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Casting porosity specification

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Porosity standards. Engineering Diecasting specifications. These are acceptable porosity standards for machined surfaces. Acceptance to these standards is on a ...
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ANALYZE OF THE POSSIBLE CAUSES OF POROSITY TYPE DEFFECTS IN ALUMINIUM HIGH PRESSURE DIECAST PARTS

Ferencz Peti1, Lucian Grama2 1SC CIE Matricon SA T 2

@ciematricon.ro, lgrama@upm.ro ABSTRACT Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity.The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process.Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium.

Depending on the type of metal being cast, a hot- or cold-chamber machine is used.Die castings are characterized by a very good surface finish (by casting standards) and dimensional consistency.The most common deffect that appear in castings is the porosity type of deffect, which can be gas porosity,shrinkage porosity or leaker.Keywords: High pressure diecasting, aluminium, porosity, mold, casting deffects

1.Introduction This paper presents the porosity type of deffects in the high pressure diecast parts and the main causes that have influence on the porosity type of deffects.Also this paper presents the most common control methods porosity type of deffects and the way to prevent and to reduce the porosity since the product / process development phase.In the paper are presented the standards in terms of porosity of the most important car's and components manufacturers.

2.Porosity type of deffects Porosity type of deffects reduces the quality of the casting usually it's tightness and strenght.

Fig.1.Porosity in the section of an aluminium diecast part The most common porosity type of deffects that appear in aluminium castings processed with high pressure diecasting technology are gas porosity, shrinkage porosity and leaker.Gas porosity can be described as trapped air in the casting which can come from several sources.It can be caused by poor shot end control, poor venting and overflow function or bad gating and runner design.In the figure 1 section of a diecast part with gas porosity.

It can be observed the specific shape mostly regular globular and the opaque shade of the gas porosities.The shrinkage porosities can be described as internal cracks in the casting which can come from several sources, mainly due to thick walls of the casting.This defect is caused by metal reducing its shrinkage with more metal before solidification.Hot spots can also cause shrinkage porosity to be concentrated in a specific zone.In the figure 2 is presented the polished section of a diecast part with shrinkage porosity.It can be observed the specific shape mostly iregular longitudinal and the bright shade of the shrinkage porosities.In the figure 3 are analyzed the dimensions of the shrinkage porosities with the microscope.42

Fig.

2.

Shrinkage porosity in the section of an aluminium diecast part

Fig.3.Analize of the dimensions of shrinkage porosities with microscope

Leaker.

Causes of leaks in casting where pressure tightness is required can be oxide folds and/or inclusion and/or porosity in conjunction with a surface defect which completes the part for a leak.A close analysis of the leaking area may reveal which of the many causes is causing the leak.In the figure 4 is shown the section of an aluminium diecast part with leaker.

Fig.4.Leaker in the section of an aluminium diecast part

3.

Main causes for porosity type of deffects in high pressure diecast aluminium parts Porosity type of deffect can be caused by several factors and process parameters
casting porosity specification
.The main causes for gas porosities can be grouped in more chategories and are the following:

- Shot end parameters: First stage velocity too low; First stage velocity too high; Change over point too early; Change over point too late; Second stage velocity too low; Intensification too late; Intensification too low;

- Metal volume: Wrong shot weight setting on ladle; Blocked pour hole; Blocked launder on dose furnace; Tube constricted on dose furnace - Clamping & Ejection: Irregular operating cycle

- Die surface: Not enough plunger lube/sticking plunger;

Die is too cold; Too much die spray;

- Vacuum & Venting: Leaking vacuum; Vacuum on too soon/late; Ineffective venting and/or overflows

- Die construction: Poor gating and runner design; Difficult casting geometry

- Metal: Metal is too hot/cold;

The main causes for shrinkage porosities are the following:

- Shot end parameters: Metal pressure too low; Wrong deceleration setting; Second stage velocity too low; Second stage velocity too high; Intensification too late; Intensification too low;

- Metal volume: Wrong shot weight setting on ladle; Blocked pour hole; Blocked launder on dose furnace; Tube constricted on dose furnace - Clamping & Ejection: Irregular operating cycle

- Die surface: Not enough plunger lube/sticking plunger; Die is too hot; Not enough die spray;

- Die construction: Poor gating and runner design; Poor thermal control/ hot & cold spots; Difficult casting geometry

- Metal: Metal is too hot/cold; Metal is contaminated and/or dirty; Metal is out of specification; Dross in holding furnace

The main causes for leaker are the following:

- Shot end parameters:

Metal pressure too low; First stage velocity too low; First stage velocity too high; Change over point too early; Change over point too late;

Wrong deceleration setting; Second stage velocity too low; Intensification too late; Intensification too low;

- Die surface: Water in cavity/leaking water channel; Leaking oil heating/ cooling unit; Too much plunger lube; Not enough plunger lube/sticking plunger; Die is too cold;

Die is too hot; Too much die spray;

