Ru Peng

Strain and texture analysis of coatings using high-energy x-rays

We investigate the internal strain and crystallographic orientation (texture) in physical-vapor deposited metal nitride coatings of TiN and CrN. A high-energy diffraction technique is presented tha ...

Modeling subsurface deformation induced by machining of Inconel 718

ABSTRACT Traditionally, the development and optimization of the machining process with regards to the subsurface deformation are done through experimental method which is often expensive and time consuming. This article presents the development of a finite element model based on an updated Lagrangian formulation. The numerical model is able to predict the depth of subsurface deformation induced in the high- speed machining of Inconel 718 by use of a whisker-reinforced ceramic tool. The effect that the different cutting parameters and tool microgeometries has on subsurface deformation will be investigated both numerically and experimentally. This research article also addresses the temperature distribution in the workpiece and the connection it could have on the wear of the cutting tool. The correlation of the numerical and experimental investigations for the subsurface deformation has been measured by the use of the coefficient of determination, R2. This confirms that the finite element model developed here is able to simulate this type of machining process with sufficient accuracy.

Residual Stresses in High Speed Turning of Nickel-Based Superalloy*

Abstract Residual stresses induced by machining operations can be critical for the performance and lifetime of the machined components. This paper investigated the influence of cutting conditions, including the use of cutting fluid, cutting speed and tool flank wear, on the residual stresses in high speed turning of Inconel 718 alloy. X-ray diffraction measurements revealed characteristic residual stress distributions with tensile stresses in a thin surface layer and compressive stresses in a much thicker subsurface layer in all investigated samples. However, the magnitude of the surface tensile stresses and subsurface compressive stresses as well as the size of the tension and compression zone depended on the cutting parameters. Surface tensile residual stresses over 1600 MPa and 1400 MPa were induced by tool flank wear and increased cutting speed, respectively, in dry turning. The effect of cutting induced local plasticity and temperature increase on the observed residual stress distributions was analyzed based on microstructural study by electron channeling contrast imaging and electron backscatter diffraction.

Residual Stresses in Shot Peened Grey and Compact Iron

Abstract Grey cast iron and compacted graphite iron with a pearlitic matrix are investigated in this study after shot peening using twelve unique combinations of parameters, namely shot size, peening intensity and coverage, followed by residual stress measurements and evaluations. Cylindrical test samples were cut out from heavy truck cylinder heads and polished on the top flat surface to decrease effects from cutting. Residual stresses and the affected depth from the different peening conditions varied between −245 MPa to −565 MPa and from 280 μm to 770 μm in depth. Resultant surface compressive stresses decrease with increasing shot size, peening intensity or coverage whereas the affected depth increases with increasing intensity. The increased affected depth is a result from the increased extent and magnitude of plastic deformation. The compacted graphite iron was more affected by shot peening than the grey cast iron, meaning that the same shot peening parameters resulted in both higher compressive stresses and larger deformation depth.

