David Schwam

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.

Hot Tearing Susceptibility and Fluidity of Semi-Solid Gravity Cast Al-Cu Alloy

Aluminum-copper alloys offer both high strength and excellent ductility suitable for a number of automotive applications to reduce vehicle weight; however, the alloys are difficult to cast because of their tendency for hot tearing. In this work, semi-solid gravity casting of an aluminum-copper alloy, B206, was conducted in constrained rod casting molds to study the feasibility of using the process to reduce or eliminate hot tearing. To demonstrate the feasibility of gravity casting of the metal slurries, a fluidity test was also conducted. Results show that the hot tearing susceptibility of the aluminum-copper B206 alloy cast in semi-solid state is lower than those cast in liquid state with high superheat temperatures. The grain size of the semi-solid cast Al-Cu samples appears to be finer than those cast in liquid state with high superheat temperatures. In addition, the metal slurries had sufficient fluidity to fill the molds even with low gravity pressures. The results suggest that semi-solid gravity casting is a feasible process to help reduce hot tearing.

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.

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.

Optimization of Permanent Mold Mechanical Property Test Bars in A356 Alloy Using a New Mold Design

Using separately cast test bars is a quick and convenient method of determining as-cast metal quality in the foundry—although such results are only representative of the section of a casting solidifying at the same rate as the test bar. Unfortunately the current standard test bar mold suffers from shrinkage porosity which detracts from best properties. In this work computer simulation has been utilized to predict and design an improved permanent mold test bar mold. A356 alloy has been melted and treated with best metal cleaning practices (degassing, filtration) in order to assess the effect of clean metal as a baseline (Phase 1) of this research prior to microstructural enhancements (modification, grain refinement, SDAS) which is Phase 2 of this research.The results show that with a knife ingate in the re-designed test-bar mold, better tensile properties with A356 separately cast test bars heat treated to T6 can be obtained on a more consistent basis throughout a varying mold temperature range. With best in-furnace clean metal practice and virgin ingot, applying filters in the test bar mold have minimal effect, but show that filtration is beneficial if melting recycled metal.Even without the additional microstructural enhancements, preliminary results in the standard heat treated T6 condition show that the new test bar mold delivers a high quality result as measured by the Quality Index method.

The Effect of Substrate Temperature on Properties of RPA Deposited Maraging 250 for Tooling Repair

During cladding, the heat input from a new overlay can induce metallurgical changes in the previously deposited layers and change their mechanical properties. This effect is encountered during cladding repair of tooling with robotic pulsed arc (RPA) and was the target of this study. The alloy used for cladding was Maraging 250 steel, an age hardenable 18% nickel alloy. A parametric study was undertaken to correlate between cladding with and without cooling of the substrate and the resulting mechanical properties. Twenty layers of Maraging 250 were deposited (a) with forced air cooling to below 250°F (b) with thirty seconds hold time between layer w/o temp control (c) with water-spray cooling to below 250°F, resulting in an average hardness of 42.2HRC, 51.8HRC and 39.1HRC respectively. A fourth block was deposited cold by forced cooling between passes to below 250°F and was subsequently aged at 900°F for three hours and air cooled. The hardness of this specimen ended at 49.5HRC, as anticipated. For die repair with Maraging 250, temperature control during deposition is therefore desirable, to target a hardness range of 42–44HRC. This hardness range provides a satisfactory combination of elongation strength and toughness for tooling application.

Application of Bayesian Analysis Method in the Design Optimization of Permanent Casting Mold

This work is a case study of applying Bayesian analysis, a statistical data method, in the design optimization of permanent test-bar mold. The permanent test-bar mold is used in casting foundry to examine the metal quality. Since the current standard test-bar mold suffers from shrinkage porosity which detracts from best properties, a modified design is recently proposed to improve the mechanical properties. In order to validate the new design, Bayesian data analysis method is utilized to analyze the experimental data from the two designs. The effects of the mold designs and casting process operational parameters on the mechanical properties of castings are compared. Main effect to the mechanical properties is identified based on the Bayesian analysis.Copyright

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.