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Technical Paper

The USAMP Magnesium Powertrain Cast Components Project

Over the past five years, the US Automotive Materials Partnership (USAMP) has brought together representatives from DaimlerChrysler, General Motors, Ford Motor Company and over 40 other participant companies from the Mg casting industry to create and test a low-cost, Mg-alloy engine that would achieve a 15 - 20 % Mg component weight savings with no compromise in performance or durability. The block, oil pan, and front cover were redesigned to take advantage of the properties of both high-pressure die cast (HPDC) and sand cast Mg creep- resistant alloys. This paper describes the alloy selection process and the casting and testing of these new Mg-variant components. This paper will also examine the lessons learned and implications of this pre-competitive technology for future applications.
Technical Paper

The Effect of Porosity Size on the Fatigue Properties in a Cast 319 Aluminum Alloy

A study was conducted to determine how the fatigue life of a 319 aluminum alloy (W319) was affected by pore size. To perform this study, two sets of microstructurally similar castings with differing levels of microporosity were created. Following room temperature fatigue testing, the pores that initiated fatigue cracks were identified and quantified. The results indicate that doubling the average initiating pore diameter yielded an approximate 30% decrease in run-out stress in the W319-T7 aluminum alloy.
Technical Paper

The Effect of Copper Level and Solidification Rate on the Aging Behavior of a 319-Type Cast Aluminum Alloy

Compositional and microstructural variations in a casting can often result in rather significant variations in the response to a given aging treatment, leading to location dependent mechanical properties. The objective of this study is to determine the effect of copper content and solidification rate on the aging behavior of a type 319 cast aluminum alloy. The nominal composition of the alloy is Al-7% Si-3.5% Cu-0.25% Mg, however, typical secondary 319 aluminum specifications allow copper levels to vary from 3-4%. Solidification rates throughout a casting can vary greatly due to, among other factors, differences in section size. To determine the effect of copper level and solidification rate on the aging response, aging curves were experimentally developed for this alloy. Three different copper levels (3, 3.5, 4%) and two solidification rates were used for this study. Aging temperatures ranged from 150-290°C with nine aging times at each temperature.
Technical Paper

The Dimensional Stability of Cast 319 Aluminum

The automotive use of cast aluminum has greatly increased during the past decade, especially for engine blocks and cylinder heads. One physical property that is important in elevated-temperature applications is long-term dimensional stability of the cast aluminum component. Certain cast aluminum alloys (like 319) can undergo dimensional changes when exposed to engine operating temperatures over long periods of time; when these changes occur, the shape of the casting is distorted and the performance of the component may be diminished. Thus, a study was conducted to measure dimensional growth changes in a cast 319-type aluminum alloy as a function of heat-treatment, exposure temperature, and exposure time at the given temperature. The results show that all three factors have a significant effect upon the dimensional stability.
Technical Paper

Progress Toward a Magnesium-Intensive Engine: The USAMP Magnesium Powertrain Cast Components Project

The US Automotive Materials Partnership (USAMP) and the US Department of Energy launched the Magnesium Powertrain Cast Components Project in 2001 to determine the feasibility and desirability of producing a magnesium-intensive engine; a V6 engine with a magnesium block, bedplate, oil pan, and front cover. In 2003 the Project reached mid-point and accomplished a successful Decision Gate Review for entry into the second half (Phase II) of the Project. Three tasks, comprising Phase I were completed: (1) evaluation of the most promising low-cost, creep-resistant magnesium alloys, (2) design of the engine components using the properties of the optimized alloys and creation of cost model to assess the cost/benefit of the magnesium-intensive engine, and (3) identification and prioritization of scientific research areas deemed by the project team to be critical for the use of magnesium in powertrain applications.
Technical Paper

Permanent Mold Casting and Creep Behavior of Mg - 4 Al - 4 X: (Ca, Ce, La, Sr) Alloys

