John F. Wallace

Die Materials for Critical Applications and Increased Production Rates

Die materials for aluminum die-casting need to be resistant to heat checking, and have good resistance to washout and to soldering in a fast flow of molten aluminum. To resist heat checking, die materials should have a low coefficient of thermal expansion, high thermal conductivity, high hot yield strength, good temper softening resistance, high creep strength, and adequate ductility. To resist the washout and soldering, die materials should have high hot hardness, good temper resistance, low solubility in molten aluminum and good oxidation resistance. It is difficult for one material to satisfy with all above requirements. In practice, H13 steel is the most popular material for aluminum die casting dies. While it is not an ideal choice, it is substantially less expensive to use than alternative materials. However, in very demanding applications, it is sometimes necessary to use alternative materials to ensure a reasonable die life. Copper-base, nickel-base alloys and superalloys, titanium-,molybdenum-, tungsten-base alloys, and to some extent yttrium and niobium alloys, have all been considered as potential materials for demanding die casting applications. Most of these alloys exhibit superior thermal fatigue resistance, but suffer from other shortcomings.

Sputtered protective coatings for die-casting dies

Abstract This investigation was undertaken to determine whether selected ion-beam-sputtered coatings on H-13 die steel would have the potential of improving the thermal fatigue behavior of the steel used as a die in aluminum die casting. The coatings were selected to test candidate insulators and metals capable of providing protection of the die surface. The studies indicate that tungsten and platinum coatings 1 μm thick reduce the thermal fatigue more than any other coating tested and are candidates to be used on a die surface to increase die life.

Effects of process parameters on quality of squeeze casting A356 alloy

Abstract This investigation has studied the influence of the various casting variables on the quality of indirect squeeze castings primarily of aluminium alloys. The variables studied include gating design, filling velocity and squeezing pressure. The quality of the die casting was assessed by an analysis of both their surface condition and internal soundness. The cavity filling patterns with fanned gates and straight gate are compared. Straight gates are prone to cause jetting of the metal stream even at low velocities while fanned gates allow use of higher fill velocity without excessive jetting. A higher metal pressure provides a more complete fill of the die including improved compensation for solidification shrinkage. The gate velocity for cavity filling process is one of the most important factors for attaining a sound casting. Either too slow velocity or too rapid gate velocity can cause such defects as misrun, indications, jetting vortex, air entrapment, etc. Furthermore, the computer models using the UES ProCast software are conducted.

Measurement of thermal degradation in epoxy composites by Fourier transform Raman spectroscopy

The ability of Raman spectroscopy to nondestructively evaluate thermal degradation in graphite reinforced epoxy composites was examined. A series of composite samples, exposed to temperatures ranging from 150 to 400 degree(s)C for periods of 2 to 20 minutes, were analyzed by Fourier transform Raman and reflectance IR spectroscopies. The intensity of the Raman and IR polymeric backbone vibration at 1600 cm-1 diminishes with increasing thermal exposures and can be correlated to failure strain and flexural strength measured by four point bending tests, as well as acoustic emission events. These data, along with IR transmission spectra of species evolved from composite pyrolysis, suggest that thermal degradation occurs in three stages: (1) polymeric fragmentation (possibly microcracking), (2) advanced polymer degradation observed as delamination between the four ply layers, and (3) final composite failure with fiber fracture.

