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CT Answers Complex Casting Questions

Nov. 5, 2021
Computed tomography is becoming essential to quality control for metalcasting operations. An NDT software expert explains how and why.

Computed Tomography (CT) is the process of collecting multiple x-ray images of an object, taken from different angles, and then assembled to produce revealing cross-sections of the object’s structure, and in that way revealing the internal conditions of the structure. The process is important in its own right – but the software that manages and evaluates the results of CT scans is equally critical. Andreas Harborth manages Volume Graphics’ development of CT analysis software for detecting defects in cast parts so that those products achieve and surpass specified quality and performance standards.

In our discussion, Harborth explained that, because CT allows designers and inspectors to view invisible dimensions of complex structures, metalcastings now comprise more than one-third of all the software developer’s business activity.

Describe Volume Graphics and its technology offering.
Volume Graphics has been developing software for non-destructive testing based on industrial computed tomography (CT) for more than 20 years. Very early on, we identified the inspection of cast components as one area for which non-destructive testing using industrial CT offers a genuine benefit.

VGSTUDIO MAX software is used in research, production and quality assurance and works equally well with different CT systems from various manufacturers.

Because CT reconstruction produces a complete representation of a component in 3D, based on a large number of 2D X-ray images, CT allows a manufacturer to answer even the most complex questions about the external and internal structures of a component and its material properties.

Our customers range among automotive, aerospace industries, defense technology, and electronics manufacturers. Because more than one-third of all customers can be associated to the foundry industry, with a steadily growing proportion, we understand the challenges of customers in this field and are constantly working to provide the appropriate solutions.

What are the current trends and important changes in casting technology Well, first I would like to briefly take up an interesting historical aspect: The diecasting process has been tried and tested for over 100 years and was established by the German-American diecasting pioneer Herman Doehler, when the Doehler Die Casting Co. was founded in 1908 after having made a breakthrough. The first diecasting machine available on the market was developed by the Americans Josep Soss and Louis H. Morin, who received the patent in 1925. (See Image 1.)

Current trends in casting technology are set by the automotive industry, which is increasingly focusing on lightweight construction and a high level of functional integration in cast components. This means, for example, that in the field of vehicle structures, a strut dome made of a thin-walled diecasting replaces a previously spot-welded steel sheet assembly with a lot of individual parts, becoming lighter and at the same time stiffer, has greater dimensional accuracy, and can be installed in a body-in-white (BIW) structure more quickly.

What are general developments in high-pressure diecasting (HPDC) processes?  Current developments in diecasting are aimed primarily at improving the productivity of a casting cell. This includes timely monitoring and control of the diecasting processes through the increased use of sensors, as well as recording and statistical evaluation of casting-process data. The aim here is always to influence the casting process at an early stage in order to prevent production of substandard-quality castings.

Another development is that even larger diecasting machines are being developed, currently with clamping forces up to 8,000 mt, in order to map even larger vehicle structures in just a single large casting, with a high-degree of functional integration.

What are the production volumes of parts inspected using Volume Graphics software?  This depends on the CT system designs used and their purpose: With CT at-line systems, sample tests accompanying series production are mainly carried out manually or semi-automatically; around 10 to 30 different castings from the current series are subjected to a CT analysis several times per shift.

With modern CT in-line systems, the CT tests are 100% integrated with the production process. Depending on casting cycle times, between 20 and 60 CT scans/hour are carried out fully automatically.

How much of the inspection process can be automated?  Using predefined test procedures and routines for automated test-report generation, at-line systems can run around 80% automatically. Only the loading and unloading of the CT system is done manually. In the case of in-line systems, the CT test sequences are carried out 100% automatically.

Describe the advantages of a 3D inspection technique (CT)?  The time required for sampling scopes of new parts as well as monitoring of casting quality during production start-ups, and in-series production, can be significantly reduced by using 3D CT inspection. Foundry technologists as well as quality managers are relieved at the time savings gained with this form of inspection and gain more time for other tasks. (See Image 2.)

One example is the elimination of manually performed porosity inspections. Porosity inspection, including documentation, carried out manually take an average of 2 to 2.5 hours. If, instead, the porosity inspection is carried out partially automatically, then with the help of a CT system around 95% of the time required can be saved.