Creep-resistant magnesium alloys for automotive powertrain applications offer significant potential for vehicle weight reduction. In this study permanent mold casting, microstructure and creep behavior have been investigated for a series of ternary magnesium alloys (Mg-4Al-4X (X: Ca, Ce, La, Sr) wt%) and AXJ530 (Mg-5Al-3Ca-0.15Sr, wt%). A permanent mold was instrumented with twelve thermocouples and mold temperature was monitored during the casting process. Average mold temperature increased from 200°C to 400°C during a typical alloy casting series (fifteen to twenty castings). The cast microstructure for all alloys consists of primary α-Mg globular phase surrounded by eutectic structure which is composed of intermetallic(s) and α-Mg magnesium phases. The primary cell size of the AXJ530 increased from 18 to 24 μm with increasing mold temperature and a similar trend is expected for all alloys.
Technical Paper

Microstructure Influence on the Corrosion of Permanent Mould Cast Magnesium Alloy AJ62 in Engine Coolant

Powertrain applications of alloy AJ62 arose from its comparative resistance to high temperature deformation among magnesium alloys. In this research, AJ62 permanent-mould cast in different section thicknesses was subjected to immersion corrosion in commercially-available engine coolant. The objective was to determine corrosion behaviour variation among casting thicknesses. Corrosion product accumulation suggests passive film formation, and unlike in other media, the film exhibits certain stability. Extreme thicknesses were used to generate polarization curves for their respective microstructures in engine coolant. Variation with casting section thickness was observed in the curves. These preliminary results indicate coarsened microstructures reduce corrosion resistance of the permanent mold cast AJ62 alloy.
Journal Article

Fusion Welding of Vacuum High Pressure Die Cast Aluminum Alloy A356 and Wrought Alloy 6061

Recently, joining of cast aluminum components with wrought and/or cast similar metals becomes an urgent task for the auto industry to develop light-weight complex and large-scale chassis and body structures for further reduction in vehicle weight. In this study, fusion-joining of vacuum high pressure die cast (VHPDC) alloy A356 subjected to T5 heat treatment and wrought alloy 6061 with the Gas Metal Arc Welding (GMAW-MIG) process was experimented in an effort to understand the effect of the MIG process on the microstructure development and tensile behaviors of the base alloys (T5 A356 and 6061), Heat Affected Zone (HAZ) and Fusion Zone (filler metal ER4043). The results of tensile testing indicated that the ultimate tensile strength (UTS), yield strength (YS) and elongation (Ef) of VHPDC T5 A356 were relatively high, compared to both wrought alloy 6061 and the filler metal (ER 4043).
Technical Paper

Finite Element Modeling of Bolt Load Retention of Die-Cast Magnesium

The use of die cast magnesium for automobile transmission cases offers promise for reducing weight and improving fuel economy. However, the inferior creep resistance of magnesium alloys at high temperature is of concern since transmission cases are typically assembled and joined by pre-loaded bolts. The stress relaxation of the material could thus adversely impact the sealing of the joint. One means of assessing the structural integrity of magnesium transmission cases is modeling the bolted joint, the topic of this paper. The commercial finite element code, ABAQUS, was used to simulate a well characterized bolt joint sample. The geometry was simulated with axi-symmetric elements with the exact geometry of a M10 screw. Frictional contact between the male and female parts is modeled by using interface elements. Material creep is described by a time hardening power law whose parameters are fit to experimental creep test data.
Technical Paper

Die Castability Assessment of Magnesium Alloys for High Temperature Applications: Part 1 of 2

This paper describes the high-pressure die castability assessment of two high temperature magnesium alloys, AE42 and the AC series alloy. AE42 is a commercially available alloy. Results showed that AE42 was a castable material for use in high-pressure die casting applications, including large transmission components. AE42 was determined to have similar operating/manufacturing costs if produced in equivalent volumes to AZ91D. The AC series alloy is an experimental alloy comprised of AM50 combined with small percentages of calcium (Ca). It was found that the castability of the AC series alloy decreased with increasing calcium content. Over 0.3% calcium content yielded poor castability performance. Selected mechanical and corrosion properties of AZ91D, AE42, AM50 and the AC series alloys were also explored.
Technical Paper

Development of Experimental Methods to Validate Residual Stress Models for Cast Aluminum Components

