The AIM "Quality of HPDC parts" Workgroup are working for a proposal of classification

 

 

Abstract: the paper gives an introduction to one of the current actions of the WG “Qualità of High Pressure DieCast (HPDC) products” within the TC “High pressure diecasting” of the Italian Association of Metallurgy. The preparation and dissemination of common tools to allow foundries to define a proper, comparable, quality standard is in fact one of the current targets of this WG. The basic tool, to which this paper refers, is a common terminology and classification of defects of HPDC products.

An initial survey of literature and industrially adopted classifications of defect in components cast in metallic dies, revealed that the geometry and origin-based approaches are often mixed giving rise to a wide range of hybrid classifications. The proposed classification of defects of HPDC products, discussed within the WG with the contribution of several foundries, is a multi-level, hybrid-type one. In the first level defects are grouped on the basis of their position (surface/internal and geometry defects) according to the typical control operations during which they can be detected and to the effects of defects on the functionality of the parts. The second level of the classification groups defect in classes accordino to their general metallurgical origin.

The proposed classification does not specify defect/cause correlations, but gives starting points to the identification of their specific causes. In order to better classify these features the analogies between the origin of internal/ surface defects are highlighted by corresponding names for level II classes: gas-related, shrinkage-related, filling-related, thermal contraction defects and undesired phases.

In addition the class of metal/die interaction defects is proposed for surface defects.

The specific defects identified are those included in the third level of the classification. In the present paper an introduction to the general classes of defects defined in level II is proposed.

The action of the WG concerning defect classification of HPDC parts will be completed with the compilation of the official document including the proposed classification and multi-language terminology equivalents.

 

It is well established that defects represent a critical feature for die-cast components. Defects could be of different types and they can differently impair properties and functionality of components, as reported in several studies concerning Al low pressure and high pressare diecast products [1, 2, 3, 4a, 5]. Recently the “Quality of high pressure diecast products” within the TC “High pressure diecasting” of the Italian Association of Metallurgy carried out a statistical analysis on the production of High Pressure DieCast (HPDC) aluminium components by Italian foundries [6]. The 47 foundries contributing data handled yearly a total of 193,000 t of Al alloys (raw materials) with a corresponding turnover of 915 Millions euros.

Figure 1a gives an idea of the application fields and of their relative economical importance for the Italian and for the foreign markets. In this survey, 5 families of die-cast products were taken into account: parts including thin sections, security components (for example brackets), products that can be assimilated to carters, to engine blocks and other components.

The customer requests and controls for each product family were investigated and results for two families are schematically presented in Figure 2. It is clear that different geometries and typical needs for each product family lead to focus the attention on different features of the parts (for example to the pressure tightness, the presence of porosità etc.) and to suitable control techniques directly or indirectly related to defects to be revealed.

A few introductive remarks on defects in cast products are needed before presenting the proposed classification of defects in HPDC products. First, it is interesting to make some comments on the results of “round-robin” tests on the presence of defects in HPDC products for different application fields published by Brevick [7]. In this study the rejection of die cast parts during their machining step or durino the following visual control and pressurized leak test was examined (Figure 3, adapted from [7]). Even considering the specific goals and the features of interest for this experimental analysis (excluding parts damaged during machining, only the machined surfaces were taken into account for detecting defects), the investigation clearly showed that most of the defects are of metallurgical type, i.e. they are related to the melting/casting/solidification and cooling steps of the manufacturing process. As a matter of fact, metallurgical defects are those taken into account in several classification of die-casting defects. Often they exclude other type of defects, directly related to operations following the ejection of the cast part from the die such as the removal of feeling channels and dross, to handling or machining of the parts. On the other hand, the example proposed by Brevick [7] shows that these operation, can also bring to surface, and reveal at visual controls defects of metallurgical origin. Focusing now the attention of metallurgical defects, their list can be widened with respect to those taken into account in the previous example, including surface defects on unmachined surfaces and other internal defects (not revealed by machining operations).

Internal defects are particularly critical for HPDC products since their detection and evaluation involves non destructive tests more costly and often less effective that controls suitable for surface defects. Another preliminary remare to be done before presenting defect classifications is that the analysis of defects performed on a specific production can assist a foundry to monitor the quality of the produced parts, to keep a quality standard, for example by rejecting parts having defects outside specified limits. Further, the analysis of defect data can provide the foundry with correlations between defect type/distribution and their origin so that it could be possible to define process modification to be adopted for improving product quality. Lastly, all the analyses of defect data can not leave out of consideration a fundamental step to be preliminary done: different types of defects need to be clearly identified. This will also help both foundries and customers in the following steps of identification of limits for rejection of die-cast parts. The compilation and dissemination of common tools to allow foundries to define a proper, comparable, qualità standard is one of the current targets of the WG “Quality of HPDC products”. The first tool to be proposed and promoted to foundries corresponds to the above fundamental step earlier defined is a common basis of language, i.e. a terminology of defects. This is the main focus of the present paper.