The complete recording of 3D characteristics of internal volume deficits allows not only quantitative recording of porosities but also a first qualitative assessment: If, for example, the determined value for the sphericity of pores is in the range >0.6, it is more a question of rounder, often smooth-walled gas pores.

If the value is below <0.5, the problem is often fissured, often rough-walled shrinkage deficits. Corresponding casting process optimizations can be derived from this information. (See Image 3.)

Another big advantage: The improved possibility of measurement of scanned castings; bore diameters and spacing, difficult-to-access internal geometries as well as component distortion can be measured quickly with high measuring accuracy. Often, the possibilities of CT are limited only by the size of the available CT systems.

How can software detect porosity/inclusions? Through the implementation of the new Volume Graphics porosity analysis, considering the new Federal Association of German Foundries BDG Reference Sheet P 203 for testing of castings using X-ray computed tomography (CT), internal volume deficits can now be determined with a porosity specification using so-called porosity keys. The result is that three-dimensional features are quickly and more precisely characterized and described.

In addition, porosity keys can be defined globally for a constructively required maximum porosity level in the casting as well as, deviating from this, for sub-areas, for example with increased demands on the dynamic component strength. Any number of porosity keys can be used for a casting and evaluated reproducibly with just one porosity analysis job. (See Image 4.)

The specification of porosity keys and their entry in 2D/3D CAD data enables transparent test regulations for suppliers and customers. Furthermore, it is possible to statistically record and evaluate results from the porosity analysis. Process capability analyses now can be carried out for any porosity parameters beyond geometric dimensions. (See Image 5.)

How is simulation being incorporated into metalcasting workflows? Almost all of our customers who use 3D CT in their foundries also carry out corresponding simulations as part of product development. These are, in particular, those for the development and optimization of casting processes in advance of implementation of series production.

The customers are foundries and component plants of automobile manufacturers (OEM) as well as their suppliers (Tier 1, Tier 2.) Castings are manufactured for the powertrain sector. In addition, there are increasing numbers of castings produced for the rapidly growing field of e-mobility: E-engine housings and rotor parts, battery housings and more.

As the subject of lightweight construction is also gaining importance again, more and more vehicle structural elements are manufactured as thin-walled castings (BIW, structural cast.)

How are simulations used to optimize product designs? Are simulations effective at optimizing production practices?  In particular, mold filling and solidification simulations are used in the development of casting design and casting process: With this, pore and shrinkage cavity distributions are determined in a data model including the runner and gating system and reduced to an acceptable level using appropriate iteration steps. This has worked effectively for decades now.

The potential for our customers: The CT analysis software can support the development of the casting process by subjecting CT data from initial casts with pores to a porosity analysis. Porosity parameters for the subsequent porosity analyzes in the series can then be derived from the results.

What are metalcasters’ greatest process / CT inspection concerns -- and what are viable solutions? The biggest concerns are that investments in CT systems are too expensive and the misperception that CT systems cannot be used to add value, but only increasingly detect “rejects.”

A viable solution is: Right from the start, establish intensive use of the functionalities of the complete CT system, e.g., to shorten significantly the time required for the initial sampling of new products, and to ensure comprehensible and incorruptible OK / NOK decisions. This will noticeably improve both trust in the company's own cast products and its ability to deliver high-quality components to customers, making the foundry significantly more productive overall.

How can industrial CT software be used to shorten metalcastings’ time-to-market? The time-to-market for castings can be shortened, in particular, if the potential of industrial CT and the functionalities in Volume Graphics CT software are rigorously used right from the start.
- Non-destructive porosity inspection of initial casting samples avoids time-consuming porosity sample cuts (by sawing), micro section preparations; the time-consuming counting and measuring of pores is no longer necessary. (See Image 6.)
- Analysis of the initial casting samples with regard to contour deviations due to component distortion or shrinkage effects leads, with the help of Volume Graphics CT software, to a factual determination of the necessary corrections required. The number of necessary correction loops, including the costly sample casts, can be significantly reduced.

Andreas Harborth manages Volume Graphics’ development of Computed Tomography (CT) analysis software for detecting defects in cast parts so that those products achieve and surpass specified quality and performance standards.