The prediction of residual stresses due to manufacturing is of high importance in product development. For the accurate prediction of residual stresses in metallic components, an understanding of the quenching process that occurs in many heat treatments is required. In this paper, the experimental techniques developed to quantify the temperature fields during quenching and to quantify the residual stresses in the quenched part are presented. The temperature fields were quantified using thermocouples embedded in the components. The residual stresses were quantified using a newly developed strain gauging, sectioning and dynamic data acquisition technique. The techniques were verified using thermal histories and residual stresses for an engine cylinder head quenched at two different quenchant temperatures. The measurements obtained were incorporated into an analytical program (finite element) to study the residual stresses produced during the quenching process.
Technical Paper

Design of As-Cast High Strength Al-Si-Cu-Ni-Sr Alloys Using the Taguchi Method

In the present study, a design of experiment (DOE) technique, the Taguchi method, was used to develop as-cast high strength aluminum alloys with element additions of Si, Cu, Ni and Sr. The Taguchi method uses a special design of orthogonal arrays to study all the designed factors with a minimum of experiments at a relatively low cost. The element factors chosen for this study were Si, Cu, Ni and Sr content in the designed aluminum-based alloys. For each factor, three different levels of weight percentages were selected (Si: 6, 9, 12%, Cu: 3, 5, 7%, Ni: 0.5, 1, 1.5% and Sr: 0.01, 0.02, 0.03%). Tensile properties such as ultimate tensile strength, yield strength and elongation at failure were selected as three individual responses to evaluate the engineering performance of the designed alloys. The results of the factor response analysis were used to derive the optimal level combinations.
Technical Paper

Creep and Bolt Load Retention Behavior of Die Cast Magnesium Alloys for High Temperature Applications: Part 2 of 2

The effect of calcium on the creep and bolt load retention (BLR) behavior of AM50 alloy has been investigated. Four AM50 alloys 0, 0.25, 0.56, and 0.88% Ca have been die-cast. BLR-tests have been conducted at 125, 150, and 175°C and preloads of 14, 21, and 28kN. Tensile and compressive creep tests were also conducted at 150°C and initial stresses from 40 to 80 MPa. Both creep and BLR were significantly influenced by calcium content, with increasing calcium content resulting in improved relaxation resistance. The BLR of AM50 with 0.88% Ca was better than that of AE42 at all temperatures although the effect of calcium was temperature dependent. Calcium did not change the sensitivity of BLR to preload, while it increased the relaxation limit (Fr) of AM50 significantly. In addition, calcium improved the creep resistance of AM50 significantly.
Technical Paper

Bolt-Load Retention Behavior of a Die Cast Magnesium-Rare Earth Alloy

The need for improved understanding of new magnesium alloys for the automotive industry continues to grow as the application for these lightweight alloys expands to more demanding environments, particularly in drivetrain components. Their use at elevated temperatures, such as in transmission cases, presents a challenge because magnesium alloys generally have lower creep resistance than aluminum alloys currently employed for such applications. In this study, a new die cast magnesium alloy, MEZ, containing rare earth (RE) elements and zinc as principal alloying constituents, was examined for its bolt-load retention (BLR) properties. Preloads varied from 14 to 28 kN and test temperatures ranged from 125 to 175°C. At all test temperatures and preloads, MEZ retained the greatest fraction of the initial imposed preload when compared to the magnesium alloys AZ91D, AE42, AM50, and the AM50+Ca series alloys.
Technical Paper

A Test Method for Quantifying Residual Stress Due to Heat Treatment in Metals

Quantification of residual stresses is an important engineering problem impacting manufacturabilty and durability of metallic components. An area of particular concern is residual stresses that can develop during heat treatment of metallic components. Many heat treatments, especially in heat treatable cast aluminum alloys, involve a water-quenching step immediately after a solution-treatment cycle. This rapid water quench has the potential to induce high residual stresses in regions of the castings that experience large thermal gradients. These stresses may be partially relaxed during the aging portion of the heat treatment. The goal of this research was to develop a test sample and quench technique to quantify the stresses created by steep thermal gradients during rapid quenching of cast aluminum. The development and relaxation of residual stresses during the aging cycle was studied experimentally with the use of strain gauges.