Solubilization and Artificial Aging (T6); This group includes products not cold worked after the heat treatment of solubilization and for which the mechanical properties or the dimensional stability, or both, are greatly improved thanks to the thermal precipitation treatment.

 

Solubilization and Natural Aging (T4); This group includes products not cold worked after the heat treatment of solubilization and for which the mechanical properties have been stabilized by aging at room temperature.

 

Solubilization and Partial Aging (T64); This group includes products that undergo thermal treatment of solubilization and partial aging to improve cold formability.

 

Aging of Artificial Aluminum (T5); This group includes products not cold worked after the hot forming process such as casting or extrusion and for which the mechanical properties are significantly improved thanks to the precipitation heat treatment.

 

Our systems are also able to perform annealing, recrystallization reclamation, reclamation and distension on parts in copper alloys by molding or casting.

Brass antidezincification:
control of the beta phase (β) residual

Antidezincification for brass: control of the residual beta phase (β)

 

Correlation is defined as the corrosion process whereby the most electronegative metal is removed selectively, this causes a deposit of the pure metal in an inconsistent form.

 

The brasses susceptible to this type of corrosion are those with a composition of more than 15% zinc. This phenomenon is called dezincification. This is where there Is a dissolution of the alloy and a reprecipitation of copper on the surface in a porous layer. Corrosion can continue due to further dissolution of the brass and growth of the dusty copper layer.

 

This phenomenon occurs particularly in waters that contain a lot of oxygen and carbon dioxide or in calm and shallow waters. Dezincification is generally uniform in slightly acidic waters with low conductivity and at room temperature while the attack is often local in neutral or weakly alkaline, saline and warm waters.

 

In the α + β biphasic brasses dezincification is more severe and occurs more often in two stages: first the β phase is attacked and then the α phase. The alpha phase (α) is a solid solution with the fcc reticulum while the beta phase (β) is a non-stoichiometric CuZn intermetallic compound with a bcc crystal structure.

 

BELOW: Partial Cu-Zn status diagram

 

Anti dezincification alloys that have a zinc content of up to 35%, which despite being brass alfa (α), ie, with stable alpha phase at room temperature, contain percentages of beta phase (β) residual that compromises the stability of the material and causes corrosion.

 

The alloys used are subject to constantly updated strict controls to ensure public. For example, for uses where there is contact with drinking water in areas subject to the 4MS standard, the CW602N (ADZ) has been replaced by the CW625N and CW626N.

 

In order to optimize the corrosion resistance characteristics of the material, an annealing heat treatment is prescribed following the hot molding, which allows the solubilization of the residual beta phase to bring the material to a state resistant to dezincification. The omission of this treatment does not allow the alloy to offer the antidiscriminating performances for which it was designed.

 

Control method of the remaining beta phase:


The control method begins with the cutting and preparation of the specimens, followed by polishing suitable for light alloys (abrasive cloths and diamond suspension which allows a suitable surface to be obtained for analysis with an optical microscope). Below the appropriate chemical attack to highlight the different phases with sufficient contrast, in this way, the quantitative analysis of the image allows in an objective way to extract the percentage of each phase in an accurate way.

 

BELOW: CuZn36Pb2As after annealing heat treatment: shows a phase-α microstucture and possible remaining phase-β. The Pb is soluble in this alloy and is presented as small precipitates on the grain edge.

 

The F.lli Temponi metallographic laboratory is able to handle any problem related to dezincification and to provide all the documentation to support the treatment.

