Age Hardening ( 8:16 am, 26.03.2011)
Age hardening or precipitation hardening produces a uniform dispersion of a fine, hard coherent precipitate in a softer, more ductile matrix. The Al %4 Cu alloy is a classical example of an age-hardenable alloy. There are three stages in the age hardening treatment.
Step1: Solution treatment: In the solution treatment the alloy is first heated above the solvus temperature and held until a homogenous solid solution alfa is produced. The step dissolves the teta precipitate and reduces any segregation present in the original alloy.
Step2: Quench: After solution treatment the alloy which contains only alfa in is structure is rapidly cooled or quenched. The atoms do not have time to diffuse the potential nucleation sites, so the teta does not form. After the quench the structure still contains only alfa. The alfa is a super saturated solid solution.
Step 3: Age: Finally the supersaturated alfa is heated below the solvus temperature. At this ageing temperature, atoms diffuse only short distances. Because the supersaturated alfa is not stable, the extra copper atoms diffuse to numerous nucleation sites and precipitates grow. Eventually if we hold the alloy for a sufficient time at the ageing temperature, the equilibrium alfa+teta structure is produced. This however is not always desirable as higher strenght may be achieved by stopping the ageing process before an equilibrium structure is produced.
(Donald R. Askeland, The Science and Engineering of Materials, Third S.I Edition, page 322)
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Cold Treatment (9:11 am 26.03.2011)
Strength can be improved in hardened steels containing retained austenite by a process known as sub-zero treatment or cold treatment. Retained austenite is converted into martensite by this treatment. This conversion of retained austenite into martensite results in increased hardness, wear resistance and dimensional stability of steel.
The process consists of cooling steel to subzero temperature which should be lower than the Mf temperature of the steel. Mf temperature for most steels lies between -30 C and -70 C. During the process, considerable amount of internal stresses are developed in the steel, and hence tempering is done immediately after the treatment
(T. V. Rajan, C. P. Sharma, Ashok Sharma, Heat Treatment: Principles and Techniques, page 121)
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Electroplated prototypes (10:01 am 26.02.1011)
For instances where it is helpful to have metal-like parts that do not necessarily need the same material properties as a final production metal part, it is possible to electroplate prototype components.
A simple though not terribly reliable approach to obtaining an electrically conducting surface on the plastic prior to electroplating is to manually paint it with conductive paints. Paints containing silver or copper powders are readily available for this purpose. This approach will work in simple cases, but the adhesion of the paint to the plastic surface during subsequent electroplating is often inadequate. The result is blistering or flaking of the plated surface.
(Ronal A. Wash, Denis Cormier , McGrawHill Machining and Metalworking Handbook , page 726)
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RTV silicone molds (10:05 am 26.03.2011)
One of the most widely used approaches to rapid tooling involves making room-temperature vulcanizing (RTV) silicone molds from a pattern made on any one of the RP/RM processes. The RTV mold may be fabricated in two pieces, or it may be a one-piece cut mold. Regardless of how the RTV mold is made, polyurethane or other two-part resins are then cast into the mold to produce a plastic part. The urethane may include dyes and/or fillers to modify its appearance and material properties. Common fillers include glass beads to reduce part weight and to reduce the volume (and cost) of the resin being used. Chopped fiberglass strands sometimes are added to increase part strength. Metal powders such as bronze are also added at times to make the part look and feel like a metal part. Provided the powder loading is sufficiently high, it is possible to buff the parts in order to achieve a somewhat shiny metallic look. Depending on the resin being cast, the workpiece geometry, and any fillers/additives, the typical RTV mold usually is good for approximately 30 to 50 castings.
(Ronal A. Wash, Denis Cormier , McGrawHill Machining and Metalworking Handbook , page 721)
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