ENHANCEMENTS IN MAGNESIUM DIE CASTING IMPACT PROPERTIES

The need to produce lighter components in transportation equipment is the main driver in the increasing demand for magnesium castings. In many automotive applications, components can be made of magnesium or aluminum. While being lighter, often times the magnesium parts have lower impact and fatigue properties than the aluminum. The main objective of this study was to identify potential improvements in the impact resistance of magnesium alloys. The most common magnesium alloys in automotive applications are AZ91D, AM50 and AM60. Accordingly, these alloys were selected as the main candidates for the study. Experimental quantities of these alloys were melted in an electrical furnace under a protective atmosphere comprising sulfur hexafluoride, carbon dioxide and dry air. The alloys were cast both in a permanent mold and in a UBE 315 Ton squeeze caster. Extensive evaluation of tensile, impact and fatigue properties was conducted at CWRU on permanent mold and squeeze cast test bars of AZ91, AM60 and AM50. Ultimate tensile strength values between 20ksi and 30ksi were obtained. The respective elongations varied between 25 and 115. the Charpy V-notch impact strength varied between 1.6 ft-lb and 5 ft-lb depending on the alloy and processing conditions. Preliminary bending fatigue evaluation indicates a fatigue limit of 11-12 ksi for AM50 and AM60. This is about 0.4 of the UTS, typical for these alloys. The microstructures of the cast specimens were investigated with optical and scanning electron microscopy. Concomitantly, a study of the fracture toughness in AM60 was conducted at ORNL as part of the study. The results are in line with values published in the literature and are representative of current state of the art in casting magnesium alloys. The experimental results confirm the strong relationship between aluminum content of the alloys and the mechanical properties, in particular the impact strength and the elongation. As the aluminum content increases from about 5% in AM50 to over 9% in AZ91, more of the intermetallic Mg17Al12 is formed in the microstructure. For instance, for 15 increase in the aluminum content from AM50 to AM60, the volume fraction of eutectic present in the microstructure increases by 35%! Eventually, the brittle Mg17Al12 compound forms an interconnected network that reduces ductility and impact resistance. The lower aluminum in AM50 and AM60 are therefore a desirable feature in applications that call for higher impact resistance. Further improvement in impact resistance depends on the processing condition of the casting. Sound castings without porosity and impurities will have better mechanical properties. Since magnesium oxidizes readily, good melting and metal transfer practices are essential. The liquid metal has to be protected from oxidation at all times and entrainment of oxide films in the casting needs to be prevented. In this regard, there is evidence that us of vacuum to evacuate air from the die casting cavity can improve the quality of the castings. Fast cooling rates, leading to smaller grain size are beneficial and promote superior mechanical properties. Micro-segregation and banding are two additional defect types often encountered in magnesium alloys, in particular in AZ91D. While difficult to eliminate, segregation can be minimized by careful thermal management of the dies and the shot sleeve. A major source of segregation is the premature solidification in the shot sleeve. The primary solid dendrites are carried into the casting and form a heterogeneous structure. Furthermore, during the shot, segregation banding can occur. The remedies for this kind of defects include a hotter shot sleeve, use of insulating coatings on the shot sleeve and a short lag time between pouring into the shot sleeve and the shot.

Mold Filling Process Control of Molten Aluminum in Permanent Molds with a Web Gating System

Web gate system of aluminum castings in permanent molds is investigated in order to improve the quality of aluminum castings produced in permanent molds. The metal flow in the mold were observed and conducted using graphite molds and real time X-ray radiography recorded at a rate of 30 images per second through those molds. The affects of web thickness on flow patterns, gas entrapment, jetting possibility are studied and discussed. Flow and solidification simulation programs were employed to predict the flow behavior under the different conditions that can prevail in permanent mold gating. The study highlights the characteristic features of web gate system used in permanent mold aluminum foundries and recommends gating procedures designed to avoid common defects, and provides direct evidence on the filling pattern and heat flow behavior in permanent mold castings.

Evaluation of Heat Checking and Washout of Heat Resistant Superalloys and Coatings for Die inserts

This project had two main objectives: (1) To design, fabricate and run a full size test for evaluating soldering and washout in die insert materials. This test utilizes the unique capabilities of the 350 Ton Squeeze Casting machine available in the Case Meal Casting Laboratory. Apply the test to evaluate resistance of die materials and coating, including heat resistant alloys to soldering and washout damage. (2) To evaluate materials and coatings, including heat resistant superalloys, for use as inserts in die casting of aluminum alloys.

Mold Materials For Permanent Molding of Aluminum Alloys

A test that involves immersion of the potential mod materials for permanent molds has been developed that provides a thermal cycle that is similar to the experienced during casting of aluminum in permanent molds. This test has been employed to determine the relative thermal fatigue resistance of several different types of mold materials. Four commercial mold coatings have been evaluated for their insulating ability, wear resistance and roughness. The results indicate that composition and structure of the mold materials have considerable effect on their thermal fatigue cracking behavior. Irons with a gray iron structure are the most prone to thermal fatigue cracking followed by compacted graphite irons with the least thermal fatigue cracking of the cast irons experienced by ductile iron. The composition of these various irons affects their behavior.