a high power interference laser beam modulator for CW power rating up to 4 kW. Metallographic cross-sections and EDX analyses of the realised welds in St37 using the contrast material nickel proved the enhancement of melt pool stirring due to DP. Moreover, roughness measurements confirmed a smoothing of the top bead surface as well as a correlation between melt pool stirring and the formation of the top bead ripple structure.

Hermetic Sealing of Stainless Steel Packages by Seam Seal Welding

Welded stainless steel packages offer a number of advantages relative to those fabricated from kovar or aluminum metals and braze sealed. They are highly resistant to corrosion, especially in aqueous or elevated temperature environments, are mechanically stronger, dramatically so at temperatures above 100°C, exhibit lower outgassing rates, and have better vibration characteristics. Since welded stainless steel packages do not require over plating to facilitate braze sealing, combined material and fabrication costs are lower than kovar or aluminum packages. Also, unlike kovar, stainless steel is nonmagnetic, which is advantageous in many electronic applications. For sealing stainless steel packages, we elected to use seam sealing rather than laser welding for two reasons. Seam sealing subjects the package contents to lower thermal excursions than laser welding because less material is melted to achieve a weld in seam sealing. Second, seam sealing confines the molten material between the cover and package, where as laser welding produces a surface filet. Consequently, there is more opportunity for molten metal to splatter or react with the atmosphere in laser welding than in seam sealing. We successfully developed a process to weld a 2 millimeter thick cover onto a box with dimensions of 75 mm long by 50 mm wide by 15 mm deep using a seam sealer. The box walls were 1 mm thick and were penetrated by a dozen glass insulated feedthrus on one side. Both cover and box were fabricated from 316L stainless steel. A combination of analytical and finite element modeling were used in conjunction with a designed experiment to optimize the process variables of roller angle, speed and pressure, weld current, pulse shape, duration and spacing, number of weld passes, and sealing atmosphere. Weld quality and seal integrity were evaluated by leak testing before and after environmental stressing, mechanical testing and metallographic cross sectioning. The effects of component dimensions and tolerances on seal integrity were also investigated. Particular attention was paid to cover flatness, flange thickness, and tightness of fit between the cover and box. The process development was concluded by conducting a qualification experiment that used the optimized process parameters with controlled variation about their nominal values. A 100% yield of sealed boxes was obtained. These test articles were then subjected to various environmental screening tests, which were all passed with no failures.