Pathway To Metal Casting Success
Contents:
This all happens very rapidly; in a matter of micro-seconds. As a new layer of metal solidifies and cools on top of the layer below, it contracts.
The new metal is constrained by the solid structure below and so its contraction sets up shear forces between the layers. Image above - laser melting of a new weld track on top of a solid substrate left. As the laser moves along the scan vector, it melts the powder, which then cools mostly through conduction of heat into the solid metal below. Once it solidifies, the cooling metal contracts right , setting up shear forces between it and the layer below. Residual stress can be destructive.
As we add layers on top of one another, the stresses build up and can result in distortion of the part, leading it to curl up at the edges and pull away from its supports:. In more extreme cases, the stress may exceed the strength of the part, leading catastrophic cracking of the component, or distortion of the build plate:.
These effects are most pronounced in parts with large cross-sections, as these tend to have longer weld tracks and there is more distance over which the shear forces can act. One way to tackle this is by varying our scanning strategy, choosing a method that is best suited to the part geometry.
When we are filling in the centre of our part, an activity known as 'hatching', we typically move the laser back and forth. The pattern that we choose affects the length of the scan vectors and hence the level of stress that we are likely to build up in the component. Strategies with shorter scan vectors will generate less residual stress:.
Official statistics attributed nearly , wartime deaths in Germany to starvation caused by the Allied blockade. Design for AM DfAM is, therefore, critical if we are to produce parts that combine exceptional performance with practical, cost-effective additive manufacture. This dynamic vision of economic evolution is very different from the tradition of neo-classical economics , which in its simplest form assumed that only a single outcome could possibly be reached, regardless of initial conditions or transitory events. There are arguably two types of path-dependent processes:. As a memorandum to the War Cabinet on 1 January stated, very few supplies were reaching Germany or its allies - either through the North Sea or through other areas such as Austria's Adriatic ports, subject to a French blockade since the first month of the war. Like any manufacturing process, AM technologies have their capabilities and their limitations. Whilst a horizontal overhang of 0.
Image above - scanning strategies and their suitability for different part types. The two most common strategies are 'meander' for thin walled parts also known as rastering , and 'stripes' for parts with thicker sections. Stripe and chessboard scanning keeps the lengths of individual scan lines shorter, reducing the build-up of residual stress.
We can also rotate the orientation of our scan vectors from one layer to the next so that stresses are not all aligned in the same plane. A rotation of 67 degrees is typically used between each layer to ensure that it is many layers before the scanning direction is exactly repeated. Heating of the build plate is another technique used to reduce residual stresses, whilst post-process heat treatments can also relieve the stresses that have built up.
With any additive layer process, the build direction is always defined as being in the Z axis - i. Note that the build orientation is not always the general use orientation. It should be chosen to produce the most stable build with minimal or no support material. In powder-bed processes, where shapes are built up layer by layer, the way these layers relate to each other is important.
As each layer is melted, it relies on the layer below to provide both physical support and a path to conduct away heat. When the laser is melting powder in an area where the layer below is solid metal, then heat flows from the weld pool down into the structure below, partially re-melting it and creating a strong weld. The weld pool will also solidify quickly once the laser source is removed as the heat is conducted away effectively. Where component features overhang those below, then at least part of the zone below the weld pool will consist of un-melted powder.
This powder is far less thermally conductive than solid metal, and so heat from the melt pool is retained for longer, resulting in more sintering of surrounding powder. The result can be additional material attached to the bottom surface of the overhanging region, meaning that overhangs can exhibit both misshapen surfaces and a rough finish. Image above - melting above solid metal enables rapid cooling left. Where melting occurs in an overhanging region above un-fused powder, then cooling takes much longer and unwanted additional material may become attached to the bottom surface of the component.
Overhanging surfaces are known as down-skins. They will generally exhibit rougher surface finish than vertical walls and upward facing surfaces. This effect is driven by the partial sintering of powder below the overhang, resulting from the slower cooling of the weld pool. Parts can often be built in multiple orientations. Our choice of orientation should ideally self-support so that we minimise build costs and post-processing.
Design for metal AM - a beginner's guide
Image above - a component can often be built in many orientations, the choice of which has a major impact on the amount of wasted support material and post-processing that will be required. It is a good idea to evaluate a range of build orientations using build preparation software early in the component design process to establish which is most promising. Once this decision has been reached, detail design can proceed on this basis. Local minima are any areas of the part that are not connected to the layer below. These require support to anchor them during the build.
If we start building without a support structure below, then the first built layer is likely to be displaced by the wiper as it doses the next layer, leading to a failed build. Local minima can be obvious such as the example shown above. They can also appear at the top of lateral and angled holes where they intersect the edge of the part shown below. As we have already discussed, down-skins tend to have inferior surface finish.
If we want to produce detail features with the best accuracy, then it is best to orientate these on the top surface of the part, also known as the up-skin. Detail features that are inset into down-skins are likely to suffer from a loss of definition. Another consideration is the orientation of the component relative to the dosing wiper. As a new layer of powder is applied and the wiper pushes it across the bed, the powder is progressively squeezed under the wiper to create the new densely-packed layer.
This creates a pressure wave in the powder bed as the material is pressed down. This can interact with component surfaces that are inclined towards the wiper, forcing powder down and pushing the front edge of the component upwards. This can cause the part to catch on the wiper, which may lead to a failed build. Note that a flexible wiper reduces this effect. Image above - interaction between the dosing wiper and the inclined edge of a component.
Pathway To Metal Casting Success - Kindle edition by Colin Croucher. Download it once and read it on your Kindle device, PC, phones or tablets. Use features. It is generally agreed that the successful treatment of a badly broken tooth with It is easier to make cast metal restorations with the aid of posts for retention and . the path of withdrawal of the post for preparing the tooth to receive the post.
Supports and inclined edges should therefore be orientated away from the wiper direction where possible. By rotating the part, the pressure wave now strikes the part at an oblique angle, reducing the likelihood of distortion.
Design for metal AM - a beginner's guide
If the rotary alignment cannot be changed, or if the part is rotationally symmetric, then supports may be needed, possibly followed by post-process machining of the affected face. As we have already discussed. Letter from Jerusalem A deeper look at the people and places of the Israeli-Palestinian conflict. Brexit WhatsApp group Sign up for all the latest news, exclusives and analysis.
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