Gas consumption in the production of aluminum profiles

It is known, that the aluminum industry consumes huge amounts of natural gas. Therefore, in the current situation with a global rise in prices and a shortage of gas, it's time to think about how to reduce its consumption.. Is it possible to save gas on the modern production of extruded aluminum profiles? We will try to do this using the example of the production of aluminum profiles from 6xxx series alloys.. For now, these are just questions.. We will try to give answers in subsequent publications..

Aluminum alloys 6xxx

Most 6xxx alloys have:

balance of Mg and Si content to form a compound of Mg particles₂Yes (6063 и 6061) or
excess Si content compared to that, which is necessary for the formation of the Mg compound₂Yes (6060 и 6062) - Figure 1.

The dividing line corresponds to the iron content 0,16 % [1].

Figure 1 – The most popular aluminum alloys of the 6xxx series [1]

The vast majority of the world's production of aluminum profiles are profiles from alloys 6060 and 6063. These alloys belong to the 6xxx series and are typical thermally hardenable aluminum alloys.. It means, that they achieve their final strength through heat treatment, and not plastic (cold) deformation (work-hardening).

The main alloying elements of these alloys are magnesium (Mg) and silicon (Si), which form the compound Mg₂Si as particles in three main forms:

β“-Mg₂Si - the smallest particles. Have a rod shape. Give the maximum contribution to strength properties.
β`-Mg₂Si - rod-shaped particles, but big size. Grow from particles of category β“. Give a minimal contribution to strength properties.
β-Mg₂Si - the largest particles. Have a cubic shape. Make no contribution to strength properties.

Production cycle of aluminum profiles from 6xxx alloys

During the manufacture of extruded aluminum profiles from 6xxx alloys, these alloys are subjected to a complex combination of thermal and thermomechanical processes.. At each stage, changes in the structure of the alloy occure, including dissolution., separation or enlargement of particles of magnesium silicide (Mg₂Si). These changes can positively or negatively affect the final mechanical properties of the finished profiles..

The profile production cycle includes (Figure 2):

Melting aluminum scrap or primary and secondary melting ingots
Casting of extrusion ingots (logs)
Homogenization (heat treatment of ingots)
Heating of ingots (billets) before extrusion
Hot extrusion (pressing)
Cooling (hardening) of profiles at the outlet of the press
Thermal hardening (aging).

Figure 2 – Thermal and thermomechanical processes in the manufacture of extruded aluminum profiles from 6xxx alloys [1]

Gas consumption in the production of aluminum profiles

Consider the consumption of natural gas on the example of a typical modern set of equipment for the production of aluminum profiles from aluminum scrap [2].

The total gas consumption per tonne of aluminum is 1135-1255 kW∙h,

including:

Two-chamber melting furnace: 660-700
Casting furnace: 30-50
Homogenising furnace: 195-205
Billet heating furnace: 175-215
Ageing furnace: 75-85

It is believed, that one cubic meter of natural gas is equivalent to 10,6 kW∙h [2].

Old equipment requires more gas

Note, that the above gas consumption figures refer to modern production, where all furnaces and thermal units are equipped with recovery and regeneration systems. This usually saves 15-20 % fuel. therefore, if the equipment in your factory does not have these advanced technologies, then gas consumption will be higher, least, on those 15-20 %.

Where can you save gas?

The amount of heat, which must be introduced into aluminum during its melting or heating of the billet before extrusion is determined by the physical characteristics of aluminum, solid and liquid (Figure 3).

Figure 3 [3]

Regeneration and recovery

If the melting and holding furnace, as well as the billet heating furnace already have regeneration and recovery systems, then it is already difficult to reduce gas consumption on them. If these systems are missing, then this is a real opportunity to save gas. However, this equipment requires a significant investment..

Homogenization of extrusion ingots

Need to take into account, that the technological modes of homogenization of aluminum extrusion ingots were developed back in the 1970s. At that time, the cost of natural gas was much lower, than currently.

The process of homogenization of ingots from 6xxx alloy at a temperature 560-585 ºС usually passes within 5-10 hours.

Goals of homogenization (Figure 4):

Dissolve all Mg₂Si in aluminum base (red line)
Convert iron particles from type β to type α (blue line).

Figure 4 [1]

Question #1:

How justified is the typical holding of alloy ingots 6063 during 5-6 hours, and then 8 hours? According to the chart 4 the main homogenization processes take place for 2-3 o'clock.

Artificial aging of alloys 6xxx

6xxx alloys are typical heat-hardenable aluminum alloys.. It means, that they achieve their final strength through heat treatment, and not plastic (cold) deformation (work-hardening).

The main alloying elements of these alloys are magnesium (Mg) and silicon (Si), which form the compound Mg₂Si in the form of particles in three main forms (Figure 3):

β“-Mg₂Si - the smallest precipitates of Mg₂Si. Have a rod shape. Give the maximum contribution to strength properties.
β`-Mg₂Si - rod-shaped particles, but big size. Grow from particles of category β“. Give a minimal contribution to strength properties.
β Mg₂Si - coarse Mg precipitates₂Si. Have a cubic shape. Do not contribute to strength properties.

Figure 5 [1]

The strength of 6xxx series alloys is directly related to the ability of the material to resist the movement of dislocations during deformation.. These dislocations are formed and move through the material when stresses are applied to it.. With increasing stresses, the number and density of dislocations, that pass through the material, increase until then, until it collapses. The peak of mechanical properties is reached at the maximum distribution density of small particles β“Mg₂Si.

Heating temperature and holding time

Final strength characteristics, for example, alloy 6063 as a result of artificial aging, it strongly depends on the temperature in the furnace and the duration (Figure 5):

The higher the temperature, the faster peak strength is reached
The higher the temperature n, the lower the strength
At temperatures above 170 ºС peak strength is practically not reached

Figure 6 [4]

Question #2:

Isn't it time to apply the modes of furnace artificial aging with a shorter cycle?

Economic technology

In the current situation with a sharp rise in the price of natural gas, it is urgent to look for ways to reduce its consumption, including, in case of production of extruded aluminum profiles. Technologies currently in use have been developed, when natural gas was very cheap. Therefore, the technological parameters and technical requirements used today may well be excessive.. They need to be analyzed and revised taking into account modern economic realities. Any high quality product must be economically justified.

Sources:

Basic Metallurgy 6000 series Extrusion Alloys, Comalco Publication
Energy-optimised route from aluminium scrap to extruded semi-finished products /G. Valder and H. Pfeifer – Otto Junker – 2014
Direct Charged Melter / D. F. Whipple – 2004
Precipitatation Aging / R.W. Hains, Alcan Canada Products, Ltd. – Extrusion Technology Seminar 1977