Residual Stresses in Uniaxial Cyclic Loaded Pearlitic Lamellar Graphite Iron

The mechanisms behind residual stress generation have been a topic of interest for quite some time since it is well-known that residual stresses can benefit the fatigue life of components. We have studied the residual stresses in lamellar graphite iron generated by fatigue damage. Cylindrical test specimens, with close to zero residual stresses of fully pearlitic lamellar graphite iron, manufactured and subjected to uniaxial load controlled cyclic loading, have been investigated. The load conditions used were: pure tension, pure compression, and alternating tension/compression over one thousand cycles. Measurements were performed using a four-circle goniometer Seifert X-ray machine equipped with a linear sensitive detector and a Cr-tube. Evaluation of the residual stresses were conducted using the sin2-method on the -Fe {211} diffraction peak together with material removal technique to obtain depth profiles. Introduction It is well known that compressive residual stresses (RS) at the surface of a specimen prolongs the fatigue life. The associated increase in strain hardening of the surface, can be equally important for the specimen fatigue life since the combination of strain and compressive RS at the surface of the sample inhibits crack nucleation and propagation. Steels, aluminium, titanium, and nickel alloys are some of the metallic materials in which work hardening at the surface can result in compressive RS and increased fatigue strength of the material [1,2]. These homogenous metallic material withstand considerable plastic deformation before final fracture compared to cast iron, especially lamellar graphite iron (LGI) which has an inhomogeneous microstructure. Lamellar graphite iron plasticises already at tensile loads ~ 40 MPa [3]. Graphite acts as notches all over the specimen volume and is the reason for its poor tensile strength. Relaxation of near surface RS due to cyclic loading are a well-known phenomenon. Local plastic deformations (microcracks) as well as pinning and un-pinning of dislocations are believed to be a source of stress relaxation [2,4–6]. Residual stress relaxation mechanisms due to cyclic loading are affected by the initial magnitude of the RS, the gradient of RS, degree of cold work, cyclic loading and material response to cyclic loading [1]. In a multiphase material, such as pearlitic LGI, the different response of the phases to cyclic loading can render a measurable shift in RS in one of the phases. In this study, we have three phases (ferrite, cementite, lamellar graphite) but only the stresses in the ferritic phase, which has the largest volume fraction in the material, are measured with laboratory X-rays, since diffraction peaks for the other phases can not be obtained. In multiphase materials, such as duplex steels, the different phases are known to have different amounts of RS and often also different signs on the stresses [7–9]. During cooling and solidification, the differences in volume contraction between phases gives a thermal mismatch, resulting in RS. The different thermal coefficients of the phases in cast iron (ferrite, cementite and graphite) are the source of RS in untreated material. Surface treatments and cyclic loading also result in plasticity mismatch between the different phases, causing large variations in RS between the phases which can also have Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 67-72 doi: http://dx.doi.org/10.21741/9781945291173-12 68 different signs. The effects of these RS in a LGI under cyclic loading have not been thoroughly investigated. In this paper, the RS generated in a pearlitic LGI under cyclic loading have been investigated to fill in the knowledge gap in RS progression in cast iron. Experimental setup Four cylindrical specimens, with a 6.3 mm diameter of over the 25 mm gauge length, of pearlitic LGI were manufactured. The specimens were manually ground using 2400 and 4000 grit SiC paper to lower the machining effects. Afterwards, the specimens were stress relieve annealed at 600 °C for one hour and cooled slowly in air to room temperature. Surface oxides were gently removed manually with 4000 grit SiC paper. The gauge length was electrolytic polished in A2-solution to achieve a stress free surface. Uniaxial testing was performed on three test samples in an Intron 8801 servo hydraulic test machine. The fourth sample was used as reference. A sinusoidal load cycle was used in pure tension (between 20 MPa and 200 MPa), pure compression (between -20 MPa to -200 MPa), and alternating tension/compression (± 150 MPa, starting in tension). The material ultimate tensile strength is 250 MPa. Each specimen was run for 1000 cycles. X-ray measurements were performed using a four-circle goniometer Seifert X-ray machine, equipped with a linear sensitive detector and a Cr-tube. Evaluation of RS were conducted using the sin2-method [10] with the -Fe {211} diffraction peak, at 2θ ≈ 156.5°. A ø 1 mm collimator was used to minimize the effects of specimen curvature. Diffraction data was obtained from four different measuring points, A‒D, ~ 90° rotation of the specimen between each point. At every depth all diffraction data was superimposed to obtain a good average value of the RS. Peak position was calculated using a double pseudo-Voigt curve fit. Electron backscatter diffraction (EBSD) for grain size determination was performed using an OXFORD detector in a Hitatchi SU-70 field emission gun scanning electron microscope (FEGSEM). The specimen was tilted to 70° with a working distance of 20 mm, acceleration voltage of 15 kV and a step size of 0.75 μm were used. The HKL software Channel 5 was used to evaluate the EBSD measurements. The angle mismatch between two neighbouring points of 10° or higher was set as definition of a grain boundary. Results The biaxial stresses for the four samples were calculated from the superimposed diffraction data (SDD), results depicted in Fig. 1. No significant differences can be observed between the samples. Figure 1: SDD of the four different specimens investigated. Residual stress fluctuations and standard deviation for the reference sample are shown in Fig. 2 (a). Small tensile residual stresses for the pure tensile loaded specimen are shown in Fig. 2 (b) at some of the measured positions. The RS in the compression and tension/compression specimens in Fig. 2 (c) and (d) only show three measuring points where the RS are tensile. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 67-72 doi: http://dx.doi.org/10.21741/9781945291173-12 69 Figure 2: Residual stress profile for the four points in the (a) reference-, (b) pure tension-, (c) pure compression-, and (d) tension/compressionspecimen. All samples show an increase in Full Width at Half Maximum (FWHM) for SDD with increasing depth. No significant changes between the test specimens can be seen in Fig. 3 when comparing the average FWHM for SDD. A B C D 0 50 100 150 200 250 300 350 60 40 20 0 20 (a) 0 50 100 150 200 250 300 350 60 40 20 0 20 (b) 0 50 100 150 200 250 300 350 60 40 20 0 20 (c) 0 50 100 150 200 250 300 350 60 40 20 0 20 Depth [μm] (d) St re ss [M Pa ]

Effect of Cooling and Shot Peening on Residual Stresses and Fatigue Performance of Milled Inconel 718

The present study highlights the effect of cooling and post-machining surface treatment of shot peening on the residual stresses and corresponding fatigue life of milled superalloy Inconel 718. It ...

Bending Fatigue Behavior of Blast Cleaned Grey Cast Iron

This paper presents a detailed study on the effect of an industrial blast cleaning process on the fatigue behavior of a grey cast iron with regard to the residual stresses and microstructural chang ...

Tensile failure of thin aluminium sheet observed by in-situ EBSD

Tensile tests on two similar 75-μm-thick aluminium sheet materials were carried out inside a scanning electron microscope equipped with an electron backscatter detector. The materials were subjected to simulated brazing prior to the test because this type of material is used for fins in automotive heat exchangers. Grain sizes were large relative to sheet thickness and ND-rotated cube and P texture components dominated the recrystallization textures; their volume fractions differed strongly in the two different materials, though. Strains over the microscope image fields were determined from positions of constituent particles or from grain sizes; the two methods gave consistent results. Grains with high Schmid factors accumulated significantly more deformation than grains with low Schmid factors. Cracks nucleated in high-Schmid factor grains, or in groups of such grains, at the specimen edges. When only low-Schmid factor grains were present at the specimen edges, the crack nucleated inside the specimen. The subsequent crack growth was intragranular and occurred at approximately 90° relative to the load direction.

Shot Peening Induced Plastic Deformation in Cast Iron – Influence of Graphite Morphology*

Abstract Graphite morphology in cast iron strongly affects the mechanical properties such as: Youngs modulus, tensile/compressive strength and cyclic mechanical behaviour. In this study, pearlitic compact graphite iron (CGI) and grey cast iron (GI) have been studied in a scanning electron microscope (SEM) before and after shot peening under different conditions. The plastic deformation behaviour of CGI and GI under different shot peening conditions has been analysed using electron backscattering diffraction (EBSD) and electron contrast channelling imaging (ECCI). EBSD and ECCI revealed that plastic deformation around graphite inclusions depends on the size and shape of the graphite. The different response of CGI and GI to shot peening is explained by the different damping properties of the graphite and the matrix capability for plastic deformation. n