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American Society for Testing and Materials

Recent advances in friction-stir welding : Process, weldment structure and properties

The metallurgy of zinc-coated steel

Welding Metallurgy of

Wire-feed additive manufacturing (AM) is a promising alternative to traditional subtractive manufacturing for fabricating large expensive metal components with complex geometry. The current research focus on wire-feed AM is trying to produce complex-shaped functional metal components with good geometry accuracy, surface finish and material property to meet the demanding requirements from aerospace, automotive and rapid tooling industry. Wire-feed AM processes generally involve high residual stresses and distortions due to the excessive heat input and high deposition rate. The influences of process conditions, such as energy input, wire-feed rate, welding speed, deposition pattern and deposition sequences, etc., on thermal history and resultant residual stresses of AM-processed components needs to be further understood. In addition, poor accuracy and surface finish of the process limit the applications of wire-feed AM technology. In this paper, after an introduction of various wire-feed AM technologies and its characteristics, an in depth review of various process aspects of wire-feed AM, including quality and accuracy of wire-feed AM processed components, will be presented. The overall objective is to identify the current challenges for wire-feed AM as well as point out the future research direction.

Wire-feed additive manufacturing of metal components: technologies, developments and future interests

A study was made of the effects of carbon content, strain, initial grain size, carbide morphology, annealing time, and temperature on the final grain size produced after the decarburization anneal. The results show that columnar grains form only in higher carbon (0.05/0.06 Pct C) steels with low initial grain sizes (<20 µm) when decarburized at 1450 °F (788 °C). Equiaxed grains, however, are obtained in low carbon (0.02/0.03 Pct C) lamination steels for all processing conditions. It was found that the formation of both equiaxed and columnar final grain size (GS
 f
 ) was dependent upon percent strain (e), initial grain size (GS
 i
 ), and carbide distribution (D
 c
 ) according to the following regression equations (all grain size measurements in µm):G5
 f
 (columnar grain) = 204 + 0.9GSi - 12.3e + 37.8D
 C
 logGS
 f
 (equiaxed grain) = 2.151 + 0.022GSi - 0.038e - 0.0005GS
 i
 · e- 0.0002(GS
 i
 )2 + 0.079D
 c

Factors affecting the final grain size of decarburized lamination steels

Iron-aluminum-based weld cladding is currently being considered as corrosion-resistant coatings for boiler tubes in coal-fired power plants. Although these alloys could potentially be good coating candidates due to their excellent high-temperature corrosion resistance, Fe-Al weld cladding is susceptible to cracking due to hydrogen embrittlement at elevated aluminum concentrations. Additions of chromium to these iron-aluminum alloys have been shown to improve the corrosion resistance of the alloys and could potentially increase the lifetimes of the coatings. The current study investigated the effect of chromium on the hydrogen cracking susceptibility of Fe-Al weld cladding.

The effect of chromium on the weldability and microstructure of Fe-Cr-Al weld cladding

The mechanisms for nodular corrosion-product development were investigatedin various high-temperature gaseous environments. Fe–Al alloys, with5–20 wt.% Al, were exposed in both oxidizing and sulfidizing[p(S2)=10−4 atm, p(O2)=10−25 atm] atmospheres at 700°Cfor times up to 100 hr. The corrosion kinetics were monitored by theuse of a thermogravimetric balance and the morphological developmentthrough light-optical and scanning-electron microscopies,energy-dispersive spectroscopy, electron-probe microanalysis,and quantitative-image analysis. Under both conditions, theelimination of nodule formation was observed by increasing thealuminum content of the alloy, above 5 and 7.5 wt.% Al for oxidizingand sulfidizing environments, respectively, which promoted the growthand maintenance of a continuous surface scale of alumina. For thosealloys that were observed to develop nodular corrosion products, theirmorphological appearance was similar in nature regardless of thecorroding species. The nodules typically consisted of an outeriron-rich product, either sulfide or oxide, that was randomly dispersedacross an alumina scale. Samples from the oxidizing atmosphere displayeda single growth-rate time constant from the kinetics data, suggesting thatthe nodule growth mechanism was by the simultaneous or codevelopment oftwo different (Fe and Al) oxides from the onset of exposure. Measurementof nodule planar diameter and depth of penetration into the alloyindicated that growth occurred through diffusional processes. Kineticsdata from the development of sulfide nodules in the reducingenvironment revealed a different type of mechanism. Multiplegrowth-rate time constants were found due to the localized mechanicalfailure of an initially formed surface scale. At early times in thesulfidizing atmosphere, a low corrosion rate was recorded as acontinuous-alumina scale afforded protection from excessive productdevelopment. However, with the mechanical failure of the scale, sulfurwas able to attack the underlying substrate through a short-circuitdiffusion mechanism that resulted in rapid weight gains from nonprotective,iron sulfide growth. The sulfide morphologies observed were very complex ascontinued growth of the nodule did not solely depend upon the diffusingspecies through the previously formed corrosion products, but also,continued mechanical failure of the oxide scale. It is suggested that thedifference in development mechanisms between the two environments may liein the relative growth rates of the nonprotective, Fe-base corrosionproducts formed.