 

Bibliography and references:
[1] Characterization of the Microstrutural Aspects of Machinable α-β Phase Brass – G. Pantazopulosand A. Vazdirvanidis, ELKEME Hellenic Research Center for Metals, Athens, Greece
[2] Metallographic etching and reagents: II. For cooper alloys, nickel, and the alpha alloys of nickel – Henry S.Rawdon and Majorie G.Lorentz
[3] http://www.ing.unitn.it/~colombo/brasature/Analisi_di_laboratorio_a.htm

Aluminum alloys for die-casting:
defects and thermal treatment

ALUMINUM ALLOYS FOR DIE-CASTING: DEFECTS AND THERMAL TREATMENT

 

1. Die casting: Definition of defects

During the high-pressure casting process (HPDC is an abbreviation of “High pressure die casting”), defects inherent to the process itself are produced due to various factors. The final properties and the mechanical behavior are a consequence of the microstructure conditions and of the defects deriving from the process before the heat treatment. Component design, alloy properties and process control are the critical parameters that directly determine the quality of the obtained microstructure and possible defects. For example, it can be considered that in the filling phase of the mold there are some extreme conditions:

 

– Complexity of the metal that involves a complexity of the mold.

– High mold speed (over 120 strokes per hour) leads to a high filling speed of the same (over 40 m / s) generating strong turbulence inside.

– High cooling speed from over 700 ° C in the melted state, up to room temperature in about 30 seconds.

 

For these reasons the HPDC (in addition to other casting processes of aluminum alloys such as gravity die casting) can be considered as “a process that generates defects”. Not only does it generate a high average waste (from 5% to 10%), but the type of measurement and the importance of the defects are different and must always be evaluated.

 

2. Classification of defects during die-casting
StaCast (New Quality and Design Standards for Aluminum Alloys Cast Products) is a European project dedicated to aluminum foundries with the aim of developing a new classification of structural defects in the castings and defining the limits of acceptability of these according to the final expected destination.

 

3. Alloys for die-casting suitable for heat treatment
There are a great variety of aluminum alloys, but not all of them are suitable for die-casting and even less for rear heat treatment in order to bring the appropriate mechanical properties and a satisfactory level of stability.

In this case, two of the major producers of die-cast aluminum alloys are used as reference: RAFFMETAL in the province of Brescia, with headquarters in Casto, and RHEINFELDEN in Germany, based in the city of the same name. Both of them have a database in their portal that allows filtering the search to find alloys suitable for die-casting and, among these, alloys that are suitable for heat treatment.

 

These alloys are in majority of the AlSi10Mg Group (EN AB and AC 43500 AlSi10MnMg), and alloys of the AlZnSiMg group (EN AB and AC 71100AlZn10Si8Mg). In the relative technical sheets, there are instructions to minimize the risk of defects during the process. This highlights the fact that in addition to the choice of an alloy suitable for die-casting, the process itself must be done with care to obtain a good pre-heat treatment result.

 

4. T5 and T6 process according to the UNI EN 1706 standard

The process defined as T5 begins in the foundry, with the cooling controlled by the press and follows with artificial aging in the oven. The T6 process, on the other hand, is merely a heat treatment cycle consisting of solubilization hardening followed by artificial aging in the furnace. The heat treatment phase of the T5 process and the T6 process are among the most requested thermal cycles for aluminum alloys obtained by die-casting. These treatments involve heating and maintaining a given temperature so that the precipitation of the phases such as Al-Mg, the aluminum solid solution, the AlFeMnSi compound, etc. are stable over time, without changing any mechanical properties.

 

During the heat treatments, the parts undergo temperature changes that produce the redistribution of the intermetallic components inside the material, but are not able to “repair” any defects in the original jet. The final result after treatment can be totally inadequate, starting from superficial defects such as blisters (Defect B2.1), up to internal or superficial cracks that can produce, in some cases, macroscopic breakage of the piece due to the redistribution of gases residues during the die-casting process.

 

To obtain a satisfactory result, the exchange of information and the collaboration between the foundry and the treatmant are key. With this article our metallurgical laboratory would like to invite the customer for an open discussion with the aim of producing an excellent product while minimizing time and waste.

 

Bibliography and references:
[1] StaCast – New Quality and Design Standards for Aluminum Alloys Cast Products FP7-NMP-2012-CSA-6-PROJECT N.319188
[2] The criteria for the selection and treatment of construction and tool steels Volume Quinto, part two – Micrography – Cibaldi Dr. Cesare

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