Ali H. Ammouri

FEM optimization of process parameters and in-process cooling in the friction stir processing of magnesium alloy AZ31B

Controlling the temperature in friction stir processing (FSP) of Magnesium alloy AZ31b is crucial given its low melting point and surface deformability. A numerical FEM study is presented in this paper where a thermo-mechanical-based model is used for optimizing the process parameters, including active in-process cooling, in FSP. This model is simulated using a solid mechanics FEM solver capable of analyzing the three dimensional flow and of estimating the state variables associated with materials processing. Such processing (input) parameters of the FSP as spindle rotational speed, travel speed, and cooling rate are optimized to minimize the heat affected zone, while maintaining reasonable travel speeds and producing uniformity of the desired grain size distribution of the microstructure in the stirred zone. The simulation results predict that such optimized parameters will result in submicron grain sized structure in the stirred zone and at the corresponding stirred surface. These simulation predictions were verified using published experimental data.Copyright

Developing a Carbon Footprint Calculator for Construction Buildings

Carbon footprint is commonly defined as the total amount of greenhouse gases produced directly or indirectly as a result of an activity. The term carbon footprint has become the standard for measuring the environmental impact of activities in several sectors (e.g., transportation, energy, construction). While there have been several studies documenting calculators that estimate the carbon footprint of individual activities (e.g., driving a car, riding an airplane), the literature describing the process of carbon footprint calculations for construction activities remains limited. The few existing tools that calculate the carbon footprint of construction buildings do not take into account some of the major variables in the design and construction process (e.g., properties of selected materials, location of suppliers). In an effort to improve the accuracy of carbon footprint calculations, this paper presents a tool that estimates the total carbon footprint of construction buildings while taking into consideration project characteristics (e.g., size, location, material choices). The calculator relies on data collected from construction material suppliers and covers the various phases of a construction project. Through a case study, the research team illustrates the use of the tool to identify the activities with high carbon emissions.

Experimentally Validated Thermo-Mechanically Coupled FE Simulations of Al/Mg Friction Stir Welded Joints

Numerical simulations of the friction stir welding of dissimilar metal joints is a daunting task given the complex issues involved such as the flow mixing action and the phase transformations. In this work, a 3D thermo-mechanical FE model is developed to simulate the dissimilar friction stir welding (DFSW) of aluminum-magnesium bi-metallic joints. The model is built using a manufacturing-processing-specific FEM software package (DEFORM 3D). Suitable constitutive laws are implemented to describe flow stress for both welded constituents: Al and Mg. The flow patterns of the stirring action from the simulations were verified against flow patterns of steel shots reported from experiments published in the literature. Also, the simulated interface patterns were found to be in agreement with microscopic images of welded sections taken from reported experiments. Furthermore, simulated temperature profiles favorably compare with temperature measurements previously published in the literature. The numerical model output includes relevant results such as material flow and volume fractions throughout the joint but most importantly in the recrystallized stir zone.Copyright

FEM analysis of the effects of cooling techniques on the microstructure of aluminum 7075 friction stir welded joints

Friction stir welding (FSW) is a proven solid-state technique for joining metal alloys. Given the low melting temperatures of light alloys, excessive heat build-up in such joints may have undesirable consequences such as melting and/or undesired grain growth. It is widely recognized that the resulting mechanical properties of a welded joint depends to a great extent on microstructure development.The aim of this paper is to improve the microstructure of friction stir welded aluminum alloy joints by utilizing two different cooling techniques. To this end, a 3D FEM model is developed to simulate the friction stir welding plunging and advancing phases. The parameters used in the FEM model were optimized for minimum simulation time and resulting in accurate simulations as compared with experimental results previously published by other workers. The work material was modeled as a visco-plastic material and dynamic recrystallization was implemented and added to the material model. Two main cooling techniques were compared: temperature controlled backing plate and another via cryogenic CO2 direct nozzle. The monitored output parameters were: temperature, stress, strain, and strain rate. Consequently, values of the Zener-Hollomon parameter, Z, were calculated and the resulting grain size distribution in the joint was found. Due to dynamic recrystallization, nano-sized grains were predicted to be generated in the cryogenically cooled weld line when compared to non-cooled one.Copyright

Carbon Footprint Calculator for Construction Projects (CFCCP)

A carbon footprint is defined as ‘The total set of greenhouse gas emissions caused directly and indirectly by an individual, event, organization or product expressed as mainly CO2 emissions [1]. Carbon footprints and their calculations have recently drawn considerable attention in order to limit Greenhouse Gas Emissions. Several carbon footprint calculators are available online to estimate the carbon dioxide emissions of individuals and households based on house, auto, and other consumption-related measures. On the other hand, there are a limited number of calculators that address construction projects. The importance of developing carbon calculators for construction projects is due to the fact that 13- 18% of the total embodied carbon footprint of any construction project [2] and 100% of the total embodied carbon footprint of any landscape project is released the year the project is built or installed.

