Pencegahan Terjadinya Retak Panas Pada Proses Pengecoran Squeeze Benda Tipis Al-Si

Solidification of molten metal in squeeze casting was done under high pressure condition. It will produce small grains and decrease porosity of product but have high probability of hot crack. Hot crack depend on silicon content, molding and pouring temperature of squeeze cast parameters. The aim of this research is to analize silicon content, melt temperature and mold temperature on hot crack to eliminate this defect on production of thin wall of Al-Si. Hydraulic pressure of 135 MPa is applied to forge molten metal of aluminum-silicon alloys. Mold temperature from 220 to 330 0C, pouring temperature from 665 to 885 0C and silicon content from 0.45 to 6.04 % weight were considered. Hot crack length and cracking index were used to indicate the dimension of hot crack. The increasing of silicon content decreases hot crack length and cracking index of thin wall. The increasing of pouring and mold temperature increases hot crack length and cracking index of thin wall. Combination of the higher silicon content, the lowest melt and mold temperature produced the flawless thin wall squeeze cast of hot crack

Evaluation of Cyclic Behavior of Aircraft Turbine Disk Alloys

An evaluation of the cyclic behavior of three aircraft engine turbine disk materials was conducted to compare their relative crack initiation and crack propagation resistance. The disk alloys investigated were Inconel 718, hot isostatically pressed and forged powder metallurgy Rene '95, and as-hot-isostatically pressed Rene '95. The objective was to compare the hot isostatically pressed powder metallurgy alloy forms with conventionally processed superalloys as represented by Inconel 718. Cyclic behavior was evaluated at 650 C both under continuously cycling and a fifteen minute tensile hold time cycle to simulate engine conditions. Analysis of the test data were made to evaluate the strain range partitioning and energy exhaustion concepts for predicting hold time effects on low cycle fatigue

Life assessment of combustor liner using unified constitutive models

Hot section components of gas turbine engines are subject to severe thermomechanical loads during each mission cycle. Inelastic deformation can be induced in localized regions leading to eventual fatigue cracking. Assessment of durability requires reasonably accurate calculation of the structural response at the critical location for crack initiation. In recent years nonlinear finite element computer codes have become available for calculating inelastic structural response under cyclic loading. NASA-Lewis sponsored the development of unified constitutive material models and their implementation in nonlinear finite element computer codes for the structural analysis of hot section components. These unified models were evaluated with regard to their effect on the life prediction of a hot section component. The component considered was a gas turbine engine combustor liner. A typical engine mission cycle was used for the thermal and structural analyses. The analyses were performed on a CRAY computer using the MARC finite element code. The results were compared with laboratory test results, in terms of crack initiation lives

Nanoparticle-enabled phase control for arc welding of unweldable aluminum alloy 7075.

Lightweight materials are of paramount importance to reduce energy consumption and emissions in today's society. For materials to qualify for widespread use in lightweight structural assembly, they must be weldable or joinable, which has been a long-standing issue for high strength aluminum alloys, such as 7075 (AA7075) due to their hot crack susceptibility during fusion welding. Here, we show that AA7075 can be safely arc welded without hot cracks by introducing nanoparticle-enabled phase control during welding. Joints welded with an AA7075 filler rod containing TiC nanoparticles not only exhibit fine globular grains and a modified secondary phase, both which intrinsically eliminate the materials hot crack susceptibility, but moreover show exceptional tensile strength in both as-welded and post-weld heat-treated conditions. This rather simple twist to the filler material of a fusion weld could be generally applied to a wide range of hot crack susceptible materials

Bending strength studies on hot-pressed silicon carbide

The 4-point bending strength of 4 grades of hot-pressed SiC was determined at different temperatures. With a transgranular mode of fracture the values for bending strength are retained up to high temperatures. For intergranular fracture the decrease of strength is governed by subcritical crack growth. The intergranular fracture is caused by a high content of silicate glassy phase at the grain boundaries of hot-pressed SiC