- Vacuum & Venting: Leaking vacuum; Vacuum on too soon/late; Ineffec
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: The researches included analyze of the influence of chemical composition, geometry and roughness of anodic layer obtained on aluminum casts.Research limitations/implications: Contributes to research on anodic layer for aluminum casting alloys.Practical implications: Conducted investigations lay out the areas of later researches, especially in the direction of the possible, next optimization anodization process of aluminum casting alloys, e.g.in the range of raising resistance on corrosion.Originality/value: The range of possible applications increases for example as materials on working building constructions, elements in electronics and construction parts in air and motorization industry in the aggressive environment.Keywords: Mechanical properties; Metallography; Computational material science; Surface treatment1.Introduction It the continuous growth of use of alloys aluminum in zqhttlqrrsq~shkF}phAnodic oxide layers which are connected fixedly with the aluminum substrate are resistant on corrosion.The corrosion resistance can be reduced by the pores and pits in layer or the presence of harmful alloy-forming elements and admixtures, particularly the copper or by impurities [10].The intermetallic phase of copper with aluminum dissolve during anodizing, which causes lowering the hardness and thickness of coats, and the enlargement the porosity [11].

The gain in thickness of the anodic layer in relation to the thickness of the formed oxide film amounts about to 0.001 Pm per 1V.A porous and conducting layer forms from the basic layer, which is dissolved by electrolyte.The basic layer is simultaneously restored by formation of aluminium oxide that proceeds with the same speed as it transforms into the surface layer.

In this way the basic layer maintains its thickness at almost constant voltage [12].

During the forming process the aluminum oxide occurring

the small increase of mass element as well as his volume.

The layer of oxides is fixedly with substrate very strongly.Dissolving of oxides layer is possible only in basic solutions or acid about larger pH than 8.8 relatively lower than 4.0 [13-15].

The goal of the work is to investigate the properties of anodic layer made on casting aluminum alloys in anodizing process and evaluation of influence of electrolyte and casting method on obtained anodic layer.2.Material and methods The analysis of geometry of surface was based on data acquired with measurement of selected fragments of casts, executed on laser profile measurement gauge MicroProf of the FRT company.

Measurements were executed for 8 samples divided on two groups.First of them was the starting material, in state directly after casting without any processing of surface.Material made up second group after apply an oxide layer by galvanic method.Investigations were carried out on EN AC-AlSi12(b) as well as EN AC-AlSi9Cu3(Fe) alloys.For both EN AC-AlSi12(b) as well as EN AC-AlSi9Cu3(Fe) alloys, high pressure and sand casting was used.The chemical composition of these alloys is showed in Table 1.Four elements were anodized:

xEN AC-AlSi12(b) high pressure cast alloy,

xEN AC-AlSi12(b) sand cast alloy,

xEN AC-AlSi9Cu3(Fe) high pressure cast alloy,

xEN AC-AlSi9Cu3(Fe) sand cast alloy.

Table 1.

Concentation of alloying elements in EN AC-AlSi12(b) and EN AC-AlSi9Cu3(Fe) alloys Elements concentration, % (mass) Alloy Si Mn Fe Zn Mg Cu Al AlSi12(b) 12.5 0.5 0.6 0.1 0.05 0.05 Rest AlSi9Cu3(Fe) 9.5 0.5 0.9 0.5 1.5 3.0 Rest To determine the influence of a kind of electrolyte onto homogeneity of pores in the oxides layer at the same conditions, the samples of EN AC-AlSi12(b) alloy were put under anodic treatment in the presence of the following electrolytes: 3% H2C2O4, 4% H3PO4, 4% H2SO4, 3%CrO3 (Table 2).Table 2.

Anodizing parameters Parameter Value Electrolyte H2SO4 with a concentration 295-315 g/l Temperature -4

2C Pulsating current 2 A/dm2 during 0.25 s 1 A/dm2 during 0.1 s Concentration of aluminum ions 6-9 g/l 3.1.Results and discussion The alloys used for investigation with similar chemical composition were cast by two methods: pressure and gravitational cast, therefore several factors as well: xchemical composition of alloys, xparameters of casting (pressure, to sand form), xattendance of layer, could have influence on the surface geometry formation.

Comparing the two- and three- dimensional surfaces figures

(Fig.1) as well roughness distribution can be clearly state that investigated samples any chemical composition influence on surface forming was found.This result can be found both for covered samples layer with oxide and materials in initial state.Fig.1.The geometrical shape of fragment of studied surface, topography of 3D surface, sand cast alloys; A) EN AC-AlSi12(b), B) EN AC-AlSi9Cu3(Fe) The surface shape geometry is clearly depending on the casting method applied.The surface images of the casts a very similar, regardless the anodizing method is used or not (Fig.2).The roughness values achieves the maximum by 80 Pm.

Using the ' character of altitudes on surfaces.Comparing the geometrical shape of studied anodic layer can be found that it is a representation of substrate surface shape.The surface configuration keeps characteristic features even after applying of anodic layer.

With other words the applying of anodic layer does not affect the geometry profile of surface.Fig.2.Profiles 2D cut out from studied surface in plane N - S,EN AC-AlSi12(b) high pressure cast alloy; A) before anodization, B) after anodization For high pressure cast materials, the maximum roughness value of surface does not exceed 15 Pm.

The roughness distribution on the whole analyzed surface is identical without of any anomalies.On the EN AC-AlSi 9Cu3(Fe) alloy surface there was observed an high of circa 5 Pm.Its formation should be considered with any da
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