Growth of Nodular Corrosion Products on Fe-Al Alloys in Various High-Temperature Gaseous Environments

The J-integral fracture toughness and tensile behavior of AL-6XN (ATI Properties Inc., Pittsburgh, PA, USA) plate material in the short-transverse (S-T) orientation was studied. The material was tested with respect to the presence or absence of microstructural ‘packets’ of brittle sigma phase (σ-phase) particles within the austenite matrix, located near the centerline of the plate. The J-integral-fracture resistance curve (J-R-curve) qualitatively indicated that the presence of the σ-phase packets is a detriment to the fracture toughness of the material in the S-T orientation. Tensile specimens containing the packets of σ-phase particles exhibited reduced yield and tensile strengths as well as pronounced reduction in ductility compared to specimens nominally free of σ-phase packets. Particle cracking was observed within the σ-phase packets, which lead to premature fracture. This limited the ability of the material to plastically deform and work-harden, thereby accounting for the observed reductions in ductility, toughness, and ultimate tensile strength.

The Influence of Centerline Sigma (σ) Phase on the Through-Thickness Toughness and Tensile Properties of Alloy AL-6XN

The influence of microstructure on the fatigue crack propagation behavior of gas metal arc welds in 316L and AL6XN austenitic stainless steels has been investigated. A constant AK (stress intensity range) testing procedure with a stress ratio value of 0.6 was first used to deconvolute stress intensity range and residual stress effects from microstructural effects as the fatigue crack propagated from the base metal into the weld metal. The results of this test demonstrated that the large grain size of the weld metal produced a rough fracture surface with improved fatigue resistance relative to the base metal. The influence of grain size on fatigue resistance was then studied in more detail by generating full fatigue curves over a wide range of AK on base metal samples that were heat treated to obtain various uniform grain sizes. Results from fatigue tests conducted on the base metal control samples were consistent with the weld metal results and showed that large grain sizes produced relatively rough fracture surfaces with improved fatigue resistance. The improved fatigue resistance occurred predominately at low stress intensity ranges where the plastic zone size is approximately equal to or less than the grain size. The improved fatigue resistance with increasing grain size was attributed to three main factors, including 1) a tortuous crack path that requires formation of a large surface area for a given length of crack propagation, 2) crack growth out of the Mode I plane, which reduces the stress intensity range available for crack growth, and 3) roughness-induced closure that shields the crack from part of the applied load. Direct crack closure measurements were used to identify the range of ΔK levels where the third factor was operable. Quantitative estimates of the AK level below which grain size effects are expected to occur are in reasonable agreement with the experimental results.

Arnold R. Marder

Papers

Factors affecting the final grain size of decarburized lamination steels

The effect of chromium on the weldability and microstructure of Fe-Cr-Al weld cladding

Growth of Nodular Corrosion Products on Fe-Al Alloys in Various High-Temperature Gaseous Environments

The Influence of Centerline Sigma (σ) Phase on the Through-Thickness Toughness and Tensile Properties of Alloy AL-6XN

The influence of microstructure on fatigue crack propagation behavior of stainless steel welds