 

The need to deal with the relatively wide range of defects that can be found in HPDC products, leads to organize them into classes (i.e. classify them) according to specific criterions. Also the proposal of a terminology document for defect identification can not leave out their classification.

Two are the main approaches for defect classification of cast components proposed in literature or currently adopted by foundries. Defects can be distinguished on the basis of their geometry/location or on the basis of their metallurgical origin or specific causes (part geometry, cast alloy, die characteristics, die lubrication, process parameters, etc.).

The main advantage offered by classifications based on the defect origin/cause approach is that strategies to be adopted in order to improbe product quality can be guessed and, in the second case, directly defined. The main disadvantage of this approach is that the origin/cause of the defect must be defined concurrently to defect identification. This is particularly critical when defects are classified on the basis of specific causes, since a single defect can be due to several concurring factors. Further, this approach is less suitable for direct application in foundries with respect to geometry/position-based approaches, where specific inspections can be proposed to reveal different defect groups taking into account the specific needs of the component. A survey of the proposed defect classifications of cast products, some of which more general, other specifically developed for an alloy class or for die-casting or even more specifically for the HPDC process, [4b, 8-11] was done. The examples proposed by Campbell [8] and cocks [9] of defect classification are presented in Figure 4. In addition to literature references, also customer or foundry specifications for aluminium alloy castings were taken into account. It was observed that generally the two approaches earlier described were mixed in different ways, generating a wide range of hybrid classifications. It was also observed that geometry defects such as distortion, bending,

regions out of tolerance, were included in defect classifications when of metallurgical type.

 

On the basis of the previous comments on the presence and classification of defects in cast products and on the specific targets of the WG, a new defects classification was elaborated.

The guidelines for the classification were presented in April 2006 [12] and the classification was completed by the WG in the following year with the precious contribution of several foundries. The proposal refers to metallurgically-based defects of HPDC products. Defects directly related to handling, finishing, machining operations following ejection from the die were excluded from the classification, even if it is well known that they could be possible causes for product rejection. In this way the range of defect type was not excessively widened.

The present classification of HPDC defects is of hybrid type and multi-level, as schematically shown in Figures 5 and 6. In the first level (level I) the defect geometry/location approach is followed, taking into account the different types of controls performed on cast parts to reveal defects: dimensional, visual inspections and controls involving the bulk material. Thus three wide groups of defects are defined in level I: geometry, surface and internal defects. Sub-surface defects are included in surface defects when they are so close to the component surface to affect surface features and to be detected by surface control methods. Level II is based on the origin-based approach.

Defects are grouped into several classes accordino to their general metallurgical origin:

- defects related to the presence of gas (gasrelated defects);

- defects related to material volume contraction during metal solidification (shrinkage defects);

- defects related to thermal contraction prevented by previously solidified metal or by the die (thermal contraction defects);

- defects related to incorrect filling of the die cavity (filling defects);

- defects related to metal/mould interaction (metal/die in the specific case of HPDC process);

- defects related to the presence of phases different from the suitable ones(undesired phases), originated by the interaction of the metal with external environment during or related to melting, casting, filling or extraction/ejection from the mould/die;

The metallurgical origin taken into account in level II is of general type and it is not directly related to the specific causes of the defects. Nevertheless, the knowledge of metallurgical origin could supply starting points for corrective actions. For this reason, a clear correspondence between classes of defects in internal and surface defects was considered useful and the same names were given to analogous level II classes of internal and surface defects.

As far as geometry defects are concerned, a geometry approach was followed also for level II and excess or lack of material as well as other unsuitable geometries were considered as different classes. The exact terminology and definition of each defect is given in level III. Frequently the term adopted for the single defect in level III better specifies the metallurgical origin of level II. In a few cases, the presented terms correspond to those adopted in European standard EN 12258-1 [12]. The final document on the classification, now in progress, will include terminology in different languages, a short description for each defect with illustrations and reference macro/micrographs to help readers and foundries in identification of defects found in cast parts.

 

It has been already explained that level II, and partly level III, are based on the metallurgical origin of defects. Further, it has also been observed that the same metallurgical origin can lead both to internal and to surface defects. These features will be highlighted in the second part of this paper, on P&TF’s December issue, where parallel level II classes as well as the single defects of level II will be presented.

 

 

 

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