Entrainment Defects

Most metals start their lives in the liquid state, and are subjected to various transfers involving pouring or other types of surface turbulence. These actions entrain (fold in) the surface film to create entrainment defects. These are principally (a) bubbles that in turn create bubble trails; (b) bifilms; doubled over surface films, that act as cracklike defects; and (c) sundry entrained debris, collectively known as exogenous inclusions. The bifilm is the subject of this perspective. It appears to be a common, but serious and almost overlooked metallurgical defect. Analysis of bifilms provides a simple, powerful and elegant concept based on an enduring legacy from turbulence during the pouring of liquid metals. Usually large populations of bifilms are introduced into metals at an early stage of their production. In general their presence has been unsuspected because although they can have large area, they can often be only nanometres thick and not easily detected by conventional non-destructive techniques. The populations of cracks in suspension in liquid metal explains many otherwise inexplicable features of cast products such as porosity, hot tearing, the morphologies of second phases, and impaired reliability of mechanical properties. The fundamental difference between such entrained defects (associated with a macroscopic unbonded interface) and defects and inclusions grown in the melt is seen to be of central significance for the failures of metals by mechanical or corrosion type mechanisms. For wrought products the continued presence of bifilms, now usually extended and elongated and mainly occupying grain boundaries, appears to offer explanations for many metallurgical phenomena. Bifilms are likely to influence the development of texture, and are the most likely source for many types of failure in the solid state. Thus the limitations to superplastic forming, cavitation in tertiary creep, pitting corrosion of various types and stress corrosion cracking are likely to be profoundly affected by bifilms. Although the effects of bifilms can be reduced by expensive post-casting operations such as hipping or working, the major future potential lies in techniques for their avoidance. Some casting operations are already taking some first steps in new technology for their avoidance, and benefiting technically and commercially.

Effect of laser parameters on microstructure, metallurgical defects and property of AlSi10Mg printed by selective laser melting

AlSi10Mg alloy has been widely used in selective laser melting (SLM). However, the formation of metallurgical defects in this material during SLM process has not been studied sufficiently. In this work, different laser parameters were adopted to fabricate the specimens. The effects of volumetric energy density (VED) on the metallurgical defect, densification, phase composition and mechanical property were also comprehensively analyzed. At low VED of 37.39[Formula: see text]J/mm3, a nearly full dense sample with density of 2.602[Formula: see text]g/cm3 can be printed. The sample with maximal tensile strength of 475[Formula: see text]MPa can be printed. While with the increase of VED, the ultimate tensile strength decreases due to the formation of micro-pores. The formation mechanisms of micro-pores including gas pores and keyhole-induced pores were disclosed from the angle of alloy smelting. Better understanding of the influence mechanisms of the laser parameters on the formation of metallurgical defects is beneficial for the production of high performance SLM parts.

Metallurgical Defect and Hydrogen Embrittlement Failure of a Gear

Circumferential cracking occurred on the spoke plate of a statically placed marine gear. Fractographic investigation indicates that two independent crack origins are present on the fracture surface. Intergranular fracture mode was mostly found in slow crack propagation region and the intergranular facets associated with ductile shearing. Metallographic observation reveals that a type of macro casting defect, spot-segregation, is generally present on the gear material. Large size macro spot-segregation defect zones just appear in two crack origin regions. Appearance of the large spot-segregation zones in the core of spoke plate supplies the favorable metallurgical condition for hydrogen-induced fracture: presence of excessive impurities of S, P and then appearance of abnormal microstructure of upper-bainite susceptible to hydrogen embrittlement. From the evidences it is concluded that the gear failed due to the hydrogen embrittlement. The possible source for hydrogen was introduced during melting and casting operating.

On the use of Positron Annihilation Spectroscopy in the Study of a Nonuniform Spatial Distribution of Metallurgical Defects

ABSTRACTMost of the Interpretation of positron annihilation results to date have been done within the context of the simple trapping model which assumes a uniform spatial distribution of defects within the specimen examined. While this assumption is often justifiable there are numerous metallurgical defect configurations which are markedly nonuniform. In this work some of the appropriate trapping models which have previously appeared in the literature will be briefly reviewed. Additional models which are currently being developed will be presented and the series of experiments which are being used to test the trapping models will be described.