A Numerical Model for Predicting the Zener-Hollomon Parameter in the Friction Stir Processing of AZ31B

Grain size determines to a large degree the mechanical properties of the friction stir processed (FSP) material. Developed in this work is a numerical (FEM) based-model for predicting values of the Zener-Hollomon parameter (Z-parameter) as function of input process parameters during friction stir processing of AZ31B. Prediction of Z values is desirable given that direct relations exist between the Z-parameter and the average grain size in the dynamically recrystallized zone (DRX). For this purpose, utilized in this work is a robust finite element model with a suitable constitutive equation and boundary conditions the results of which have been previously validated against published experimental data. A virtual test matrix constituting of 16 cases (4 spindle speed, N, x 4 feed, f) was run. Based on resulting state variables of strain rates and temperatures at a representative point within the stir zone, a statistically-validated power equation model was developed that relates Z-parameter values to input parameters of speed and feed. The results of the numerically developed power equation were validated against experimental results. This model can be readily used in future control frameworks to FSP produce AZ31B sheets of a predefined target grain size.

Indirect Tool-Wear Maps for Tool Condition Monitoring in Dry Metal Drilling Operations

To avoid damage to work and/or machine, real-time tool condition monitoring is necessary in automatic and sustainable manufacturing operations. In particular, metal machining with NC machine tools can benefit handsomely from the identification of dull tools in real-time so that they can be replaced. This requirement is especially true in dry (sustainable) drilling operations where heat buildup represents a major challenge.

Current Rise Index (CRI) Maps of Machine Tool Motors for Tool-Wear Prognostic

Quantitative polar maps of tool-wear of cutting tools (here, chisel drills) undergoing dry machining are charted based only on transducers reporting electrical current measurements from machine (spindle and feed drives) motors. Associated with these maps are qualitative descriptions of the various modes of tool-wear afflicting the drill tools. These tool-wear maps are based a novel wear criterion developed here that relies on the % increase in motor (spindle and feed drive motors) RMS current values and is dubbed the Current Rise Index (CRI). For verification in a drilling operation application, this index is found to positively track the progressive increase in tool-wear. Utilizing this CRI and the associated polar plots, monitoring of cutting tools may be achieved simply by machine tool operator via visual monitoring of polar CRI maps generated in real-time. Naturally, such plots lend themselves to automated prognosis by common control techniques utilized in machine tool operations. Such maps may also serve as indirect means of predicting tool-wear in automated cutting operations.Copyright

Pre-cooling Effects on the Resulting Grain Size in Friction Stir Processing of AZ31B

The effects of in-process cryogenic LN cooling on the resulting grain size of the friction stir processing (FSP) of twin roll cast (TRC) magnesium alloy AZ31B. Sheets (3 mm-thick) of TRC AZ31B were friction stir processed using a wide range of processing parameters (mostly tool feed and spindle speed). The tool rotational speed was varied between 600 RPM and 2,000 RPM while the tool feed rate varied between 75 mm/min and 900 mm/min. Thrust force and torque values were experimentally measured using a 4-component dynamometer. Temperature measurements were monitored during the different tests using Infrared sensors and thermocouples. The microstructure of processed samples was observed using optical microscopy. It was found that thrust force and torque values of the pre-cooled samples were 5% higher than those of the room temperature samples due to the material hardening induced by the cooling effect. Finer and more homogenous microstructure was observed for the pre-cooled samples when compared with samples processed at room temperature. The average grain size of pre-cooled samples was predicted using a relation -previously introduced by the authors- that relate grain size and the Zener-Hollomon parameter for TRC AZ31B. This equation was found to correctly predict grain diameter for in-line cooled FSP AZ31B samples at temperatures lower than room temperature.

Comparison of Material Flow Stress Models toward More Realistic Simulations of Friction Stir Processes of Mg AZ31B

Utilizing a proper material model for describing the mechanical behavior of any material is key for a successful simulation of friction stir processing (FSP) where temperature, strain, and strain rate gradients vary abruptly within, and when moving away, from the stirring zone. This work presents a comparison of how faithfully do three different constitutive equations reproduce the state variables of strain, strain rate, and temperature in an FEM simulation of a test-case FSP (1000 rpm spindle speed, and 90 mm/min feed). The three material models considered in this comparison are namely: Johnson-Cook (JC), Sellars-Tegart (ST), and Zerilli-Armstrong (ZA). Constants for these constitutive equations are obtained by fitting these equations to experimental mechanical behavior data collected under a range of strain rates and temperatures of twin-rolled cast wrought AZ31B sheets.It is widely recognized that JC-based models over predicts stress values in the stir zone whereas ST-based models are incapable of capturing work hardening outside of the stir zone. Therefore, a ZA model, being a physical based-HCP specific model, is hereby investigated for its suitability as a material model that would overcome such drawbacks of JC-and ST-based models. The equations from the constitutive models under consideration are fed into an FEM model built using DEFORM 3D to simulate the traverse phases of a friction stir process. Amongst these three material models, comparison results suggest that the HCP-specific ZA model yield better predictions of the state variables: strain, strain rate, and temperature, and, consequently, the estimated values for flow stresses.