Volume 50, Issue 10

Microstructural development, post-weld ageing response and mechanical properties of gas tungsten arc and CO 2 laser welded Al-6 %Cu (AA2219) alloy were investigated in the present study. The fusion zone of laser weld consists of three parts: the fine grains in the middle part, the columnar zone and equiaxed zone adjacent to fusion boundary. In contrast, fusion zone of gas tungsten arc weld consists of two parts: the columnar zone in the middle part and the equiaxed zone adjacent to fusion boundary. The size of the equiaxed grains in the fusion zone was the least in the laser welds when compared to gas tungsten arc welds. Laser welds have exhibited higher hardness, post weld ageing response, yield strength, ductility and fatigue strength when compared with gas tungsten arc welds. Fine equiaxed grain morphology and discontinues eutectic distribution in the laser weld fusion zone could be attributed to the improved tensile and fatigue properties of laser welded samples. ‘Thermal pinning effect’, due to non isothermal conditions, has arrested grain growth in heat affected zone in spite of temperatures close to the melting point of aluminum in heat affected zone.

The aim of this work was to develop a reproducible method of preparation for green and brown bodies which, after injection moulding and sintering, would permit an entire MIM component to be microscopically examined for internal inhomogeneities such as porosity and segregation. The particular challenge was to prepare the fragile MIM microstructure prior to the sintering process free of artefacts. Three different methods of preparation (variations in the polishing substrates and suspensions used) were compared. To evaluate the quality of the resulting preparation, the microscopically determined volume fraction of metal particles was chosen. This was determined after set preparation times using quantitative microstructural analysis (QMA/QGA) and evaluated during the various stages and times of the preparation process. The feedstock composition which had a defined volume fraction of 68 % was used as the reference value. The preparation of the brown sintered body proved to be the most successful. The volume fraction determined microscopically measured 67 %. The duroplastic infiltration mounting resin penetrates into the open pore structure effectively stabilising and fixing the powder particles. In contrast the preparation of the green body proved to be much more difficult as the breaking out of the metal powder particles from the comparatively soft thermo-plastic binder was not able to be prevented completely. The volume fraction measured microscopically was 62 % which is about 6 % less than the actual value. The chemical removal of the binder from the surface of the green body significantly improved the overall result of the preparation as it was then possible to effectively fix the powder particles by subsequent infiltration with the duroplastic mounting resin, as in the case of the brown body. In addition to the qualitative appraisal of the microstructural specimens, quantitative microstructural analysis proved to be a valuable instrument for systematically and objectively evaluating the results of the preparation and for developing the preparation processes. Using the methods of preparation developed it is now possible to carry out qualitative examinations on almost artefact free microstructural specimens prior to the lengthy and costly thermal processing.

This paper presents results of quantitative analysis of ferrite grains in low carbon steel C15V with microalloying addition of vanadium. Thermomechanical treatment in the intercritical (α+γ) region was performed using the dilatometer DIL805A. On the basis of dilatometric researches the critical temperatures of α ↔ γ transformation for C15V steel during heating with v 5−8 =0.5 K/s and cooling with v 8−5 =1 K/s were estimated. The steel deformation was carried out with application of two different variants: deformation in (α+γ) region after cooling from full austenitization temperature and deformation in (α+γ) region after heating from ambient temperature (ferritic-bainitic microstructure). The effects of these treatments were observed by means of a light optical microscope in bright and polarized field. The quantitative analysis of microstructure was investigated using Image Pro Plus (IPP) program. Characteristic parameters for ferrite grains such as: field area, average diameter, shape factor were estimated. For each of these parameters, histograms and maps of average grain diameter distribution were performed using STATISTICA 9 and IPP programs. It was observed that deformation in the intercritical range (α+γ) promotes the occurrence of many ferrite nucleus in the obtained structures, especially at grain boundaries. For low carbon steel this results in formation of very fine microstructure.

Cold-drawn and cold-bent seamless Alloy 617 pipes were received by a gas turbine engine manufacturer for final assembly. They exhibited multiple cracking on the surface. The extrados of bending zones were most affected by this condition. It was determined that the metallurgical root cause of the problem was inadequate heat treatment by the supplier of the semi-finished product. The cracking upon cold bending the seamless pipes was caused by embrittlement of the pipes’ surface due to nitridation and a further drop in formability as a result of grin boundary embrittlement by secondary carbide precipitates.