Elevated temperature crack growth

Critical gas turbine engine hot section components such as blades, vanes, and combustor liners tend to develop minute cracks during early stages of operations. The ability of currently available path-independent (P-I) integrals to correlate fatigue crack propagation under conditions that simulate the turbojet engine combustor liner environment was determined. To date, an appropriate specimen design and a crack displacement measurement method were determined. Alloy 718 was selected as the analog material based on its ability to simulate high temperature behavior at lower temperatures in order to facilitate experimental measurements. Available P-I integrals were reviewed and the best approaches are being programmed into a finite element post processor for eventual comparison with experimental data. The experimental data will include cyclic crack growth tests under thermomechanical conditions, and, additionally, thermal gradients

A study on surface cracking in extrusion of aluminium alloy AA2014

Surface cracking is generally recognised as one of the main defects occurring during the process of aluminium extrusion, especially in the case of the so called hard aluminium alloys. Previous experiments suggest that this type of defect is caused by the rise in temperature as the process proceeds. Some experiments indicate that the surface quality is good even though the temperature may be high during extrusion. It is also well known that crack criteria have been adopted to explain the cracking that occurs in extrusion, blanking and rolling, etc. In this study, a finite element method (FEM) is used in different ways to predict surface cracking during hot extrusion. The crack criteria are integrated into the FEM code FORGE12.0. The effectiveness of these criteria in predicting surface cracking in the case of hot extrusion is discussed. The FEM simulation also provides some other quantitative data, such as the temperature rise during extrusion from different initial temperatures. In addition, the principal stresses at the die land area at different extrusion stages are also shown

Fatigue and fracture: Overview

A brief overview of the status of the fatigue and fracture programs is given. The programs involve the development of appropriate analytic material behavior models for cyclic stress-strain-temperature-time/cyclic crack initiation, and cyclic crack propagation. The underlying thrust of these programs is the development and verification of workable engineering methods for the calculation, in advance of service, of the local cyclic stress-strain response at the critical life governing location in hot section compounds, and the resultant crack initiation and crack growth lifetimes

Dynamical Instabilities of Quasi-static Crack Propagation Under Thermal Stress

We address the theory of quasi-static crack propagation in a strip of glass that is pulled from a hot oven towards a cold bath. This problem had been carefully studied in a number of experiments that offer a wealth of data to challenge the theory. We improve upon previous theoretical treatments in a number of ways. First, we offer a technical improvement of the discussion of the instability towards the creation of a straight crack. This improvement consists of employing Pad\'e approximants to solve the relevant Weiner-Hopf factorization problem that is associated with this transition. Next we improve the discussion of the onset of oscillatory instability towards an undulating crack. We offer a novel way of considering the problem as a sum of solutions of a finite strip without a crack and an infinite medium with a crack. This allows us to present a closed form solution of the stress intensity factors in the vicinity of the oscillatory instability. Most importantly we develop a {\em dynamical} description of the actual trajectory in the regime of oscillatory crack. This theory is based on the dynamical law for crack propagation proposed by Hodgdon and Sethna. We show that this dynamical law results in a solution of the actual track trajectory in post critical conditions; we can compute from first principles the critical value of the control parameters, and the characteristics of the solution like the wavelength of the oscillations. We present detailed comparison with experimental measurements without any free parameter. The comparison appears quite excellent. Lastly we show that the dynamical law can be translated to an equation for the amplitude of the oscillatory crack; this equation predicts correctly the scaling exponents observed in experiments

Life prediction and constitutive behavior

One of the primary drivers that prompted the initiation of the hot section technology (HOST) program was the recognized need for improved cyclic durability of costly hot section components. All too frequently, fatigue in one form or another was directly responsible for the less than desired durability, and prospects for the future weren't going to improve unless a significant effort was mounted to increase our knowledge and understanding of the elements governing cyclic crack initiation and propagation lifetime. Certainly one of the important factors is the ability to perform accurate structural stress-strain analyses on a routine basis to determine the magnitudes of the localized stresses and strains since it is these localized conditions that govern the initiation and crack growth processes. Developing the ability to more accurately predict crack initiation lifetimes and cyclic crack growth rates for the complex loading conditions found in turbine engine hot sections is of course the ultimate goal of the life prediction research efforts. It has been found convenient to divide the research efforts into those dealing with nominally isotropic and anisotropic alloys; the latter for application to directionally solidified and single crystal turbine blades